Master Thesis by Isabel Kerschgens June 2012, University of Amsterdam
Abstract
Mitragynine, paynantheine and speciogynine belong to the group of corynanthe alkaloids, a large class of biologically active indole alkaloids. Present in the leaves of the Asian plant Mitragyna speciosa (Rubiaceae) they have been used by Thai and Malaysian natives as a substitute for opium as well as for their stimulating activity. Besides the use as a drug, the plant has found application in medicine in the treatment of coughing, diarrhea, muscle pain and hypertension. Interestingly, mitragynine has a stronger analgesic effect than morphine, so that it has been suggested as a useful compound in the treatment of opiate addiction in replacement therapy. About the biological activity of paynantheine and speciogynine there are very little studies reported. Three syntheses of mitragynine have been developed, two starting from enantiopure starting materials and a formal synthesis using organocatalysis. Syntheses of paynantheine and
speciogynine have not been reported so far. Our approach to the three alkaloids proceeds via an asymmetric Pictet-Spengler reaction catalyzed by organic bifunctional cinchona alkaloids. This strategy allows fast and highly selective formation of the tetrahydro-β-carboline skeleton. The second key-step in the synthesis is a Tsuji-Trost allylic alkylation precedented in earlier work of our group. Based on achiral starting materials, a fast and enantioselective excess to mitragynine, paynantheine and speciogynine was established. Additionally, our method allows to design new unnatural derivatives which exhibit improved biological properties.
i Table of Contents
1. Introduction ………………………………………………………………………………………………………. 1
1.1. Mitragyna Speciosa ……………………………………………………………………………………….. 1
1.2. Structure of Mitragynine, Paynantheine and Speciogynine …………………………………. 3
1.3. Biological Activity of (-)-Mitragynine ……………………………………………………………… 3
1.4. Reported Syntheses of (-)-Mitragynine …………………………………………………………….. 4
1.4.1. Synthesis by Takayama et al. …………………………………………………………………….. 5
1.4.2. Synthesis by Cook et al. ……………………………………………………………………………. 6
1.4.3. Synthesis by Ma et al. ………………………………………………………………………………. 9
1.5. Aim and Motivation of the Investigations ……………………………………………………….. 10
2. Synthesis of (-)-Mitragynine, (+)-Paynantheine and (+)-Speciogynine ………………… 11
2.1. Synthetic Strategy and Outline of This Thesis …………………………………………………. 11
2.2. Synthesis of the Components for the Pictet-Spengler Reaction ………………………….. 13
2.2.1. Synthesis of the Tryptamine …………………………………………………………………….. 13
2.2.2. Synthesis of the Aldehyde ……………………………………………………………………….. 14
2.3. Asymmetric Pictet-Spengler Reaction …………………………………………………………….. 15
2.3.1. Use of Binol-Phosphoric Acids as Organocatalysts …………………………………… 17
2.3.2. Use of Bifunctional Cinchona Alkaloids as Organocatalysts ………………………. 20
2.3.3. Optimization of the Thiourea-Catalyzed Pictet-Spengler Reaction ………………. 24
2.3.4. Mechanistic Considerations Towards the Substrate-Catalyst Interaction …….. 26
2.3.5. Necessity of Acid for the Pictet-Spengler Reaction …………………………………….. 29
2.4. Synthesis of the α-Keto-Ester ………………………………………………………………………… 30
2.5. Tsuji-Trost Allylic Alkylation ……………………………………………………………………….. 32
2.6. Final Steps Towards (-)-Mitragynine ……………………………………………………………… 34
2.7. Final Steps Towards (+)-Paynantheine and (+)-Speciogynine ……………………………. 36ii
3. Conclusion ……………………………………………………………………………………………………….. 39
3.1. Summary …………………………………………………………………………………………………….. 39
3.2. Comparison with Other Syntheses …………………………………………………………………. 40
3.3. Conclusion ………………………………………………………………………………………………….. 41
4. Abbreviations …………………………………………………………………………………………………… 43
5. Experimental ……………………………………………………………………………………………………. 45
6. Acknowledgements …………………………………………………………………………………………… 73
References ………………………………………………………………………………………………………………. 751
1. Introduction
1.1. Mitragyna Speciosa
Mitragyna speciosa is a tree growing in the tropical climate of Thailand and Malaysia whereas the leaves and extracts are commercially sold under the name ”Kratom”. The plant is classified as a representative of the family Rubiaceae which is also known as the “coffee family”. With only 10 species worldwide the genus of Mitragyna is relatively small.[1] Its popularity is based on its unique biological activity, so that the leaves have been traditionally consumed by natives of Thailand and Malaysia for over hundred years. The extract of the plant has dose-dependent effects: a stimulating activity with small doses while higher amounts lead to euphoric and sedative effects.[1] Therefore, it has been applied as a substitute for opium by natives and has as well been considered as a helpful agent for replacement therapy in the western world. The leaves, either fresh or dried, and the resin were consumed mainly orally to release their relaxing or stimulating activity.[2, 3] Despite the use of Mitragyna speciosa as a drug, the plant has also found medicinal application in the treatment of coughing, diarrhea, muscle pain and hypertension.
[4]
Figure 1: A picture of Mitragyna speciosa by the Dutch botanist Pieter Korthals
Responsible for the biological activity of the tree are the compounds which are present in the leaves or the resin. These compounds (mainly alkaloids) are released during the consumptiona to exhibit their desired effects. The alkaloid content of the leaves is about 0.5%[8] whereas the isolation of 44 different compounds has been reported over the last 87 years. The exact distribution of alkaloids varies depending on the region and each specific plant but mitragynine is generally obtained as the major constituent. In 2004 Takayama investigated the distribution of alkaloids with a plant growing on the campus of the Chulalongkorn University of Bangkok (Scheme 1).
[9] Scheme 1: Alkaloid distributiona
Brewing the leaves in hot water and serving them as a tea, chewing the fresh leaves or smoking the resin.
mitragynine 66.2%
paynantheine 8.60%
speciogynine 6.60%
7α-hydroxy-7H-mitragynine 2.00%
speciociliatine 0.80%
other 15.8%3
More than half of the amount of isolated alkaloids turned out to be mitragynine with 66.2%, followed by paynantheine with 8.6% and speciogynine with 6.6%.
1.2. Structure of Mitragynine, Paynantheine and Speciogynine
The naturally occurring indole alkaloid (-)-mitragynine and its analogues (+)-paynantheine and (+)-speciogynine belong the class of corynanthe alkaloids. First isolated in 1965 by the group of Shellard[10] the final structure of mitragynine was confirmed by an X-ray analysis of the group of Zacharias with a mitragynine hydroiodide salt.[11] The isolation of paynantheine and speciogynine followed soon.[12, 13] Structurally, they all consist of an aromatic indole system bearing a methoxy-group at the 4-position. In addition to that, there are two sixmembered rings both sharing nitrogen-4 and carbon-3. In total there are three stereocenters. Mitragynine and speciogynine only differ in the configuration of stereocenter C-20, so that both molecules are diastereomers of each other. Paynantheine and speciogynine have the same configuration at C-20 but paynantheine is bearing a vinyl-group at this position instead of an ethyl-group.
Figure 2: Structures of (-)-mitragynine, (+)-speciogynine and (+)-paynantheine
1.3. Biological Activity of (-)-Mitragynine
Although there are studies which deal with metabolism of paynantheine and speciogynine[14,
15] concrete studies on the biological activity are not reported so far. On the other hand, main
alkaloid mitragynine has been investigated in more detail. In vivo and in vitro studies
indicated that mitragynine is a central nervous system stimulant[3, 16] and primarily acts on µ-
opioid receptors.[17, 18]4
Studies about the relationship between structure of the molecule and biological activity are
summarized in a review by McCurdy et al.
[1] and are shown in Scheme 2.
Scheme 2: Structure and activity in (-)-mitragynine
First of all, the methoxy-group on the C-9 is essential for the biological activity. When the
methoxy group is replaced by a longer alkoxy-group, the biological activity is abolished.
When the group is removed completely to give the natural product corynantheidine an
antagonistb
is produced. This shows that modulation of this functionality can dramatically
alter the biological properties of the molecule. Secondly, oxidation and introduction of a
hydroxyl-group on the C-7 carbon with phenyliodine bis(trifluoroacetate)[19] give the 46-fold
higher active 7α-hydroxy-7H-mitragynine (for the structure see Scheme 1).[8] Finally, loss of
the basic character of the tertiary amine through oxidation as well as disruption of the β-
methoxyacrylate moiety abolish the activity.c
1.4. Reported Syntheses of (-)-Mitragynine
Since the characterization of (-)-mitragynine through the crystal structure in 1965[11] there
have been two syntheses of enantiopure mitragynine and one formal synthesis reported. The
first one has been published in 1995 by Takayama et al. using chiral starting materials
generated by enzymes.[20] Later, Cook et al. developed the second synhesis using a chiral
b
A receptor antagonist is drug that does not provoke a biological response itself upon binding to a receptor, but
blocks or dampens agonist-mediated responses. Mitragynine on the contrary, belongs to the class of agonists.
c
Unfortunately, the origin (references) of the semi-synthesis/SAR-studies are not mentioned in the review. 5
auxiliary.[8] The first organocatalytic approach has been published very recently by Ma et al.
with a formal synthesis of (-)-mitragynine.[21]
1.4.1. Synthesis by Takayama et al.
Takayama’s synthesis starts with commercially available 6-chloronicotinic acid which is
converted into the racemic acetate 2 and subjected to enzymatic hydrolysis to afford the
resulting secondary alcohol 3 and acetate 4 in 100% ee (Scheme 3). In the next step the
enantiopure alcohol was condensed with bromide 5
d
in heated benzene and catalytic amount
of sodium iodide to give pyridinium salt 6 in 56% yield. Reduction of the salt resulted in an
allylic alcohol with two separable diastereomers 7 and 8. Although two diastereomers have
been formed it was assumed that through a Claisen-rearrangement and their corresponding
chair-like transition states, the newly formed stereocenter can be controlled by the absolute
configuration of C-19. Moreover, it was believed that the undesired configuration of
stereocenter C-3 of diastereomer 8 could be converted into the desired one after the Claisenrearrangement.
Both diastereomers 7 and 8 were subjected to the Claisen-rearrangement with
trimethyl orthoacetate and catalytic amounts of benzoic acid to give the corresponding
acetates 11 and 12. The configuration of C-3 in acetate 12 was inverted to 11 with an
oxidation-reduction sequence via a 3,4-dehydroimmonium salt.
Next, a formyl-group was introduced in 11 at C-16 to give the resulting product 13 in 56%
yield.e
The formyl group was converted into the dimethyl acetal and then treated with KOtBu
to generate the corresponding enol ether 14 in the trans configuration. Stereoselective
hydrogenation with PtO2 gave (-)-mitragynine in 9 steps.
d
5-step synthesis, yields are not mentioned.
e
The formyl group was obtained in the enol-form with the undesired cis-configuration. 6
Scheme 3: Total synthesis of (-)-mitragynine published by Takayama et al.
1.4.2. Synthesis by Cook et al.
James Cook from the University of Wisconsin-Milwaukee spent a lot of research on the
chemistry of tryptophanesters. He found out that enantiopure tryptophan esters such as 157
undergo a Pictet-Spengler reaction with aldehydes to condensation product 17f
with the estergroup
and the R2-substituent trans to each other.[22, 23] With this trans-preference the
stereochemistry of C-3 can be controlled in a diastereoselective fashion. Tryptophan esters are
easily available from the naturally occurring amino acid tryptophan and the condensation
product 17 resembles the core structure of mitragynine to great extend. Therefore, for the
synthesis of mitragynine it would be the easiest to start from 4-methoxy-substituted
tryptophan esters and to control the stereochemistry of the created stereocenter C-3 in
condensation product 18 through the preferred trans relationship of the ester group and the
R2-substituent. Unfortunately, methoxy-substituted tryptophans are not available from nature.
For the synthesis of (-)-mitragynine, as a consequence, first the methoxy-substituted
tryptophan ester had to be synthesized.
Scheme 4: Tryptophan ester are desired compounds for the Pictet-Spengler reaction
To realize a synthesis of mitragynine, Cook et al. envisioned a chiral auxiliary approach for
the preparation of enantiopure methoxy substituted tryptophan ester 18 using a modified
Schöllkopf auxiliary (a bis-lactim derived from the amino acid valine). The synthesis starts
with commercially available 3-methoxy-anilin which is easily converted into iodine 20.
[25] to a TMS propargyl-substituted
Schöllkopf auxiliary.[26] At the same time the Boc-group on the indolic nitrogen was removed
and the desired indole derivative 22 was obtained in 82% yield and >95% ee. Removal of the
auxiliary went nicely in one step through hydrolysis with hydrochloric acid to give the desired
methoxy-substituted tryptophanester 16. Employing a sterically demanding substituent on the
amine in three steps, the substrate 25 for the Pictet-Spengler reaction was obtained. With
acetic acid and aldehyde 26 the Pictet-Spengler reaction was carried out in a
diastereoselective fashion to the trans isomer 27. From trans-tetrahydro-β-carboline 27 the
two thiophenol groups were removed and a Ni(COD)2 mediated cyclization was carried out
with intermediate 28 to give 29. From cyclized product 29 the ester-group was removed in
f
This reaction will be later discussed extensively, but for now it is only important to know that a tryptamine
reacts with an aldehyde to a system as 17. 8
three steps and the double bond of 30 selectively hydrogenated with Crabtree’s catalyst. Five
more steps from 31 gave (-)-mitragynine in >95% ee and an overall number of 22 steps.
Scheme 5: Total synthesis of (-)-mitragynine by Cook et al. 9
1.4.3. Synthesis by Ma et al.
In 2011 Ma et al. published a formal synthesis of (-)-mitragynine using organocatalysis for
the introduction of chirality.[21] The Key feature of their work is a chiral fragment synthesized
via an organocatalyzed Michael-addition between 32 and n-butanal with O-TMS-protected
diphenylprolinol 34. 35 serves as a building block for the synthesis of several natural products
as well as for (-)-mitragynine. The organocatalyzed Michael-addition went in good yield,
unfortunately the ee dropped significantly when the reaction was performed on a 1 mmol
scale (from 91% to 82% ee). Having fragment 35 at hand, this was coupled to 4-methoxysubstituted
tryptamine via a reductive amination. 36 was then selectively hydrogenated and
further subjected to saponification and decarboxylation to end up with lactam 37. After
debenzylation through Pd/C-catalyzed hydrogenation to give a primary alcohol, IBXoxidation
to the aldehyde, Pinnick oxidation to an acid and esterification, 37 was converted
into 38. Bischler-Napieralski cyclization followed by reduction afforded intermediate 31,
from which the synthesis could be finalized following Cook’s route (see Scheme 5).
Scheme 6: Formal synthesis of (-)-mitragynine published by Ma et al. 10
1.5. Aim and Motivation of the Investigations
Aim of this master project was to develop an efficient asymmetric synthesis of (-)-
mitragynine, (+)-paynantheine and (+)-speciogynine.
Due to its unique biological activity (-)-mitragynine promises to be an interesting compound
for medicinal applications. In contrast to mitragynine, there are hardly no studies known
which deal with the medicinal evaluation of paynantheine and speciogynine. An easy access
to these compounds through a chemical synthesis would facilitate the studies of these
compounds. No synthesis for these compounds has been reported yet. Moreover, with a
flexible synthetic strategy, not only the natural product itself but also derivatives with
structural variation can be accessible. These compounds might have an increased or even
different biological activity and are therefore interesting to investigate.
In addition to the accessibility of these compounds, chemists always aim for the development
of new synthetic methods. Especially, the field of organocatalysis has grown exponentially
over the last fifteen years. The use of small organic molecules functionalized as catalysts to
perform a diversity of chemical reactions is strongly desired since they bring a number of
advantages compared to metal- or biocatalysts.g
Since the pioneer work in the early 2000s[27-
34] many asymmetric organocatalytic methods have been developed. Based on these
achievements it is a challenge to apply these methods in the total synthesis of complex natural
products.
g
Broad substrate scope, less toxic, easy handling. 11
2. Synthesis of (-)-Mitragynine, (+)-Paynantheine and (+)-
Speciogynine
2.1. Synthetic Strategy and Outline of This Thesis
The retrosynthetic analysis of mitragynine, paynantheine and speciogynine is depicted in
Scheme 7. Due to their strong structural resemblance it was envisioned that all three
molecules can be generated through a common route which splits at a certain point later in the
synthesis. This strategy has been applied before by our group in the synthesis of (-)-
corynantheidine, (+)-corynantheine and (+)-dihydrocorynantheine.[35]
Starting the retrosynthesis, speciogynine 39 can be obtained from paynantheine 40 with
hydrogenation of the vinylic substituent at C-20. Paynantheine 40 and mitragynine 41 both
result from the corresponding α-keto-esters 43 and 42. These two diastereomers are the
products of a Tsuji-Trost reaction which functions as an intramolecular ring closing operation
of intermediate 44.
Scheme 7: Retrosynthesis I 12
α-Keto-ester 45 can be obtained from the corresponding thioacetal 46 which is the product of
an asymmetric Pictet-Spengler reaction between tryptamine 47 and aldehyde 48 (Scheme 8).
The Pictet-Spengler reaction should be carried out with a chiral organocatalyst. Tryptamine
47 was synthesized from commercially available 4-hydroxy-indole.
Scheme 8: Retrosynthesis II
The structure of this thesis is oriented along the retrosynthesis starting with the commercially
available compounds and ending with the natural products mitragynine, paynantheine and
speciogynine. First, the synthesis of tryptamine 35 starting from 4-hydroxy-indole and
aldehyde 36 will be described. The next section deals with the key step of the synthesis, the
asymmetric Pictet-Spengler reaction. Before the separation of the route in the two
diastereomers via a Tsuji-Trost reaction, the transformation of the thioacetal into the ketone
will be described. The last section deals with the finalization of the three syntheses. 13
2.2. Synthesis of the Components for the Pictet-Spengler Reaction
2.2.1. Synthesis of the Tryptamine
The synthesis of tryptamine 47 starts with commercially available 4-hydroxy-indole which
was first methylated with iodomethane and potassium carbonate.[36] Next, 4-methoxy-indole
50 was formylated at the 3-position of the indole ring through a Vilsmeier-Haack reaction
with N-chlorosuccinimide, triphenylphosphine and N,N-dimethylformamide to give aldehyde
51.
[37] Aldehyde 51 was treated with ammonium acetate and nitromethane to form first the
Henry-product which immediately eliminates water to give the conjugated and therefore
highly stable nitro alkene 52. This was subsequently reduced to amine 53 with the help of
lithium aluminum hydride.[38]
Scheme 9: Synthesis of the 4-methoxy-tryptamine
The synthesis of the functionalized amine 47, was continued with protection of 53 using pnosyl-chloride
followed by alkylation with bromide 55 and deprotection after a Fukuyama
protocol.[39, 40] The bromide 55 was synthesized in one step from (E)-1,4-dibromobut-2-en.[35]
Scheme 10: Synthesis of the functionalized tryptamine with help of a Fukayama protocol14
2.2.2. Synthesis of the Aldehyde
The synthesis of the aldehyde starts with installation of a thioacetal moiety in α-position to the
ester group. This was realized with substitution of both chlorides of dichloroacetate 43 by
ethanethiol (Scheme 11).[41]
Scheme 11: Synthesis of methyl bis(ethylthio)acetate
From 58 it was envisioned that after deprotonation a Michael-addition on acrolein would
furnish aldehyde 48 (Scheme 12). As a first attempt Triton-B®
, a quaternary ammonium
hydroxide base, was applied with dimethoxyethane as a solvent for 2 h at room temperature
(method A).[42] Unfortunately, the yield of 18% was rather disappointing. Therefore, another
method was tried using tetrabutylammonium hydrogen sulfate in combination with potassium
carbonate (method B).[43] The high reactivities of acrolein and the formed aldehyde both easily
undergoing polymerization reactions are probably responsible for the low yield. In a third
attempt to synthesize aldehyde 48 the same bases as for method B were applied in the
Michael-addition this time on methyl acrylate instead on acrolein, followed by reduction of
the resulting ester 61 with DIBAL-H. Although the overall yield for this method was higher
than for method B, finally it was decided to synthesize the aldehyde with direct addition on
acrolein due to the simplicity of the procedure and the fact that 20% starting material 58 was
recoverable from the reaction. 15
Scheme 12: Different strategies for the synthesis of aldehyde 48
2.3. Asymmetric Pictet-Spengler Reaction
The Pictet-Spengler reaction is an acid catalyzed ring closing reaction between a tryptamine
and an aldehyde to form tetrahydro-β-carbolines.a
Discovered in 1911 by Amé Pictet and
Theodor Spengler[44] and extended to indole bases by Tatsui in 1928[45] the Pictet-Spengler
reaction still remains one of the most important methods for the preparation of these tricyclic
systems. The mechanism of the reaction starts with condensation of tryptamine 62 with an
aldehyde to form first the corresponding hemi-aminal which looses water to form imine 65.
Protonation of the imine gives the iminium-ion intermediate 66 on which ring-closure occurs
a
The reaction can also be applied to phenylethylamines to form tetrahydroisoquinolines. 16
initiated by the electron rich indole system in a Mannich-type fashion. This step is not only
enantiodiscriminating but has as well been reported as the rate-determining step.[46] With loss
of a proton, 67 re-aromatizes to the tetrahydro-β-carboline 68.
Scheme 13: Mechanism of the Pictet-Spengler reaction with unsubstituted tryptamines
For substituted tryptamines the mechanism is slightly different. Because of the substituent on
the Nb-nitrogen the iminium-ion formation does not occur via protonation, but through loss of
the hydroxyl-group of the hemi-aminal. Therefore, acid is not necessarily required for this
process.
Scheme 14: Mechanism of the Pictet-Spengler reaction with substituted tryptamines
Since the tetrahydro-β-carboline core structure is present in many natural products, the PictetSpengler
reaction became a useful tool for the preparation of these systems.[47, 48] After two
decades of research using auxiliaries and chiral starting materials to create enantiopure 17
tetrahydro-β-carbolines the first catalytic and at the same time organocatalyzed approach was
published in 2004.[49] The group of Jacobsen used a thiourea as an organocatalyst for the
enantioselective synthesis of these compounds. Other publications utilizing organocatalysts
with high enantioselectivity in the Pictet-Spengler reaction followed.[50-52] Because of the
applicability and usefulness of the method, it was envisioned to use organocatalysis in the
synthesis of (-)-mitragynine, (+)-paynantheine and (+)-speciogynine as well.
2.3.1. Use of Binol-Phosphoric Acids as Organocatalysts
First applied by List in 2006, BINOL-phosphoric acids evolved to widely applicable catalysts
for the Pictet-Spengler reaction.[50] The versatile variation of the chiral backbone allows steric
and electronic tuning depending on the required properties for the reaction. The interaction of
the catalyst with the substrate is depicted in Scheme 15. BINOL phosphoric acids are suitable
catalysts for the reaction because the anionic form of the acid is able to coordinate to the
iminium ion intermediate via ion attraction. The backbone of the catalyst creates a chiral
environment around the substrate in which ring-closure occurs selectively from one side. The
interaction of the organocatalyst with the substrate and the resulting stereochemistry are
therefore established in the rate-determining step.
Scheme 15: Interaction of BINOL-phosphoric anion with the iminium-ion intermediate
In our group, BINOL-phosphoric acids have been applied in the synthesis of several natural
products (Scheme 16).[53-55] In the synthesis of arboricine, yohimbine as well as
corynantheine, the Pictet-Spengler reaction was carried out with very good
enantioselectivity. Later in the syntheses, when a Boc-group was installed on the indolic
nitrogen, the intermediates were crystallized to higher ee to furnish the end-products in high
enantiomeric purity. 18
Scheme 16: Natural products synthesized through an asymmetric Pictet-Spengler reaction
To determine the applicability of BINOL-phosphoric acids in the synthesis of mitragynine,
paynantheine and speciogynine three different catalysts (Scheme 17) were screened in the
Pictet-Spengler reaction. Catalyst A and B are sterically different since the backbone of
catalyst B is hydrogenated, whereas catalyst C differs in the electronegativity of the
substituent at the 3-position.
Scheme 17: Pictet-Spengler reaction with BINOL-phosphoric acids
1. Variation of the Catalyst
The reactions were performed with 2 mol% catalyst, 1.2 equivalents of aldehyde and
molecular sieves as a drying agent at room temperature for 24 h. The results of the reactions
are shown in the table below. Unfortunately, the catalysts seemed to be completely 19
unsuccessful in introducing chirality to the reaction. The measured ees were very low and
although all catalysts had the same chirality, different enantiomers were obtained.
entry catalyst drying agent temperature ee [%]
1 A MS 4 Å 0 °C -3
2 B MS 4 Å 0 °C 7
3 C MS 4 Å 0 °C 7
Table 1: Variation of the catalyst
2. Variation of the Temperature
Next it was tried to improve the results of the previous experiments by variation of the
temperature (Table 2). Again low ees were obtained and the enantiomeric outcome seemed
random.
entry catalyst drying agent temperature ee [%]
1 A MS 4 Å -10 °C -2
2 B MS 4 Å -10 °C -6
3 C MS 4 Å -10 °C 0
4 A MS 4 Å -78 °C 11
Table 2: Variation of the temperature
3. Variation of Drying Agents
During the investigations on the phosphoric acid catalyzed Pictet-Spengler reaction it was
noticed that the reaction already proceeds when no chiral acid was used. It was discovered
that the molecular sieves were responsible for the fast product formation and obviously had
sufficient catalytic activity for our system. Originally, the molecular sieves were intended to
work as a drying agent to keep away the water which might interfere in the interaction of the
catalyst with the substrate. As a consequence, it was concluded that when no drying agent or
other drying agents were used, the BINOL-phosphoric acids were able to catalyze the process.
Other drying agents such as MgSO4 and Na2SO4 (entries 1 and 2 in Table 3) were expected to
lack any catalytic activity. Unfortunately, for MgSO4 hardly no product was formed and for
Na2SO4 the ee was really low. When no drying agent was applied and the reaction was
performed with BINOL-phosphoric acids only the ee was unsatisfactory as well.b
At this stage
no further attempts were made using phosphoric acids in this reaction.
b
Even when the catalyst loading was increased by twice the amount, no change in the ee was observed. 20
entry catalyst drying agent temperature ee [%]
1 B MgSO4 -10 °C –
2 B Na2SO4 -10 °C -9
3 B – 0°C -10
Table 3: Variation of drying agents
2.3.2. Use of Bifunctional Cinchona Alkaloids as Organocatalysts
Inspired by the work of Jacobsen who used thiourea catalysts in his Pictet-Spengler
reactions,[49, 56, 57] the bifunctional catalyst developed by Takemoto[58] was applied to the
reaction. The catalyst contains a thiourea functionality which is able to increase the
electrophilicity of carbonyl groups through coordination. Additionally, a tertiary amine is able
to abstract hydrogens and thus donate electrons due to its Lewis-basic character.
Figure 3: Takemoto catalyst
The catalyst was applied in 20 mol%, a normal catalyst loading for thiourea catalysts.
Fortunately, the catalyst was extraordinarily good and gave an ee of 81% (Scheme 18).
Scheme 18: Takemoto’s catalyst in the Pictet-Spengler reaction
This result encouraged us to investigate the bifunctional catalyzed Pictet-Spengler reaction in
more detail (Table 4). When the catalyst loading of Takemoto’s catalyst was reduced to 10
mol%, the ee dropped by 20% (entry 1). Other catalysts such as b,
[59] carrying the thiourea at
the quinoline part, or d,
[60] bearing a benzimidazole at this position, were not successful and 21
gave a very low enantioselectivity as well as an unclean reaction. Catalyst c developed by the
Soós group gave by far the best result.[61] The observed ee of 89% was even higher than the ee
obtained with the Takemoto catalyst. To find out whether the basic nitrogen or the thiourea
group was responsible for the catalysis, catalysts e-g[62, 63] and one of Jacobsen’s
commercially available thiourea catalysts h were applied.[57, 64]
entry catalyst cat. loading solvent time temperature yielda
ee [%]
1 a 0.1 toluene 3 d rt 47 -60
2 a 0.2 toluene 4 d rt 57 -81
3 b 0.2 toluene 3 d rt 29 -7
4 c 0.2 toluene 3 d rt 90 -89
5 d 0.2 toluene 2 d rt 71 -5
6
b
e 0.2 toluene 5 d rt→ 70°C 57 -11
7
c
f 0.2 toluene/DCM 2 d rt 58 -2
8
b
g 0.2 toluene 5 d rt→ 70°C 63 -6
9 h 0.2 toluene 2 d rt 68 53
10d
h 0.2 toluene 2 d rt 99 10
a
determined by weight, b
after 4 days very little conversion was obtained, the solution was heated to 70°C for 4 h; c due to the
insolubility of the catalyst in pure toluene 50% DCM were added; d
BzOH was used as an additive (20 mol%).
Table 4: Catalyst screening 22
Basic catalysts e and g gave low ees and the reactions were slow so that heating was required
to get some product formation. In these cases product formation occurs just by addition of
enough activation energy but not necessarily catalyzed by the bifunctional catalyst. Catalyst f
showed a really unclean reaction and low ee as well. The Jacobsen catalyst gave an
enantiomerically enriched product of 53% ee but an unclean mixture was observed (entry 9).
When benzoic acid is added as a co-catalyst the ee dropped to 10% (entry 10). Obviously, the
best results were obtained using bifunctional catalysts like a and c which carry a thiourea
group and a basic tertiary amine. The good stereoinduction and the similar results of both
catalysts probably result from the close proximity of the thiourea group to the quinuclidine
system. The functional groups are close together and the system allows less flexibility than
catalyst b or d. Catalyst c can be synthesized from the natural product quinidine 76, an
alkaloid isolated from the Chinchona bark, a number of small trees native to South
America.[65] The pseudoenantiomer of quinidine, quinine 75 is also isolated from the same
tree species and only the position of the vinyl group keeps them apart from being exact
enantiomers. From both pseudoenantiomers catalysts 78 and 79 can be obtained by a
Mitsunobu-Staudinger sequence were the alcohol is converted into an amine followed by
addition of the amine to an isothiocyanate to introduce the thiourea functionality.[61] This
process is easily scalable, so that several grams of the catalyst were accessible.
Scheme 19: Synthesis of the Soós catalyst 23
With both pseudoenantiomeric forms of the catalyst at hand we were then able to synthesize
both enantiomers in the Pictet-Spengler reaction. From the reaction with molecular sieves 4 Å
we had access to the racemic product.c
In Scheme 20 the chromatograms of the chiral HPLC
measurements are depicted. Using catalyst 78 derived from quinine the intermediate leading
to natural (-)-mitragynine becomes accessible. With the other catalyst 79, the route towards
(+)-mitragynine can be established, offering the possibility to investigate the difference in
biological activity between the two enantiomers.
N
H
N
OMe
OBoc
EtS CO2Me SEt
H
(-)-mitragynine
90% ee
N
H
N
OMe
OBoc
EtS CO2Me SEt
H
(±)-mitragynine
78
79
Scheme 20: Enantiomers available from the Pictet-Spengler reaction
c
The molecular sieves also functioned nicely as a catalyst up to a 6 mmol scale. 24
2.3.3. Optimization of the Thiourea-Catalyzed Pictet-Spengler Reaction
Having obtained good results with the Soós catalyst it was tried to further enhance the ee
through variation of the conditions. It was measured that with catalyst 79 the order of elution
in the chiral HPLC was the same as for the corynanthe alkaloids.d
Because of that it was
assumed that with catalyst 79, the right enantiomer is obtained. When the optical rotation was
determined later, it turned out that an opposite direction was observed than for the
intermediate of the (-)-corynantheidine-synthesis.[55] There is a close analogy of optical
rotations between the natural products mitragynine and corynantheidine, paynantheine and
corynantheine as well as for speciogynine and dihydrocorynantheine (Scheme 21). Therefore,
it can be concluded that both Pictet-Spengler products 80 and 81 should have the same optical
rotation. Hence, the enantiomers of mitragynine and corynantheidine elute in reversed order
in HPLC. To synthesize the enantiomer 81 which leads to (-)-mitragynine catalyst 78 is
required instead of 79. This was found, after the optimization experiments were carried out.
Scheme 21: Resemblance of optical rotation and structure
The optimization experiments were performed with catalyst 79 as shown in Table 5. All
reactions were carried out over 24 h and the ee was determined by chiral HPLC. Important to
mention: in all experiments the same batches of tryptamine, aldehyde and catalyst were used
to exclude a deviation of the results depending on the starting materials. Entry 1 of
Table 5 shows the initial experiment carried out in toluene at room temperature for 24 h
where an ee of 89% was observed. Firstly, the temperature of the reaction was changed
d
Very similar natural product series, recently synthesized by our group. 25
(entries 2-4). Surprisingly the ee decreased with lower temperatures. When the temperature
was increased a slightly lower ee was observed which shows that room temperature seems to
be the optimal temperature for the reaction. Variation of the solvent (entries 5-8) did not result
in an increase of the ee. Ethers such as THF and n-dibutylether gave the lowest
enantioselectivity probably due to disturbance of the catalyst-substrate interaction through the
lone-pair bearing oxygens. The addition of drying agents to absorb water which is released
from the reaction (entries 9-11) did not result in significant improvement of the enantiomeric
excess. Usually, water strongly disturbs the catalysis as was often observed for BINOLphosphoric
acids.[51] In our case, water seems not to have a strong influence on the ee. Finally,
optimization of the catalyst loading was studied. Although the catalyst is easy to make it
would be advantageous to reduce the catalyst loading to less than 20 mol%. With a lower
amount of catalyst, the enantioselectivity decreased significantly (entries 12 and 13). Using a
higher catalyst loading (entry 14), however, did not improve the ee in comparison to the initial
experiment with 20 mol%. In addition to the experiments shown in the table, the dependence
of the ee on the equivalent of aldehyde and the molarity of the reaction were investigated. It
was observed, that the amount of aldehyde did not have any influence on the selectivity of the
reaction, so did the concentration. Previous reports stated that there is a strong dependence of
the enantiomeric excess on the concentration caused by possible complexation of the
catalyst.[66] Using solutions of different molarity, however, did not verify this result. Overall,
the reaction showed strong reproducibility and robustness. 26
entry temperature solvent drying agents cat. loading conversion [%] ee [%]
1 rt toluene – 20 mol% 99 89
2 -20°C toluene – 20 mol% 99 82
3 0°C toluene – 20 mol% 99 80
4 40°C toluene – 20 mol% 99 86
5 rt DCM – 20 mol% 91 84
6 rt n-dibutylether – 20 mol% 95 78
7 rt THF – 20 mol% 90 63
8 rt CHCl3 – 20 mol% 90 83
9 rt toluene MS 4 Å 20 mol% 99 90
10 rt toluene MgSO4 20 mol% 99 92
11 rt toluene Na2SO4 20 mol% 99 91
12 rt toluene – 10 mol% 99 78
13 rt toluene – 15 mol% 99 85
14 rt toluene – 30 mol% 99 90
Table 5: Optimization of the Pictet-Spengler reaction
2.3.4. Mechanistic Considerations Towards the Substrate-Catalyst Interaction
Switching to another catalyst system from BINOL-phosphoric acids to bifunctional Cinchona
alkaloids carrying a thiourea group implements a completely different interaction between the
catalyst and the substrate. While deprotonated phosphoric acids form anion pairs with the
iminium ion, this is not possible with a neutral thiourea. In their reports Jacobsen and
coworkers formulated a catalytic cycle involving thioureas.[57] In contrast to our substrate
Jacobsen used unsubstituted tryptamines which do not carry a substituent on the Nb-nitrogen.
Additionally, Jacobsen applied Brønsted acids as co-catalysts which act in combination with
the thiourea catalyst. In the first catalytic step, the thiourea is coordinating to the Brønsted
acid to form the active species of the catalyst 84 (Scheme 22). When the imine enters the
catalytic cycle it is protonated by the catalyst species to from a chiral counter-ion complex 87.
This one consists of the iminium ion and the thiourea carrying the anion of the Brønsted acid
via anion-bonding. The thiourea creates a chiral environment in which the ring-closure takes
place, similar to the catalysis with phosphoric acids. When the attack occurred, ring-closed 27
product 88 rearomatizes to the tetrahydro-β-carboline 89 with reformation of the Brønsted
acid and the thiourea catalyst.
Scheme 22: Proposed catalytic cycle for thiourea catalysts by Jacobsen
Comparing Jacobsen’s and our catalytic system, there are certainly differences, but also
similarities. The most important difference is the presence of a substituent on the Nb-nitrogen.
In contrast to Jacobsen’s Pictet-Spengler reaction our substrate does not need to be activated
by an acid but by simple secondary iminium ion formation. Our presumption is that the
thiourea assists in this imine formation. In our case the imine is formed through abstraction of
the hydroxyl anion from hemi-aminal 90. This abstraction is facilitated by the thiourea via
hydrogen-bonding. A chiral counterion-complex is formed resembling the one proposed by
Jacobsen. In proximity of the thiourea catalyst ring-closure occurs and after re-aromatization
of intermediate 93, the tetrahydro-β-carboline, the thiourea catalyst and water are formed. 28
Scheme 23: Proposed catalytic cycle
When we deal with the process of thiourea catalysis it is necessary to think about the question
why there were such tremendous problems involved with BINOL-phosphoric acid catalysts.
Our conjecture is that the explanation could be found if one had a closer look on the ratedetermining
step of the catalysis. In contrast to the natural products which were successfully
synthesized by our group with the help of BINOL-phosphoric acid catalysis (Scheme 16) in
this system the indole ring is strongly activated by an electron-donating methoxy group
(Scheme 24). Through the strongly nucleophilic indole ring the ring-closure takes place very
fast so that it is probably not the ring-closure anymore which is rate-determining, but the
formation of the iminium ion. Exactly this formation of the iminium ion is facilitated by a
thiourea catalyst (Scheme 23) which gives additional rigidity by two hydrogen bonds.
Moreover, the assumption is supported by the fact that molecular sieves catalyzed the reaction
as well. This is easy to understand since they support abstraction of the hydroxyl-group from
the hemi-aminal through their water-binding properties. To conclude, having the methoxygroup
present on the indole ring consequently changes the rate-determining step of the
reaction and another catalyst system is required. 29
Scheme 24: Strong activation through electron-donating methoxy-group
2.3.5. Necessity of Acid for the Pictet-Spengler Reaction
During the scale-up of the Pictet-Spengler reaction it was discovered, that a freshly prepared
aldehyde resulted in significant lower ee and yield as an aldehyde which was made half a year
ago and stored in the fridge under argon. The same result was observed when an older batch
of aldehyde was columned and used immediately in the reaction. This result was highly
surprising since a possible oxidation of the aldehyde over time (Scheme 25) was always
considered disadvantageous for the enantioselectivity.e
In our case however, acid is absolutely
required for a good stereoselective reaction.f
Scheme 25: Oxidation of the aldehyde
The source of acid can be either partly oxidized aldehyde or added benzoic acid. The results
available are of a rather qualitative nature.
An aldehyde batch which was freshly made gave an ee of 52% (90% with an old batch) and a
yield of 60% (80-90% with and old batch). If the aldehyde was exposed to air for two days at
room temperature, the ee increased to 66%. Two further days resulted in an ee of 81%, when
benzoic acid (20 mol%) was added the ee was even increased to 88%. Obviously there is a
strong dependence between acid and enantioselectivity. A possible reason for this necessity to
achieve a selective reaction might be a protonation of the quinuclidine-nitrogen. Through
protonation, additional hydrogen is available which can assist in beneficial interaction with
e
Carboxylic acids are able to catalyze the Pictet-Spengler reaction as well which usually results in a decrease of
the ee.
f
Although the aldehyde was stored under argon, the compound surely was exposed to air when the container was
opened and closed. 30
the substrate through hydrogen-bonding (Scheme 26). It is hard to predict with which group
the hydrogen-bonding takes place but a possible interaction would be with the abstracted
hydroxyl-anion. It is also possible that internal hydrogen binding takes place in the catalyst,
forcing the catalyst in a special conformation which is beneficial for the stereoinduction.
Scheme 26: Protonation of the catalyst
2.4. Synthesis of the α-Keto-Ester
The next step towards the desired corynanthe alkaloids was the deprotection of the thioacetal.
At first instance the indole-nitrogen of 46 was protected with a Boc group (Scheme 27). This
is necessary since the indole nitrogen is more nucleophilic than the Nb-nitrogen and,
unprotected, forms irreversible ring-closure on the α-keto-ester instead the Nb-nitrogen.
Additionally, the indole ring is stabilized against oxidation during hydrolysis.[55] Bocprotected
tetrahydro-β-carboline 81 is transformed to salt 100 with addition of AgOTf. The
salt is hydrolyzed in the next step to give the α-keto-ester. 31
Scheme 27: Synthesis of the α-keto-ester
To shorten the synthesis it was attempted to use an aldehyde containing the keto-functionality
already, so that Boc-protection and the two following steps are not necessary anymore.
Previous investigations in our group experienced great instability of the corresponding methyl
ester. The analogous tButyl-ester however, promised to be more stable to the conditions and
could later be transformed to the desired methyl ester 45. The aldehyde was applied in the
Pictet-Spengler reaction with 20 mol% bifunctional catalyst (Scheme 28).
Scheme 28: Pictet-Spengler with α-keto-ester
Unfortunately, instead of the corresponding Pictet-Spengler product 103, enamine 104 was
obtained forming a stable, conjugated and irreversible intermediate. A short cut of the
synthesis using aldehydes carrying a keto-group already was therefore not possible. 32
2.5. Tsuji-Trost Allylic Alkylation
The final ring was installed via a Tsuji-Trost allylic alkylation. In general, a palladium
complex is added to the alkene which coordinates to the allylic fragment with loss of the
OBoc group to from 102 (Scheme 29). A base is added to deprotonate the α-position of the
ketone to form the enolate 103 which undergoes nucleophilic attack on the allylic fragment.
As a product a six-membered ring bearing a vinyl-substituent is formed.
Scheme 29: Mechanism of the Tsuji-Trost allylic alkylation
The stereoselectivity in the reaction is determined by the already existing stereocenter. After
formation of enolate 103 there are two conformations possible. The first one having the single
bond in a s-trans configuration leading to diastereomer 107 and the second one with the single
bond in a s-cis configuration which results in stereoisomer 110. In the intermediate with s-cis
configuration unfavorable steric interactions occur making the formation of isomer 110 rather
unlikely. On the other hand, intermediate 106 does not show this unfavorable interaction
while having an s-trans configuration. Therefore, the formation of isomer 107 is clearly
favored and leads to preferred (S)-configuration of stereocenter C-15. Influence on
stereocenter C-20 is rather difficult since there is no stereoselective induction in close
proximity of the substrate available. 33
Scheme 30: Diastereoselection of stereocenter C-15
In the reaction we used allylpalladium(II) chloride dimer as the catalyst precursor. The
precursor was combined with a bidentate phosphine ligand to from the active catalyst species.
As a base, N,N-diisopropylethylamine (Hünig’s base) in combination with Cs2CO3 was
applied. The reaction gave the two desired diastereomers in a ratio of 4:1 with predominance
of the cis-isomer which leads to (-)-mitragynine.
Scheme 31: Tsuji-Trost reaction using DiPEA as a base
As another base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was used since its basicity is
slightly higher and it was expected to change the rate of enolate formation. This possibly
changes the ratio between the observed isomers as well (Scheme 32). The resulting ratio was
about 1:1 which is favorable because an equal amount of material is obtained to finish both
routes towards mitragynine, paynantheine and speciogynine. Unfortunately, a third isomer
113 was observed disturbing the separability of the two favored isomers. Exposing the two 34
isomers 111 and 112 to catalytic amounts of DBU and following the mixtures by NMR
revealed that the unfavored isomer was formed from 111 in the basic medium. The transisomer
112, however, remained unaffected by addition of base. From that it was concluded
that the unwanted isomer is formed through an isomerization process from cis-isomer 111 and
ends up with inversion of stereocenter C-15. Unfortunately, the structure could not be
confirmed analytically, since the isomer was always observed in mixtures with 111 or 112 and
never isolated separately.
The attempts to use different ligands with increased bite angles in the reaction with DiPEA
did not lead to significant improvement of the cis/trans ratio. On the contrary, with increased
bite angle, the reaction rate decreased significantly and product formation stopped completely
at an angle of 111.7° (Xantphos). Dppm with a smaller bite-angle than dppe (72° compared to
85° for dppe) or P(OMe)3 as a monodentated ligand barely resulted in product formation.
Scheme 32: Tsuji-Trost reaction with DBU as a base
2.6. Final Steps Towards (-)-Mitragynine
The synthesis towards mitragynine went on with conversion of ketone 111 to enol ether 114.
This was realized with a Wittig reaction using (methoxymethyl)triphenylphosphonium
chloride as an ylid-precursor. The reaction was performed in high yield furnishing the Z-35
isomer as the only product. At this stage partial crystallization of the racemate occurred so
that the filtrate was observed with an increased ee of 98%.
Scheme 33: Synthesis of enol ether 114 leading to mitragynine
To remove the Boc-protecting group on the indolic nitrogen, enol ether 114 was exposed to a
TFA/DCM mixture (Scheme 34). Besides deprotection, isomerization of the enol ether to the
right configuration took place. Through protonation of the ester to intermediate 117, the
double bond turns into a single bond allowing free rotation. After loss of a proton the
thermodynamically more stable E-configuration is obtained.
Scheme 34: Isomerization and deprotection of enol ether 114
To obtain a good yield in the reaction it is important to work very dry and to use high quality
TFA. Due to the hygroscopicity of TFA there are traces of water present which easily
hydrolyze the enol ether. An attempt to remove traces of water from the reaction was to add
the anhydride of TFA (TFAA) to the reaction mixture so that possible water present reacts
with the anhydride to form acid again (Scheme 35). Addition of TFAA prevents the 36
hydrolysis of the enol ether very well, but the equivalent of TFAA needs to be chosen
carefully. Using an excess of TFAA leads to acylation of the indole ring, yielding the very
stable but undesired product 120 (Scheme 35). Through the electron donating methoxy group
the substrate is obviously strongly activated and undergoes Friedel-Crafts acylation very
easily. This process was prevented if the quantity of TFAA was reduced significantly to
catalytic amounts.
Scheme 35: Addition of TFAA to the reaction mixture
The last step of the synthesis represented hydrogenation of the vinyl-group which went in
quantitative yield to furnish (-)-mitragynine in 14 steps, 98% ee and an overall yield of 13%
from commercially available 4-hydroxyindole.
Scheme 36: Hydrogenation of the vinyl-group
2.7. Final Steps Towards (+)-Paynantheine and (+)-Speciogynine
The synthesis of paynantheine and speciogynine proceeded the same way as for mitragynine.
From α-keto ester 112 via a Wittig reaction enol ethers E-116 and Z-116 were synthesized
(Scheme 37). To obtain two isomers in this step is at first instance not relevant because both
can be used in the next deprotection/isomerization step. For crystallization, however, it was
indeed important since a strong difference was observed for the two isomers. Unfortunately,
only E-116 was crystallizable to 99% ee but with Z-116 neither the racemate nor the 37
enantiomer crystallized. Because of that only the minor isomer observed from the Wittigreaction
was obtained in enantiopure form.
Scheme 37: Wittig reaction of α-keto ester 112
For the next step both products E-116 and Z-166 can be used synthesize paynantheine which
was performed with racemic material in 75% yield. To yield the end-products in high ee, the
reaction was repeated only with E-116 which was crystallized to 99% ee. Using TFA and
catalytic amounts of TFAA furnished paynantheine in 13 steps and an overall yield of 4.3%.
The last step towards speciogynine was realized through hydrogenation.
Scheme 38: Deprotection and isomerization towards paynantheine and speciogynine 38 39
3. Conclusion
3.1. Summary
Within the scope of this thesis, the enantioselective total synthesis of the three natural
products mitragynine, paynantheine and speciogynine was realized. Over an asymmetric
Pictet-Spengler reaction an ee of 90% (89% scale-up) was introduced in the route which was
later increased by crystallization.
Scheme 39: Summary of the synthesis towards mitragynine, paynantheine and speciogynine 40
After a Tsuji-Trost reaction two diastereomers were obtained each leading to the natural
products mitragynine or paynantheine and speciogynine. Mitragynine was made in 14 steps
with an ee of 98% in the final product. Paynantheine and speciogynine were synthesized with
an ee of 99%.
3.2. Comparison with Other Syntheses
To formulate a precise comparison of our synthesis of (-)-mitragynine with the already
published routes (presented in the introduction: section 1.4) is rather difficult because the
overall aim of research has been different in each case. However, I will try to point out some
characteristics of each synthesis and ours. When Takayama published the first synthesis, the
focus was not to develop the most efficient way but to make the molecule in first instance.
Using enzyme-catalyzed hydrolysis of an acetate, an enantiopure alcohol was obtained. The
synthesis is relatively short, but the observed yields remain rather low, so that an overall yield
of 3% is obtained for 9 stepsa
compared to 13% yield with our synthesis. Although the
enantiodetermining step results in higher ee than our organocatalyzed Pictet-Spengler
reaction, enzymes always feature limited substrate specificity and the question of availability
arises. For the biological evaluation of unnatural derivatives, the synthetic use of enzymes
might be problematic. Our catalyst on the contrary allows more substrate-flexibility and is
based on the Cinchona bark, a cheap and renewable resource. The synthesis by Cook
published 14 years later envisioned the synthesis of several natural products applying a chiral
auxiliary strategy. From an intermediate in the synthesis several natural products including
mitragynine were reached. Although the observed enantioselectivity of the asymmetric step
was higher than with our Pictet-Spengler reaction (95% ee compared to 90% ee), we were
able to exceed the enantiomeric purity of the final product later by crystallization. Overall,
Cooks synthesis is significantly longer than ours (23 steps compared to 14), unfortunately the
overall yield could not be determined due to missing information about the yields of single
steps. The most recent published formal synthesis by Ma et al. uses an organocatalytic
approach. A lower ee of 81% was achieved in the scale-up and the synthesis was not finished,
so that the overall yield could not be determined.
a
Synthesis of bromine 5 not included. 41
3.3. Conclusion
The synthesis of (-)-mitragynine, (+)-paynantheine and (+)-speciogynine has been realized
using an organocatalytic Pictet-Spengler reaction. It is therefore the first completed synthesis
of mitragynine using organocatalysis and the first synthesis of paynantheine and speciogynine
in general. With our route, new analogues can be synthesized which might show increased or
different biological activity.
Additionally, we developed the first Pictet-Spengler reaction catalyzed by a bifunctional
organocatalyst reaching an ee of 90%. Strongly activated substrates with electrondonating
substituents on the indole system might not be catalyzed by BINOL-phosphoric acids. In
these cases, a bifunctional catalyst bearing a thiourea might be a possible solution. This turns
the class of bifunctional Chincona alkaloids to a new attractive catalyst species for the PictetSpengler
reaction on which further research is worthwhile. 43
4. Abbreviations
Ac acetyl
aq. aqueous
BINOL 1,1′-bi-2-naphthol
Bn benzyl
Boc tert-butylcarbonyl
b.p. boiling point
bs broad signal (NMR)
Bu butyl
Bz benzoyl
cat. catalyst
d doublet (NMR)
d day(s) (reaction)
DBU 1,8-diazabicyclo[5.4.0]undec-7-ene
DCM dichloromethane
DIAD diisopropyl azodicarboxylate
DIBAL-H diisobutylaluminium hydride
DiPEA N,N-diisopropylethylamine
DMAP 4-dimethylaminopydridine
DME 1,2-dimethoxyethane
DMF N,N-dimethylformamide
DMSO dimethyl sulfoxide
DPPA diphenylphosphoryl azide
dppe 1,2-bis(diphenylphosphino)ethane
dppm 1,2-bis(diphenylphosphino)methane
E entgegen
ee enantiomeric excess
equiv. equivalent(s)
Et ethyl
h hours
Hz Hertz 44
HPLC high-performance liquid chromatography
HRMS high-resolution mass spectroscopy
IBX 2-iodoxybenzoic acid
IR infrared
L ligand
LAH lithium aluminium hydride
LDA lithium diisoproylamine
m meta
m multiplet (NMR)
Me methyl
min minute(s)
MP melting point
MS molecular sieves
ND not determined
NMR nuclear magnetic resonance
NOE nuclear Overhauser effect
Ns nosyl
o ortho
p para
ppm parts per million
PE petroleum ether
Ph phenyl
rt room temperature
s singlet (NMR)
t tert
t triplet (NMR)
Tf trifluoromethanesulfonate
TFA trifluoroacetic acid
TFAA trifluoroacetic anhydride
THF tetrahydrofuran
TLC thin layer chromatography
TMS trimethylsilyl
Z zusammen 45
5. Experimental
General remarks:
All 1H-NMR and 13C-NMR (APT) spectra were recorded with a Bruker Avance 400
spectrometer (1H 400 MHz, 13C 100 MHz) at room temperature. IR spectra were obtained
using a Bruker IFS 28 FT-spectrophotometer. Optical rotations were measured with a PerkinElmer
241 polarimeter. Analytical thin layer chromatography was performed using Merck
TLC plastic roll 500 x 20 cm silica gel F254. Flash chromatography was carried out on
Biosolve 60 Å (0.032 – 0.063 mm) silica gel. Ees were determined on Chiracel®
OD-H
(Chiral Technologies Europe, 0.46 cm x 25 cm) columns. Melting points were measured with
a Leitz-Wetzlar melting point microscope and are uncorrected. Mass spectra and accurate
mass measurements were performed using a JEOL JMS-SX/SX 102 A Tandem Mass
Spectrometer.
All reactions were carried out in oven-dried glassware with magnetic stirring under nitrogen
atmosphere. Tetrahydrofuran (THF) was freshly distilled from sodium and benzophenone.
Toluene was stored under 4 Å molecular sieves. Commercial reagents and solvents were
purchased from Biosolve, Sigma-Aldrich, Fluka or Acros and used as received. 4-hydroxyindole
was purchased from AK Scientific Inc. (100g, 215 $). Powdered 4 Å molecular sieves
(Fluka) were dried at 200°C and 0.1 mbar.
(E)-4-bromobut-2-enyl tert-butyl carbonate
11 g (0.1 mol) KOtBu and 0.5 g 18-crown-6 was dissolved in 250 mL anhydrous DMSO. A
CO2-stream generated from dry ice was directed through the solution with two thick needles
and the mixture was stirred with a mechanical stirrer for 1 h. After formation of a thick gel,
21.4 g (0.1 mol) of 1,4-dibromobutene dissolved in 40 mL THF was given to the mixture in
one portion. After 2 h of stirring the reaction was quenched with half saturated NH4Cl
solution. The layers were separated and the aqueous phase was extracted with Et2O (3x). The
organic layers were combined and washed with water (2x) and the organic phase was dried 46
with MgSO4 and the solvents evaporated. A column of thickness: 66 mm and length: 150 mm
was packed with EtOAc:PE, 6:94; Eluent EtOAc:PE, 6:94 (1L) and EtOAc:PE, 8:92 (1L).
The product was isolated as a colorless oil.
Yield: 43% (10.9 g, 0.043 mol)
1H-NMR (400 MHz, CDCl3) δ = 6.02 (m, 1H), 5.90 (m, 1H), 4.60 (d, 2H, J=5.70 Hz), 3.97 (d,
2H, J=7.40 Hz), 1.51 (s, 9H) ppm.
For the remaining analytical data see:
M. J. Wanner, E. Claveau, J. H. Van Maarseveen, H. Hiemstra, Eur. J. Org. Chem. 2011, 17,
13680-13683.
methyl 2,2-bis(ethylthio)acetate
4.6 g (0.2 mol) sodium was placed in a three-neck flask with a dropping funnel and a
condenser under nitrogen-atmosphere. The flask was chilled in an ice-bath and 100 mL of
methanol was added slowly. When the sodium dissolved completely, 15 mL (0.2 mmol)
ethanethiol was added dropwise. The ice bath was removed and 10.39 mL (0.1 mol) methyl
dichloroacetate was slowly given to the mixture and the suspension was stirred for 48 h at
room temperature. The mixture was quenched with 75 mL H2O and 150 mL of diethyl ether.
The ether layer was separated and washed with water (50 mL) and saturated NaCl solution
(50 mL). The organic phase was dried over MgSO4, the solvent removed and the resulting oil
purified by distillation: 60°C (1.4∙10-1 Torr); Lit: 125-127°C (5 Torr).
Yield: 87% (16.87 g, 0.087 mol)
1H-NMR (400 MHz, CDCl3) δ = 4.40 (s, 1H), 3.80 (s, 3H), 2.74 (m, 4H), 1.30 (t, 6H, J=7.43
Hz) ppm.
13C-NMR (100 MHz, CDCl3) δ = 13.9, 24.9, 49.8, 52.5, 169.5 ppm.
IR: ν = 2967, 2929, 1731, 1434, 1260, 1137 cm-1
.
For the remaining analytical data see:
L.M. Lerner, J. Org Chem. 1976, 41, 2228-2229. 47
Synthesis of aldehyde 48
Michael-addition
2.91 g (15 mmol) of methyl 2,2-bis(ethylthio)acetate was dissolved in 50 mL of acetonitrile.
8.29 g (60 mmol) finely powdered potassium carbonate and 0.255 g (0.75 mmol)
tetrabutylammonium hydrogen sulfate was given to the mixture. Then, freshly distilled
acrolein was added in two portions one at the beginning of the reaction (2.5 mL, 38.5 mmol)
and one after 30 min (2.5 mL, 38.5 mmol). The suspension was stirred at room temperature
for 1 h in total. The inorganic solid was removed by filtration, the solvent evaporated and the
mixture purified via column chromatography (EtOAc:PE, 1:4/1:3). The product was obtained
as a colorless oil.
Yield: 29% (1.09 g, 4.35 mmol)
1H-NMR (400 MHz, CDCl3) δ = 9.83 (s, 1H), 3.80 (s, 3H), 2.64 (m, 2H), 2.73 (m, 4H), 2.34
(m, 2H), 1.24 (t, 6H, J=7.5 Hz) ppm.
For the remaining analytical data see:
J. Gonzalez, F. Sánchez, T. Torres, Synthesis, 1983, 911-913.
0.969 g (5 mmol) of methyl 2,2-bis(ethylthio)acetate was dissolved in 16 mL of acetonitrile.
2.76 g (20 mmol) of finely powdered K2CO3 salt and 0.085 g (0.25 mmol) of
tetrabutylammonium hydrogen sulfate was added. 0.5 mL (5.5 mmol) of methylacrylate was
given to the mixture and the suspension was stirred at 80°C for 90 min. The inorganic salts 48
were filtered and washed with acetonitrile. The solvent was evaporated and the product
obtained as a colorless oil via distillation. b.p. 104°C, 8∙10-1 Torr.
Yield: 66% (0.9538 g, 3.40 mmol)
1H-NMR (400 MHz, CDCl3) δ = 3.80 (s, 3H), 3.70 (s, 3H), 2.65 (m, 4H), 2.57 (m, 1H), 2.34
(m, 1H), 1.24 (t, 6H, J=7.51) ppm.
For the remaining analytical data see:
J. Gonzalez, F. Sánchez, T. Torres, Synthesis, 1983, 911-913.
DIBAL-H reduction
0.954 g (3.40 mmol) 61 was dissolved in 14 mL DCM and the solution was cooled to -78°C.
3.74 mL (3.74 mmol) of DIBAL-H as a 1M solution in hexane was slowly added and the
mixture was stirred for 2 h at -78°C. The reaction was quenched with methanol and 16 mL of
saturated potassium sodium tartate solution (Rochelle salt). The mixture was stirred
vigorously for 30 min. The layers were separated and the aqueous phase was washed with
DCM (4x). The organic layers were combined and the solvent removed. Column
chromatography with EtOAc:PE 1:4/1:3 furnished the product as a colorless oil.
Yield: 70% (0.5934 g, 2.37 mmol)
4-methoxy-1H-indole
44.9 g (325 mmol) K2CO3 and 28.4 g (200 mmol) MeI were added to a solution of 13.31 g
(100 mmol) 4-methoxy-1H-indole in 200 mL acetone. The suspension was stirred under
reflux for 48 h and at room temperature for further 48 h. The mixture was filtered over celite
and the solvent evaporated. Due to limited solubility of the mixture in the eluent, the 49
compound was absorbed on silica gel during evaporation. Purification through flash
chromatography using (EtOAc:PE, 1:2) gave the product as white crystals.
Yield: 89% (13.068 g, 89 mmol)
Melting point: 64 – 67 °C
1H-NMR (400 MHz) δ = 8.16 (s, 1H), 7.37 (t, 1H, J=8.0Hz), 7.11 (m, 2H), 6.93 (t, 1H,
J=2.6Hz), 6.77 (d, 1H, J=7.8Hz), 4.14 (s, 3H) ppm.
13C-NMR (100 MHz) δ = 153.2, 137.1, 122.6, 122.6, 118.4, 104.4, 99.5, 99.4, 55.2 ppm.
IR: ν = 3408, 1615, 1588, 1502, 1464, 1439, 1361 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C9H10ON: 148.0718; found: 148.0770.
4-methoxy-1H-indole-3-carbaldehyde
40.1 g (153 mmol) triphenylphosphine was dissolved in 640 mL dry THF and 20.41 g (153
mmol) N-chlorosuccinimide was added in portions. The suspension was stirred vigorously for
30 min at room temperature. Then, 23.5 mL (306 mmol) DMF was added to the reaction and
the mixture was stirred under reflux for 1 h. Further, 7.5 g (51 mmol) indole was added and
the mixture was stirred under reflux for 1 h. The reaction mixture was cooled down to room
temperature and the THF was evaporated. 640 mL H2O was added to the mixture and it was
stirred under reflux for 1 h. The mixture was cooled down and basified with 10% NaOH. The
aquaous phase was extracted with 200 mL (4x) EtOAc and the organic layers combined and
evaporated. Column chromatography with EtOAc:PE, 1:1 gave the product as orange crystals.
Yield: 81% (7.239 g, 41.3 mmol)
Melting point: 151-154°C
1H-NMR (400 MHz, CDCl3) δ = 10.53 (s, 1H), 8.79 (s, 1H), 7.95 (d, 1H, J=3.07 Hz), 7.24 (t,
1H, J=8.07), 7.10 (d, 1H, J=8.17 Hz), 6.75 (d, 1H, J=7.85 Hz), 4.03 (s, 3H) ppm
13C-NMR (100 MHz, CDCl3) δ = 187.6, 153.7, 137.6, 128.4, 123.1, 118.3, 115.6, 105.3,
101.6, 54.8 ppm. 50
IR: ν = 3246, 1648, 1463, 1386, 1363, 1328 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C10H10O2N:176.0667; found: 176.0712.
(E)-4-methoxy-3-(2-nitrovinyl)-1H-indole
1.76 g (22.8 mmol) NH4OAc and 2.0 g (11.4 mmol) aldehyde were dissolved in 67 mL
nitromethane and the suspension was heated under reflux for 1 h. The mixture was cooled
down to room temperature and the solvent evaporated. The solid was dissolved in a small
amount of methanol and precipitated slowly with water. The solid was filtrated over celite and
dried under vacuum. The product was obtained as a red solid.
Yield: 96% (2.3985 g, 10.99 mmol)
Melting point: 185-188 °C
1H-NMR (400 MHz, CDCl3) δ = 8.67 (bs, 1H), 8.52 (d, 1H, J=13.38 Hz), 7.98 (d, 1H,
J=13.38), 7.61 (d, 1H, J=2.73), 7.25 (d, 1H, J=8.04), 7.07 (d, 1H, J=8.19 Hz), 6.712 (d, 1H,
J=7.92 Hz), 4.04 (s, 3H) ppm.
13C-NMR (100 MHz, CDCl3) δ = 152.9, 138.4, 134.5, 131.9, 130.5, 123.5, 114.4, 107.9,
105.1, 101.3, 54.3 ppm.
IR: ν = 3285, 2940, 1687, 1612, 1511, 1481, 1440, 1304 cm
-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C11H11O3N2: 219.0725, found: 219.0773.
Remark: When the mixture is stirred too long, deprotonated nitromethane adds to the double
bond of the product to form the di-nitro-substituted product. 51
2-(4-methoxy-1H-indol-3-yl)ethanamine
58.53 g (225 mmol) LAH was dissolved in 90 mL of dry THF and cooled to 0°C. 4.456 g
(20.4 mmol) (E)-4-methoxy-3-(2-nitrovinyl)-1H-indole was dissolved in 220 mL dry THF
and given to the mixture with a dropping funnel. After 3 h of reflux the flask was placed in an
ice bath and first water (1.3 g / g LiAlH4
), then 15% aqueous NaOH (1.3 g / g LiAlH4
) and
finally again water (3.25 g / g LiAlH4
) was carefully added with a dropping funnel. The
mixture was stirred vigorously for 15 min and filtrated. The residue was washed with Et2O
(5x) and the combined organic layers evaporated. The product was obtained as a brown solid.
Yield: 99% (3.848 g, 20.2 mmol)
Melting point: 110-105°C
1H-NMR (400 MHz, CDCl3) δ = 8.06 (bs, 1H), 7.11 (t, 1H, J=7.95 Hz), 6.99 (d, 1H, J=7.70
Hz), 6.91 (d, 1H, J=2.15 Hz), 6.51 (d, 1H, J=7.74 Hz), 3.94 (s, 3H), 3.03 (s, 4H) ppm.
13C-NMR (100 MHz) δ =154.4, 138.1, 122.1, 121.2, 116.9, 112.9, 104.5, 98.7, 54.7, 42.8,
30.6 ppm.
IR: δ = 2932, 1585, 1507, 1463, 1436, 1361 cm-1
.
N-(2-(4-methoxy-1H-indol-3-yl)ethyl)-4-nitrobenzenesulfonamide
2.25 mL (16.1 mmol) triethylamine was added to a solution of 2.55 g (13.4 mmol) amine 53
in 50 mL DCM. The mixture was cooled down to 0°C and 3.27 g (14.7 mmol) p-nitrotosylchloride
was added in portions. The suspension was stirred for 2 h at 0°C. The mixture
was extracted with H2O (2x) and sat. NaHCO3 (1x) solution. The organic layers were 52
combined and the compound was absorbed on silica gel. Column chromatography with
EtOAc:PE, 1:2/1:1 gave the product as an orange solid.
Yield: 87% (4.37 g, 11.6 mmol)
Melting point: 136-140 °C
1H-NMR (400 MHz, CDCl3) δ = 7.89 (m, 2H), 7.54 (m, 2H), 7.08 (t, 1H, J=7.99 Hz), 6.88 (d,
1H, J=8.18 Hz), 6.76 (d, 1H, J=2.31 Hz), 6.46 (d, 1H, J=7.78 Hz), 3.92 (s, 3H),3.40 (m, 2H),
3.00 (m, 2H) ppm.
13C-NMR (100 MHz, CDCl3) δ = 153.2, 148.2, 145.3, 137.6, 126.7, 122.6, 121.3, 121.5,
116.1, 110.6, 104.5, 98.2, 54.4, 44.1, 26.1 ppm.
IR: ν = 3406, 1528, 1349 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C17H18O5N3S: 376.0922; found: 376.0971.
(E)-tert-butyl 4-(2-(4-methoxy-1H-indol-3-yl)ethylamino)but-2-enyl carbonate
4.10 g (29.7 mmol) of finely powdered K2CO3 and 2.73 g (10.9 mmol) bromoalkene were
added to a solution of 3.71 g (9.9 mmol) N-(2-(4-methoxy-1H-indol-3-yl)ethyl)-4-
nitrobenzenesulfonamide in 33 mL DMSO. After stirring for 4 h at room temperature 3.0 mL
(29.7 mmol) of thiophenol was given to the mixture. After 2 h the reaction was quenched with
water and the aqueous phase was extracted with EtOAc/NH4Cl. The organic layers were
combined and washed with water (3x). After removal of the solvent the mixture was purified
by column chromatography (EtOAc:PE, 1:1; EtOAc; MeOH:EtOAc, 1:9; EtOAc with 5%
MeOH and 5% NEt3). The product was obtained as a brown highly viscous oil.
Yield: 94% (3.35 g, 9.3 mmol)
1H-NMR (400 MHz, CDCl3) δ = 8.64 (bs, 1H), 7.09 (t, 1H, J=7.95 Hz), 6.95 (d, 1H, J=8.14
Hz), 6.87 (s, 1H), 6.49 (d, 1H, J=7.72 Hz), 5.73 (m, 1H), 5.87 (m, 1H), 4.51 (d, 2H, J=6.2Hz), 53
3.92 (s, 3H), 3.30 (d, 2H, J=5.86 Hz), 3.10 (t, 2H, J=6.74 Hz), 2.97 (t, 2H, J=6.74 Hz), 1.50
(s, 9H) ppm.
13C-NMR (100 MHz, CDCl3) δ = 154.6, 153.2, 138.1, 133.5, 125.3, 122.5, 121.1, 117.1,
113.6, 104.5, 99.1, 81.9, 66.9, 54.9, 50.5, 50.1, 27.6, 26.9 ppm.
IR: ν = 2932, 1740, 1368 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C20H29O4N2: 361.2083; found: 361.2122.
Organocatalyzed Pictet-Spengler reaction
0.033 g (0.055 mmol) of catalyst and 0.0067 g (0.055 mmol) benzoic acid was added to a
solution of 0.1 g (0.277 mmol) tryptamine 47 in 5 mL of toluene. Next, 0.083 g (0.33 mmol)
of aldehyde was given to the mixture and the reaction was stirred for 24 h at room
temperature. The solvent was evaporated and the resulting oil purified by column
chromatography using EtOAc:DCM:PE, 1:4:4. The product was observed as a colorless oil.
Yield: 90% (0.147 g, 0.25 mmol)
ee: 89%
Optical rotation: [α]
= -19.6° (c = 1.03, CHCl3)
HPLC: major enantiomer 18.22 min
minor enantiomer 23.40 min
(Chiralcel®
OD-H, eluent: n-heptane:iso-propanol = 90:10, flow:
0.6 mL/min) 54
1H-NMR (400 MHz, CDCl3) δ = 7.79 (bs, 1H), 7.04 (t, 1H, J=7.9 Hz), 6.94 (d, 1H, J=8.0 Hz),
6.49 (d, 1H, J=7.6 Hz), 5.91 (m, 1H), 5.76 (m, 1H), 4.58 (d, 2H, J=6.1Hz), 3.91 (s, 3H), 3.78
(s, 3H), 3.67 (t, 1H, J=5.5Hz), 3.34 (dd, 1H, J=6.0 Hz, J=14.1Hz), 3.17 (m, 2H), 3.00 (m, 1H),
2.82 (m, 2H), 2.58 (m, 4H), 2.09 (m, 2H), 1.97 (m, 2H), 1.52 (s, 9H), 1.21 (dt, 6H, J=7.5 Hz,
J=13.8 Hz) ppm.
13C-NMR (100 MHz, CDCl3) δ = 171.3, 154.3, 153.3, 137.1, 133.6, 132.3, 126.4, 122.0,
117.2, 108.5, 104.2, 99.6, 82.1, 67.0, 65.0, 56.4, 55.2, 54.6, 53.0, 46.0, 32.0, 29.1, 27.8, 24.0,
23.4, 20.6, 13.6, 13.3 ppm.
IR: ν = 3393, 2931, 1723 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C30H45N2O6S2: 593.2674; found: 593.2722.
Boc-protection of 46
0.37 g (1.70 mmol) di-tert-butyl dicarbonate and 0.035 g (0.28 mmol) DMAP was added to a
solution of 0.673 g (1.13 mmol) tetrahydro-β-carboline 46 in 20 mL toluene. The mixture was
heated to 40°C and stirred for 1 h. Conversion was checked on TLC. The solvent was
evaporated and the product isolated via column chromatography using EtOAc:DCM:PE =
1:4:4.
Yield: 99% (0.779 g, 1.12 mmol)
ee: 89%
Optical rotation: [α]
= -21.7° (c = 1.03, CHCl3)
HPLC major enantiomer 19.52 min
minor enantiomer 9.90 min
(Chiralcel®
OD-H, eluent: n-heptane:iso-propanol = 95:5, flow:
0.5 mL/min) 55
1H-NMR (400 MHz, CDCl3) δ = 7.72 (d, 1H, J=8.4Hz), 7.15 (t, 1H, J=8.2Hz), 6.65 (d, 1H,
J=8.0Hz), 5.90 (m, 1H), 5.73 (m, 1H), 4.58 (d, 2H, J=6.3Hz), 4.15 (dd, 1H, J=2.3Hz,
J=10.6Hz), 3.89 (s, 3H), 3.76 (s, 3H), 3.32 (dd, 1H, J=6.5Hz, J=13.7Hz), 3.22 (dd, 1H,
J=6.5Hz, J=13.9Hz), 3.14 (m, 1H), 2.94 (m, 2H), 2.80 (dd, 1H, J=4.7Hz, J=16.4Hz), 2.69 (m,
4H), 2.42 (m, 1H), 2.08 (m, 1H), 1.94 (m, 1H), 1.80 (m, 1H), 1.68 (s, 9H), 1.51 (s, 9H), 1.24
(t, 6H, J=7.5Hz) ppm.
13C-NMR (100 MHz, CDCl3) δ = 171.2, 153.9, 153.3, 150.2, 137.6, 134.5, 134.4, 125.8,
124.1, 118.8, 114.0, 108.8, 103.3, 83.5, 81.9, 67.0, 65.3, 57.3, 55.2, 54.8, 52.7, 41.4, 33.2,
30.0, 28.1, 27.7, 23.8, 23.7, 19.1, 13.4, 13.3 ppm.
IR: ν = 2974, 2933, 1726, 1438, 1394, 1369, 1325 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C35H53N2O8S2: 693.3199; found: 693.3248.
Deprotection of thioacetal 81 to salt 110
0.5623 g (0.811 mmol) 1,2,3,4-tetrahydro-β-carboline was dissolved in 9 mL anhydrous
DCM. 0.334 g (1.3 mmol) silver trifluoromethanesulfonate was added in two portions, one at
the beginning of the reaction and the second after 60 min of stirring. After 20 h of stirring at
room temperature the precipitated AgSEt was removed by filtration over celite and the solvent
was evaporated. The pyrrolidinium salt (as a mixture of diastereomers) was obtained as a
brown foam in quantitative yield. The salt was hydrolyzed in the next step.
Yield: >99 %
1H-NMR (major diastereomer, 400 MHz, CDCl3) δ = 7.57 (d, 1H, J=8.5Hz), 7.25 (t, 1H,
J=8.3Hz), 6.68 (d, 1H, J=8.0Hz), 6.20 (m, 1H), 5.94 (dt, 1H, J=5.3Hz, J=15.5Hz), 5.35 (t, 1H,
J=9.0Hz), 4.58 (d, 2H, J=5.2Hz), 4.26 (dd, 1H, J=6.2Hz, J=12.4Hz), 4.05 (d, 2H, J=7.3Hz),
4.00 (s, 3H), 3.89 (s, 3H), 3.82 (m, 1H), 3.72 (dd, 1H, J=5.1Hz, J=18.8Hz), 3.12 (m, 5H), 2.84
(m, 1H), 2.40 (m, 1H), 1.68 (s, 9H), 1.48 (s, 9H), 1.30 (t, 3H, J=7.4Hz) ppm.
IR: ν = 1733 cm-1
. 56
Hydrolysis of salt 100
0.633 g (0.81 mmol) pyrrolidinium salt was dissolved in 10 mL DMSO and 2.4 mL H2O was
added. A stream of nitrogen gas was directed through the solution and it was stirred for 45
min at 75°C. The reaction was quenched with water (100 mL) and aqueous NaHCO3 solution
and the aqueous phase was extracted with EtOAc. The organic layers were washed with
water, the solvents evaporated and the residue purified by flash chromatography (EtOAc:PE,
1:5/1:4.5/1:4).
Yield: 73% (0.347 g, 0.59 mmol)
ee: 89%
Optical rotation: [α]
= -41.9° (c = 0.95, CHCl3)
1H-NMR (400 MHz, CDCl3) δ = 7.73 (d, 1H, J=8.4Hz), 7.18 (t, 1H, J=8.2Hz), 6.66 (d, 1H,
J=8.0Hz), 5.80 (m, 1H), 5.68 (m, 1H), 4.53 (m, 2H), 4.04 (m, 1H), 3.92 (s, 3H), 3.89 (s, 3H),
3.20 (dd, 1H, J=7.9Hz, J=13.4Hz), 3.07 (m, 2H), 2.81 (m, 3H), 2.58 (m, 2H), 2.29 (m, 2H),
1.69 (s, 9H), 1.51 (s, 9H) ppm.
13C-NMR (100 MHz, CDCl3) δ = 188.9, 161.6, 153.8, 153.0, 149.9 137.0, 132.6, 131.5,
127.0, 124.4, 118.4, 113.9, 108.7, 103.2, 83.6, 66.5, 58.1, 55.1, 54.2, 52.3, 38.8, 36.3, 32.5,
28.0, 27.5, 18.8 ppm.
IR: ν = 2977, 1726, 1576, 1495, 1438, 1394, 1369, 1321 cm
-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C31H43N2O9: 587.2924; found: 587.2972. 57
Tsuji-Trost cyclization to 111 and 112
0.017 g (0.042 mmol) bis(diphenyphosphino)ethane was added to a solution of 0.007 g (0.02
mmol) allylpalladium(II) chloride dimer in 2 mL of anhydrous THF under argon. The solution
was stirred for 15 min before it was added to a solution of 0.233 g (0.396 mmol) α-keto-ester
101 in 5 mL of dry THF followed by 0.258 g (0.793 mmol) Cs2CO3 and 0.135 mL (0.793
mmol) DiPEA. The reaction mixture was stirred for 20 h at room temperature before it was
quenched with aqueous NH4Cl solution and extracted with EtOAc. The combined organic
layers were dried over Na2SO4, filtrated and the solvent removed. Purification was finalized
by column chromatography using EtOAc:PE, 3:1/2:1. The products were obtained in a ratio of
cis:trans = 4 : 1 and an overall yield of 78%.
cis-isomer:
yield: 62% (0.114 g, 0.245 mmol)
ee: 89%
optical rotation: [α]
= -145.8° (c = 1.07, CHCl3)
1H-NMR (400 MHz, CDCl3) δ = 7.63 (d, 1H, J=8.4Hz), 7.15 (t, 1H, J=8.2Hz), 6.63 (d, 1H,
J=8.0Hz), 6.10 (td, 1H, J=9.9Hz, J=17.2Hz), 4.99 (m, 2H), 3.87 (s, 3H), 3.86 (s, 3H), 3.56 (dt,
1H, J=3.7Hz, J=12.4Hz), 3.03 (m, 5H), 2.88 (m, 2H), 2.67 (m, 1H), 2.25 (d, 1H, J=13.3Hz),
1.77 (dd, 1H, J=12.7Hz, J=23.8Hz), 1.63 (s, 9H) ppm.
13C-NMR (100 MHz, CDCl3) δ = 194.7, 161.4, 154.0, 150.5, 138.2, 137.6, 134.3, 124.5,
118.7, 117.2, 116.6, 108.5, 103.5, 83.7, 60.9, 59.8, 55.3, 52.7, 50.9, 49.5, 40.8, 28.2, 27.0,
25.1 ppm.
IR: ν = 2978, 2942, 2800, 1724, 1606, 1579, 1495, 1439, 1405, 1394, 1369, 1358, 1323 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C26H33N2O6: 469.2294; found: 469.2340. 58
trans-isomer:
yield: 16% (0.029 g, 0.063 mmol)
ee: 89%
optical rotation: [α]
= -38.9° (c = 0.86, CHCl3)
1H-NMR (400 MHz, CDCl3) δ = 7.69 (d, 1H, J=8.3Hz), 7.15 (t, 1H, J=8.2Hz), 6.63 (d, 1H,
J=7.9Hz), 5.60 (m, 1H), 5.06 (m, 2H), 4.30 (d, 1H, J=10.6Hz), 3.87 (s, 3H), 3.84 (s, 3H), 3.49
(m, 1H), 3.12 (m, 3H), 2.87 (m, 5H), 2.27 (ddd, 1H, J=2.5Hz, J=3.5Hz, J=12.8Hz), 1.68 (s,
9H) ppm.
13C-NMR (100 MHz, CDCl3) δ = 194.7, 161.5, 153.9, 150.1, 137.8, 137.5, 133.7, 124.3,
118.5, 116.9, 115.8, 108.6, 103.4, 83.8, 83.5, 60.2, 59.5, 57.0, 55.2, 52.7, 49.3, 46.3, 37.6,
28.6, 28.0, 24.5 ppm.
IR: ν = 2977, 2940, 2837, 1726, 1606, 1579, 1496, 1440, 1407, 1394, 1369, 1318 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C26H32N2O6: 469.2294, found: 469.2340.
Wittig reaction with cis-isomer 111
2.6 mL of a 1M solution of KOtBu in THF was added to a solution of 0.89 g (2.59 mmol)
(methoxymethyl)triphenylphosphonium chloride in 6 mL THF at room temperature. The
solution was cooled to -78°C and stirred for 15 min before it was added to a solution of 0.405
g (0.864 mmol) keton 111 in 10 mL THF at -78°C. The cooling bath was removed and after 1
hour of stirring at room temperature, the reaction mixture was quenched with NH4Cl (5 mL)
and EtOAc (5 mL). After stirring the quenched mixture vigorously for 12 h the mixture was
extracted with EtOAc (3x), the combined organic layers were dried over Na2SO4. Removal of
the solvent and purification by column chromatography (EtOAc:PE, 1:3, 1:2) furnished the Zisomer
as a brown solid.
Yield: 98% (0.419 g, 0.84 mmol) 59
1H-NMR (400 MHz, CDCl3) δ = 7.73 (d, 1H, J=8.4Hz), 7.18 (t, 1H, J=8.2Hz), 6.66 (d, 1H,
J=8.0Hz), 6.075 (s, 1H), 6.05 (m, 1H), 5.07 (dd, 1H, J=2.1Hz, J=10.4Hz), 4.95 (dd, 1H,
J=1.8Hz, J=17.3Hz), 3.89 (s, 3H), 3.78 (s, 3H), 3.75 (s, 3H), 3.02 (m, 4H), 2.88 (m, 2H), 2.65
(m, 2H), 2.00 (d, 1H, J=12.0Hz), 1.64 (s, 9H), 1.50 (m, 1H) ppm.
13C-NMR (100 MHz, CDCl3) δ = 166.8, 156.9, 153.9, 150.4, 138.4, 138.2, 134.4, 124.5,
118.6, 117.0, 116.6, 110.4, 108.2, 103.5, 83.7, 61.9, 61.6, 60.8, 55.3, 51.2, 50.8, 42.6, 39.2,
31.8, 28.1, 25.2 ppm.
IR: ν = 2944, 2838, 2798, 2751, 1727, 1692, 1645, 1579, 1495, 1438, 1360, 1328 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C23H29O4N2: 497.2607; found: 497.2649.
Crystallization:
The product was dissolved in a minimal amount of ethyl acetate and diluted with petroleum
ether. After standing for 24 h at room temperature the crystals were removed by filtration.
Crystals: 4% ee, 0.0485 g, MP: 151-155°C; Filtrate: 98% ee, 0.371 g, 86% yield from 111.
optical rotation: [α]
= -179.9° (c = 0.97, CHCl3)
HPLC major enantiomer 15.22 min
minor enantiomer 12.73 min
(Chiralcel®
OD-H, eluent: n-heptane:iso-propanol = 95:5, flow:
0.6 mL/min)
Wittig reaction with trans-isomer 112
The reaction was performed as described for 114 using 0.109 g (0.233 mmol) of ketone 112
and three equivalents of phosphonium ylid. Purification through column chromatography
gave both the Z-and E-isomer in a ratio of 2.5 : 1. 60
Z-isomer:
Yield: 57% (0.064 g, 0.13 mmol)
ee: 89%
optical rotation: [α]
= +2.8° (c = 0.5, CHCl3)
1H-NMR (400 MHz, CDCl3) δ = 7.70 (d, 1H, J=8.2Hz), 7.13 (t, 1H, J=8.2Hz), 6.62 (d, 1H,
J=7.9Hz), 6.37 (s, 1H), 5.54 (m, 1H), 5.02 (m, 2H), 4.14 (d, 1H, J=10.20 Hz), 3.86 (s, 3H),
3.75 (s, 3H), 3.71 (s, 3H), 3.14 (m, 3H), 2.97 (m, 1H), 2.77 (m, 2H), 2.64 (dq, 1H, J=3.8Hz,
J=11.4Hz), 2.44 (td, 1H, J=3.6Hz, J=12.0Hz), 2.11 (ddd, 1H, J=2.6Hz, J=3.4Hz, J=12.9Hz),
1.73 (m, 1H), 1.65 (s, 9H) ppm.
13C-NMR (100 MHz, CDCl3) δ = 166.8, 157.1, 154.0, 150.2, 139.2, 138.0, 134.6, 124.4,
118.7, 116.1, 115.4, 110.7, 108.6, 103.5, 83.6, 61.9, 60.8, 60.3, 58.6, 55.3, 51.1, 47.0, 42.4,
42.2, 34.5, 28.1, 24.6, 14.1 ppm.
IR: ν = 2937, 1724, 1320 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C28H37O6N2: 497.2607; found: 497.2652.
E-isomer:
Yield: 21% (0.024 g, 0.047 mmol)
1H-NMR (400 MHz, CDCl3) δ = 7.78 (d, 1H, J=8.4Hz), 7.28 (s, 1H), 7.13 (t, 1H, J=8.2Hz),
6.62 (d, 1H, J=8.0Hz), 5.53 (m, 1H), 4.96 (m, 2H), 4.20 (d, 1H, J=10.9Hz), 3.87 (s, 3H), 3.78
(s, 3H), 3.65 (s, 3H), 3.25 (m, 1H), 3.05 (m, 4H), 2.80 (m, 3H), 2.12 (q, 1H, J=12.4Hz), 1.84
(d, 1H, J=12.8Hz), 1.64 (s, 9H) ppm.
13C-NMR (100 MHz, CDCl3) δ = 159.4, 154.0, 150.2, 139.9, 138.1, 134.7, 124.4, 118.7,
115.3, 115.1, 112.2, 108.6, 103.5, 83.6, 61.3, 60.9, 58.1, 55.4, 51.0, 46.3, 38.5, 30.8, 28.0,
24.6 ppm.
IR: ν = 1726, 1703, 1637, 1438, 1359, 1327 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C28H37O6N2: 497.2607; found: 497.2652.
Crystallization of 116:
116 was dissolved in a minimal amount of ethyl acetate and diluted with petroleum ether.
After standing for 24 h at room temperature the crystals were removed by filtration.
Crystals: 73% ee, 0.0109 g, MP: 184-187°C; filtrate: 97% ee, 0.0051 g. 61
The filtrate resulting from the first crystallization was evaporated and the remaining grass
dissolved and the crystallization procedure repeated.
Crystals: 99% ee, 0.005 g, MP: 183-187°C; filtrate: 0.0001 g.
optical rotation: [α]
= +53.7° (c = 1.08, CHCl3)
HPLC major enantiomer 18.69 min
minor enantiomer 16.39 min
(Chiralcel®
OD-H, eluent: n-heptane:iso-propanol = 95:5, flow:
0.6 mL/min)
Synthesis of (-)-dehydro-mitragynine 115
0.4 equiv. trifluoroacetic anhydride was added to 5 mL good quality TFA under anhydrous
conditions. The acid-solution was given to a solution of enolether (0.0336 g, 0.068 mmol) 114
in 15 mL DCM under argon. The reaction was stirred for 17 h at room temperature before it
was quenched with Et2O and neutralized with aqueous NaHCO3. The aqueous phase was
extracted with Et2O, the organic layers combined and dried over Na2SO4. Purification by
column chromatography using EtOAc:PE, 1:2/1:1 gave the product as a yellow solid.
Yield: 61% (0.0164 g, 0.041 mmol)
MP: 84-87°C
Optical rotation: [α]
= -104° (c = 0.93, CHCl3)
1H-NMR (400 MHz, CDCl3) δ = 7.70 (bs, 1H), 7.35 (s, 1H), 7.00 (t, 1H, J=7.9Hz), 6.90 (d,
1H, J=8.0Hz), 6.46 (d, 1H, J=7.7Hz), 6.32 (dt, 1H, J=9.9Hz, J=17.1Hz), 4.91 (m, 2H), 3.88 (s, 62
3H), 3.69 (s, 3H), 3.68 (s, 3H), 3.23 (bd, 1H, J=11.2Hz), 3.07 (m, 2H), 2.94 (m, 3H), 2.72 (dd,
1H, J=2.9Hz, J=11.2Hz), 2.55 (m, 2H), 2.42 (bd, 1H, J=7.6Hz), 1.86 (bd, 1H, J=12.8Hz) ppm.
13C-NMR (100 MHz, CDCl3) δ = 169.0, 160.2, 154.4, 139.4, 137.1, 133.3, 121.8, 117.4,
114.2, 111.0, 107.8, 104.1, 99.6, 61.4, 61.2, 60.9, 60.3, 55.2, 53.5, 51.1, 44.5, 39.0, 30.2, 23.7
ppm.
IR: ν = 3364, 2936, 2838, 2791, 2752, 1698, 1643, 1597, 1570, 1507, 1460, 1435, 1350, 1309,
cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C23H29O4N2: 397.2083; found: 397.2122.
Friedel-Crafts acylation of 114
1.3 mL TFAA was added to 13 mL TFA under anhydrous conditions. The TFA/TFAAsolution
was given to a solution of enol ether (0.665 g, 1.339 mmol) 114 in 40 mL DCM and
stirred for 20 h at room temperature. The reaction was quenched with Et2O and neutralized
with NaHCO3 under strong stirring. The aqueous layer was extracted with Et2O (3x), the
organic layers combined and dried over Na2SO4. Column chromatography using EtOAc:PE
1:2/1:1 furnished product 120 as an orange foam.
Yield: 74% (0.489 g, 0.99 mmol)
Optical rotation: [α]
= -187.6° (c = 1.18, CHCl3)
1H-NMR (400 MHz, CDCl3): δ = 9.95 (s, 1H), 7.84 (m, 1H), 7.39 (s, 1H), 6.59 (d, 1H,
J=8.8Hz), 6.32 (dt, 1H, J=10.0Hz, J=17.0Hz), 4.95 (m, 2H), 4.02 (s, 3H), 3.72 (s, 3H), 3.71 (s,
3H), 3.28 (d, 1H, J=11.1Hz), 3.12 (m, 2H), 2.97 (m, 3H), 2.76 (dd, 1H, J=3.1Hz, J=11.2Hz),
2.57 (m, 2H), 2.47 (dd, 1H, J=2.8Hz, J=6.5Hz), 1.99 (dt, 1H, J=2.4Hz, J=12.9Hz) ppm.63
13C-NMR (100 MHz, CDCl3): 168.8, 161.4, 160.2, 139.2, 137.8, 135.1, 128.9, 128.9, 118.5,
117.6, 115.7, 114.4, 110.6, 108.7, 107.8, 100.7, 61.3, 61.2, 60.7, 55.6, 53.1, 51.1, 44.3, 38.9,
30.0, 23.4 ppm.
IR: ν = 2940, 2846, 2791, 2747, 1700, 1646, 1600, 1561, 1510, 1461, 1436, 1391, 1363, 1310
cm
-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C25H28O5N2F3: 493.1906; found: 493.1949.
Deprotection and isomerization to furnish paynantheine
Isomerization with racemic material:
1.6 mL TFA was added to a mixture of racemic Z-116 and E-116 (0.162 mmol) in 4.8 mL
anhydrous DCM and was stirred for 12 h at room temperature. The reaction was quenched
with Et2O and aqueous NaHCO3. The aqueous phase was extracted with EtOAc, the organic
layers combined and dried over Na2SO4. Purification by column chromatography using
EtOAc:PE, 1:2/1:1 gave (±)-paynantheine as a brown solid.
Yield: 75% (0.035 g, 0.089 mmol)
Isomerization with enantioenriched (99% ee) material:
N
Boc
OMe
N
CO2Me
MeO
N
H
OMe
N
CO2Me
MeO
H
TFA/ TFAA
DCM
H
(+)-paynantheine
E-116 40
99% ee
1.9 µL TFAA was added to 1.7 mL TFA under anhydrous conditions. The TFA/TFAA
solution was given to a solution of 0.023 g (0.048 mmol) E-116 in 5.2 mL dry DCM. The
solution was stirred for 17 h at room temperature. The reaction was quenched with Et2O and
aqueous NaHCO3. The aqueous phase was extracted with EtOAc, the organic layers 64
combined and dried over Na2SO4. Purification by column chromatography using EtOAc:PE,
1:2/1:1 gave (+)-paynantheine as a brown solid.
Yield: 96% (0.0181 g, 0.046 mmol)
ee: 99%
optical rotation: [α]
= +20.2° (c = 0.91, CHCl3)
Lit.: [α]
= +29.4° (c = 1.2, CHCl3)
[67]
1H-NMR (400 MHz, CDCl3) δ = 7.73 (bs, 1H), 7.33 (s, 1H), 7.00 (t, 1H, J=7.9Hz), 6.87 (d,
1H, J=8.1Hz), 6.46 (d, 1H, J=7.8Hz), 5.58 (m, 1H), 4.98 (m, 2H), 3.87 (s, 3H), 3.77 (s, 3H),
3.69 (s, 3H), 3.26 (bd, 1H, J=11.6Hz), 3.17 (m, 1H), 3.02 (m, 4H), 2.75 (td, 1H, J=3.5Hz,
J=11.7Hz), 2.58 (td, 1H, J=4.2Hz, J=11.2Hz), 2.27 (t, 1H, J=11.4Hz), 2.14 (dd, 1H, J=12.0Hz,
J=24.2Hz), 1.95 (d, 1H, J=12.5Hz) ppm.
13C-NMR (100 MHz, CDCl3) δ = 159.8, 154.4, 139.4, 137.4, 133.0, 121.8, 117.5, 115.4,
107.8, 104.3, 99.7, 61.5, 61.3, 60.0, 55.3, 53.2, 51.3, 42.8, 33.4, 23.7 ppm.
IR: ν = 3370, 2940, 2847, 2799, 2751, 1703, 1637, 1596, 1569, 1509, 1461, 1436, 1353, 1336,
1317 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C23H29O4N2: 397.2083; found: 397.2122.
Hydrogenation towards mitragynine
0.011 g (0.028 mmol) of enol ether was dissolved in 2 mL of EtOAc. 2 mg of Pd/C-catalyst
was added and the suspension was stirred under H2 atmosphere (1 atm.) for 12 h. Filtration
over celite furnished (-)-mitragynine as a brown solid.
Yield: 99% (0.011 g, 0.028 mmol)
optical rotation: [α]
= -112° (c = 0.66, CHCl3)
Lit.: [α]
= -126 (c = 1.2, CHCl3)
[10]
1H-NMR (400 MHz, CDCl3) δ = 7.73 (bs, 1H), 7.46 (s, 1H), 7.02 (t, 1H, J=7.9Hz), 6.92 (d,
1H, J=8.0Hz), 6.48 (d, 1H, J=7.7Hz), 3.90 (s, 3H), 3.75 (s, 3H), 3.73 (s, 3H), 3.13 (m, 2H), 65
3.05 (m, 3H), 2.94 (m, 1H), 2.53 (m, 3H), 1.79 (m, 2H), 1.66 (m, 2H), 0.89 (t, 3H, J=7.3Hz)
ppm.
13C-NMR (100 MHz, CDCl3) δ =169.2, 160.5, 154.5, 137.2, 133.7, 121.8, 117.6, 111.5,
107.8, 104.2, 99.7, 61.5, 61.2, 57.7, 55.3, 53.8, 51.3, 40.7, 39.9, 29.9, 23.9, 19.1, 12.8 ppm.
IR: ν = 3367, 2933, 2849, 2796, 2747, 1703, 1643, 1624, 1597, 1569, 1508, 1461, 1435, 1373,
1350, 1310 cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C23H30O4N2: 399.2239; found: 399.2291.
(E)-methyl-2-((2S,3R)-3-ethyl-8-methoxy-1,2,3,4,6,7,12,12b-octahydroindolo[2,3-
a]quinolizin-2-yl)-3-methoxyacrylate
0.0342 g (0.086 mmol) of (+)-paynantheine was dissolved in 6 mL of EtOAc and 6 mg of
Pd/C-catalyst was added and the suspension was stirred for 12 under H2 atmosphere for 12 h.
Filtration over celite furnished (+)-speciogynine as a brown solid.
Yield: 99%
optical rotation: [α]
= +22.8 (c = 0.89, CHCl3)
Lit.: [α]
= +26.8°(c = 0.85, CHCl3)
[67]
1H-NMR (400 MHz, CDCl3) δ = 7.81 (bs, 1H), 7.36 (bs, 1H), 6.99 (t, 1H, J=7.9Hz), 6.86 (d,
1H, J=8.1Hz), 6.45 (d, 1H, J=7.7Hz), 3.87 (s, 3H), 3.58-3.81 (bs, 6H), 3.28-2.93 (m, 5H),
2.78-2.5 (m, 2H), 2.35-2.20 (m, 1H), 2.15-1.82 (m, 3H), 1.50-1.36 (m, 1H), 1.12-0.97 (m,
1H), 0.87 (t, 3H, J=7.4Hz) ppm.
13C-NMR (100 MHz, CDCl3) δ = 159.9, 154.4, 137.4, 133.1, 121.8, 117.5, 107.7, 104.3, 99.7,
61.7, 60.9, 60.3, 55.3, 53.5, 51.5, 39.9, 38.7, 33.7, 30.6, 29.7, 24.4, 23.7, 11.3 ppm.
IR: ν = 2936, 2851, 2802, 2750, 1698, 1635, 1597, 1569, 1509, 1461, 1435, 1356, 1336, 1320,
cm-1
.
HRMS (FAB): m/z calcd. for (M+H)+
C23H31O4N2: 399.2239; found: 399.2289. 66
Recorded spectrum of (-)-mitragynine (CDCl3, 400 MHz): 67
Copy of 1H-NMR spectrum (CDCl3, 300MHz) of (-)-mitragynine published by Cook et al.[8]68
Recorded spectrum of (+)-paynantheine (CDCl3, 400 MHz): 69
Recorded spectrum of (+)-speciogynine (CDCl3, 400 MHz): 70
Chiral HPLC chromatogram of the Pictet-Spengler product 46 in the scale-up 71
Chiral HPLC chromatogram of racemic 46. 73
6. Acknowledgements
Finishing the final stages towards my master degree, I have to thank some people. Thank you,
Henk Hiemstra, for giving me the opportunity to work in this group and the trust in me to
finish this wonderful project. I always felt in very good hands and never regretted coming to
Amsterdam for my master studies. You have provided me with opportunities to experience
the work of a scientist and I am convinced that I grew a lot on that. Thank you as well for
helping me finding a good place for my future.
Martin, thank you so much for being such a good supervisor! I learned incredibly much from
you and hope that during my future research, I am able to apply your way of thinking. Not
only chemically but also personally we came along with each other very well and it was
always a pleasure to work next to you.
Thanks also got to Jan van Maarseveen and Steen Ingemann for accepting me in this group
and for a chat every now and then in the lab. Ron Wever is kindly acknowledged for being my
second reviewer. Suze, Guido, Sjoerd, Luuk, Berend and Brian, thank you for the nice study
atmosphere, not only during this project but also in the courses last year. I would like to thank
the whole group for their kindness and for helping me with daily problems.
Jan Geenevasen, Jan Meine Ernsting and Han Peters are kindly acknowledged for the help
with the NMR-machines and the performance of mass analysis.
Ich möchte meinen Eltern danken, die mich während meines Studiums immer unterstützt
haben. Ohne Euch wäre ich heute nicht an diesem Punkt. Danke, Philipp, dafür dass du mit
mir die letzten zwei Jahre geteilt hast! 75
References
[1] J. E. Adkins, E. W. Boyer, R. McCurdy, Current Topics in Medicinal Chemistry 2011, 11,
1165-1175.
[2] K. L. Jansen, C. J. Prast, J. Psychoactive Drugs 1988, 20, 455-457.
[3] K. L. Jansen, C. J. Prast, J. Ethnopharmacology 1988, 23, 115-119.
[4] S. A. Suwanlert, Bull. Narcotics 1975, 27, 21-27.
[5] W. P. Korthals, Verhandelingen over de Natuurlijke Geschiedenis der Nederlandsche
overzeesche bezittingen; Kruidkunde, Leiden, 1839.
[6] E. J. Shellard, J. Ethnopharmacology 1989, 25, 123-124.
[7] Herbal drug update: Kratom., Nat. Drug Intell. Center 2005, 4, 4.
[8] J. Ma, W. Yin, H. Zhou, X. Liao, J. M. Cook, J. Org. Chem. 2009, 74, 264-273.
[9] H. Takayama, Chem. Pharm. Bull 2004, 52, 916-928.
[10] A. H. Beckett, E. J. Shellard, A. N. Tackie, Planta Med. 1965, 13, 241-246.
[11] D. E. Zacharias, R. D. Rosenstein, G. A. Jeffrey, Acta Crystallogr. 1965, 18, 1039-1043.
[12] A. H. Beckett, E. J. Shellard, J. D. Phillipson, C. M. Lee, Planta Med. 1966, 14, 277-288.
[13] A. H. Beckett, E. J. Shellard, J. D. Phillipson, C. M. Lee, J. Pharm. Pharmacol. 1965,
17, 753-755.
[14] A. A. Philipp, D. K. Wissenbach, A. A. Weber, J. Zapp, H. H. Maurer, Journal of
Chromatography B 2011, 879, 1049-1055.
[15] A. A. Philipp, D. K. Wissenbach, A. A. Weber, J. Zapp, S. W. Zoerntlein, J.
Kanogsunthornrat, H. H. Maurer, Analytical & Bioanalytical Chemistry 2010, 396, 2379-
2391.
[16] K. S. Grewal, J. Pharmacol. 1932, 46, 251-271.
[17] K. Matsumoto, M. Mizowaki, T. Suchitra, H. Takayama, S. Sakai, N. Aimi, H.
Watanabe, Life Sci. 1996, 59, 1149-1155.
[18] K. Matsumoto, M. Mizowaki, T. Suchitra, Y. Murakami, H. Takayama, S. Sakai, N.
Aimi, H. Watanabe, Eur. J. Pharmacol. 1996, 317, 75-81. 76
[19] H. Ishikawa, H. Takayama, N. Aimi, Tetrahedron Lett. 2002, 43, 5637-5639.
[20] H. Takayama, M. Maeda, S. Ohbayashi, M. Kitajima, S. Sakai, N. Aimi, Tetrahedron
Lett. 1995, 36, 9337-9340.
[21] X. Sun, D. Ma, Chem. Asian J. 2011, 6, 2158-2165.
[22] F. Ungemach, M. DiPierro, R. Weber, J. M. Cook, J. Org. Chem. 1981, 46, 164-168.
[23] E. D. Cox, L. K. Hamaker, J. Li, P. Yu, K. M. Czerwinski, L. Deng, D. W. Bennett, J. M.
Cook, W. H. Watson, M. Krawiec, J. Org. Chem. 1997, 62, 44-61.
[24] Y. Kondo, S. Kojima, T. Sakamoto, J. Org. Chem. 1997, 62, 6507-6511.
[25] R. C. Larock, E. K. Yum, J. Am. Chem. Soc. 1991, 113, 6689-6690.
[26] C. Ma, X. Liu, X. Li, J. Flippen-Anderson, S. Yu, J. M. Cook, J. Org. Chem. 2001, 66,
4525-4542.
[27] P. I. Dalko, L. Moisan, Angew. Chem. , Int. Ed. 2001, 40, 3726-3748.
[28] B. List, Synlett 2001, 1675-1686.
[29] E. R. Jarvo, S. J. Miller, Tetrahedron 2002, 58, 2481-2495.
[30] P. I. Dalko, L. Moisan, Angew. Chem. , Int. Ed. 2004, 43, 5138-5175.
[31] J. Seayad, B. List, Org. Biomol. Chem. 2005, 3, 719-724.
[32] G. Lelais, D. W. C. MacMillan, Aldrichimica Acta 2006, 39, 79-87.
[33] B. List, Chem. Commun. (Cambridge, U. K. ) 2006, 819-824.
[34] M. Marigo, K. A. Jorgensen, Chem. Commun. (Cambridge, U. K. ) 2006, 2001-2011.
[35] M. J. Wanner, E. Claveau, J. H. Van Maarseveen, H. Hiemstra, Chemistry 2011, 17,
13680-13683.
[36] M. D. Lebar, K. N. Hahn, T. Mutka, P. Maignan, J. B. McClintock, C. D. Amsler, A. van
Olphen, D. E. Kyle, B. J. Baker, Bioorg. Med. Chem. 2011, 19, 5756-5762.
[37] O. Sugimoto, M. Mori, K. Moriya, K. Tanji, Helv. Chim. Acta 2001, 84, 1112-1118.
[38] S. Butini, S. Gemma, G. Campiani, S. Franceschini, F. Trotta, M. Borriello, N. Ceres, S.
Ros, S. S. Coccone, M. Bernetti, A. M. De, M. Brindisi, V. Nacci, I. Fiorini, E. Novellino, A.
Cagnotto, T. Mennini, K. Sandager-Nielsen, J. T. Andreasen, J. Scheel-Kruger, J. D.
Mikkelsen, C. Fattorusso, J Med Chem 2009, 52, 151-169.
[39] T. Fukuyama, C. Jow, M. Cheung, Tetrahedron Lett. 1995, 36, 6373-6374. 77
[40] T. Fukuyama, M. Cheung, C. Jow, Y. Hidai, T. Kan, Tetrahedron Lett. 1997, 38, 5831-
5834.
[41] L. M. Lerner, J. Org. Chem. 1976, 41, 2228-2229.
[42] G. Massiot, T. Mulamba, J. Levy, Bull. Soc. Chim. Fr. 1982, 241-248.
[43] J. Gonzalez, F. Sanchez, T. Torres, Synthesis 1983, 911-913.
[44] A. Pictet, T. Spengler, Ber. Dtsch. Chem. Ges. 1911, 11, 2030-2036.
[45] G. Tatsui, Yakugaku Zasshi 1928, 48, 453-459.
[46] B. Robinson, G. F. Smith, J. Chem. Soc. 1960, 4574-4578.
[47] P. Yu, T. Wang, J. Li, J. M. Cook, J. Org. Chem. 2000, 65, 3173-3191.
[48] P. D. Bailey, P. D. Clingan, T. J. Mills, R. A. Price, R. G. Pritchard, Chem. Commun.
(Camb) 2003, (22), 2800-2801.
[49] M. S. Taylor, E. N. Jacobsen, J. Am. Chem. Soc. 2004, 126, 10558-10559.
[50] J. Seayad, A. M. Seayad, B. List, J. Am. Chem. Soc. 2006, 128, 1086-1087.
[51] M. J. Wanner, R. N. van der Haas, K. R. de Cuba, J. H. van Maarseveen, H. Hiemstra,
Angew. Chem. Int. Ed Engl. 2007, 46, 7485-7487.
[52] N. V. Sewgobind, M. J. Wanner, S. Ingemann, R. de Gelder, J. H. van Maarseveen, H.
Hiemstra, J. Org. Chem. 2008, 73, 6405-6408.
[53] M. J. Wanner, R. N. Boots, B. Eradus, R. de Gelder, J. H. van Maarseveen, H. Hiemstra,
Org. Lett. 2009, 11, 2579-2581.
[54] B. Herle, M. J. Wanner, J. H. van Maarseveen, H. Hiemstra, J. Org. Chem. 2011, 76,
8907-8912.
[55] M. J. Wanner, E. Claveau, J. H. van Maarseveen, H. Hiemstra, Chemistry 2011, 17,
13680-13683.
[56] I. T. Raheem, P. S. Thiara, E. A. Peterson, E. N. Jacobsen, J. Am. Chem. Soc. 2007, 129,
13404-13405.
[57] R. S. Klausen, E. N. Jacobsen, Org. Lett. 2009, 11, 887-890.
[58] T. Okino, Y. Hoashi, Y. Takemoto, J. Am. Chem. Soc. 2003, 125, 12672-12673.
[59] T. Marcelli, R. N. van der Haas, J. H. van Maarseveen, H. Hiemstra, Angew. Chem. Int.
Ed Engl. 2006, 45, 929-931.
[60] S. Telderman, Master Thesis, Universiteit van Amsterdam, 2012. 78
[61] B. Vakulya, S. Varga, A. Csámpai, T. Soós, Org. Lett. 2005, 7, 1967-1969.
[62] W. Braje, J. Frackenpohl, P. Langer, H. M. R. Hoffmann, Tetrahedron 1998, 54, 3495-
3512.
[63] Y. Liu, B. Sun, B. Wang, M. Wakem, L. Deng, J. Am. Chem. Soc. 2009, 131, 418-419.
[64] Y. Lee, R. S. Klausen, E. N. Jacobsen, Org. Lett. 2011, 13, 5564-5567.
[65] T. Marcelli, Master Thesis, Universiteit van Amsterdam, 2007.
[66] H. S. Rho, S. H. Oh, J. W. Lee, J. Y. Lee, J. Chin, C. E. Song, Chem. Commun. (Camb)
2008, (10), 1208-1210.
[67] S. Horie, F. Koyama, H. Takayama, H. Ishikawa, N. Aimi, D. Ponglux, K. Matsumoto,
T. Murayama, Planta Med. 2005, 71, 231-236.
