The Asymmetric Total Synthesis of ( + )-and (-)-Trypargine via Noyori Asymmetric Transfer Hydrogenation

Uma síntese total e eficiente da (+) e (-)-tripargina, alcalóide b-carbolínico isolado da pele da rã africana K. Senegalensis, foi realizada em 6 etapas e 38% de rendimento global, a partir da triptamina, tendo como base a construção do sistema b-carbolínico via reação de BischlerNapieralski e a redução enantiosseletiva do intermediário diidro-b-carbolínico via reação de transferência de hidrogênio assimétrica usando o protocolo de Noyori.

The presence of a b-carboline and a guanidine moieties in the structure of (-)-trypargine (1) has led to preliminary evaluation of its biological profile.Neither a full account of the biological properties nor its biosynthetic origin have been reported although one may envision trypargine (1)  to be formed from tryptophan and the 2-keto carboxylic acid derived from arginine.It has been reported to be toxic to mice (LD 50 = 16.9 mg kg -1 , intravenous administration) although complete details of this study are lacking. 5,6Ireland and co-workers 3 have failed to observe significant cytotoxicity in fractions containing trypargine alkaloids against human colon tumor cells.Additionally, (-)-trypargine (1) has been reported to block voltage gated sodium channels in squid axon membrane. 7ur interest to further explore the biological profile of trypargine and analogues led us to consider the available methodologies for chirality transfer in order to develop a short and asymmetric route to these compounds which would secure enough quantities to carry out biological assessment.Our approach to a catalytic and asymmetric synthesis of (+)-trypargine (1) relied on our previous results on the enantioselective total syntheses of arborescidines when we successfully employed the Bischler-Napieralski protocol to assembly the 3,4-dihydro-b-carboline moiety, followed by the Noyori catalytic asymmetric hydrogen transfer reaction to reduce the intermediate prochiral Vol. 20, No. 8, 2009   imine. 8Recently, Drabowicz and co-workers 9 disclosed the total synthesis of (+)-trypargine (1) via the latter approach which prompted us to disclose our results in this topic.

Results and Discussion
Our approach to (R)-trypargine (1) was based on the construction of the dihydro-b-carboline moiety via the Bischler-Napieralski reaction of tryptamine derivative 4 (Scheme 1).Among the routes evaluated to secure the preparation of intermediate 4, two emerged as the most efficient: introduction of the 3-aminopropionyl group via Curtius rearrangement on 5-ketopentanoyl tryptamine (3, Scheme 1, step b) or via N-acylation of tryptamine (2)  with N-allyloxycarbonyl-4-aminobutyric acid (6, Scheme 1, step d).
1][12] The isocyanate intermediate can be isolated when the reaction is carried out in non-nucleophilic solvent or can be intercepted by water, amines or alcohols to give the corresponding, amines, ureas or carbamates.
Starting from commercially available tryptamine (2), reaction with glutaric anhydride in acetone at room temperature provided the corresponding amide 3 in quantitative yield (Scheme 2).The Curtius rearrangement was carried out by treating a solution of amide 3 in acetone with methyl chloroformate in order to give rise to the corresponding mixed anhydride at 0 o C which was then converted to the corresponding acyl azide 7 upon treatment with aqueous sodium azide at rt. Acyl azide 7 was heated in toluene for 2 h and the reaction was quenched by the addition of allylic alcohol to provide, after chromatography on silica gel, allyl carbamate 4 in 73% overall yield from tryptamine (2).
Alternatively, allyl carbamate 4 was prepared in 88% yield via acylation of tryptamine (2) with N-carboallyloxy-4-aminocarboxylic acid 6 and catalytic amount of N-hydroxybenzotriazol (HOBt) and 1-ethyl-3-(3dimethylaminopropyl)carbodiimide (EDCI). 13Despite the slight increase in the overall yield of allyl carbamate 4, the use of expensive reagents in the method employing 4-amino butyric acid makes the Curtius rearrangement the method of choice for large scale preparation.
With the preparation of allyl carbamate 4 secured, the formation of dihydro-b-carboline 9 was succesfully carried out upon treating 4 with 3 equivalents of POCl 3 in toluene/acetonitrile (7:3, v/v) under reflux, to give 9 in 76% yield after column chromatography.However, it proved to be more efficient to filter the crude reaction mixture through a pad of silica gel in order to remove most of the contaminants and proceed to the next step, the asymmetric transfer hydrogenation under the conditions described by Noyori and coworkers.Using the protocol, (R)-(+)-10 was obtained in 75% overall yield from allyl carbamate 4 (Scheme 3).
The Bischler-Napieralski reaction is a cyclodehydration which takes place when an acyl derivative of a b-arylethylamine is treated with a dehydrating agent. 14,15echanistic studies carried out by Fodor and coworkers have shown that the formation of imidoyl chloride and nitrilium intermediates is promoted by several dehydrating agents such as POCl 3 , PCl 5 and SOCl 2 . 16,17In our case, the formation of the imidoyl chloride or nitrilium salt from the indole amine would be followed by an aromatic electrophilic substitution to afford dihydro-b-carboline 9 (Scheme 4).
The asymmetric reduction of the prochiral imine 9 was efficiently carried out using the asymmetric transfer hydrogenation as devised by Noyori and co-workers. 18The reaction is catalyzed by chiral N-sulfonated diamine-Ru(II)-h 6 arene complexes and has become the method of choice for the enantioselective reduction of cyclic imines due to the high yield and enantiomeric excess usually attained and the simplicity of the experimental protocol. 19,20Despite the advantages of this methodology, relatively few examples of asymmetric synthesis of natural products containing the tetrahydro-b-carboline core have appeared since our first disclosure of the application of Noyori asymmetric transfer hydrogenation in the total synthesis of arborescidines A, B and C. 8,9,[21][22][23] The 16 electron catalytic active species II was generated upon treatment of pre-catalyst RuCl(S,S)-H 2 NCHPhCHPhNTs)(h-p-cymene) (I) in DMF at 80 o C in the presence of Et 3 N. 24,25 Then, a solution of prochiral imine 9 in DMF was added, followed by a 5:2 formic acid-triethylamine azeotropic mixture and the reaction mixture was kept at room temperature for 8 h.Under these conditions tetrahydro-b-carboline 10 was isolated in 75% overall yield from allyl carbamate 4.However, when longer reaction time was employed the hydrogenation of the allyloxy group present in 10 was also observed.
The proposed mechanism involves the six-membered arrangement IV which displays an hydrogen bond between the N-H in the chiral ligand and the nitrogen in the imine and hydride transfer from the ruthenium hydride species to the imine via an out-of-plane interaction between Ru-H and the C=N bonds.According to the working model proposed by Noyori and co-workers, 18,19 the use of (S,S)-DPEN as the chiral ligand would direct the asymmetric transfer hydrogenation to take place at the Si face of prochiral imine 9 leading to (R)-10 as the major enantiomer as depicted in Scheme 5.
Considering that the sense of chirality in the Noyori reduction would be validated by comparing the specific  optical rotation of synthetic trypargine (1) with that described for trypargine isolated from natural sources, we focused on the determination of the enantiomeric ratio of tetrahydro-b-carboline 11 which was shown to be 98:2 after 19 F-NMR spectroscopic analyses of the Mosher amide prepared from (R)-(-)-a-methoxy-a-(trifluoromethyl) phenyl acetic acid (MTPA).
Our choice for the allyloxycarbonyl group in our synthetic scheme evolved after screening several different protecting groups (carbomethoxy, terc-butoxycarbonyl, benzyloxycarbonyl, 9-fluorenylmethoxycarbonyl and trifluorocarbonyl groups) as it proved to be compatible with the acidic conditions during the Bischler-Napieralski reaction using POCl 3 , the Noyori protocol for the transfer hydrogenation and eventually it was easily removed under very mild conditions.In fact, by employing the conditions described by Mandal and McMurry, amine (R)-11 was isolated in 90% yield after 30 min at rt in the presence of Et 3 SiH and catalytic amount of Pd/C in ethanol. 26he final step toward (R)-trypargine (1) was the introduction of the guanidine moiety which was carried out with the method described by Bernatowicz and co-workers 4,9 which makes use of 1H-pyrazol-1-carboxyamidine hydrochloride (12) in DMF and diisopropylethylamine.After column chromatography on basic alumina and treatment with methanolic HCl, (R)-trypargine hydrochloride (1 • HCl) was isolated in 77% yield.Its physical (melting point and specific optical rotation) and spectroscopic data ( 1 H-and 13 C-NMR data) nicely matched those described in the literature.
Natural (S)-(-)-trypargine (1) was also prepared in 6 steps and 38% overall yield from tryptamine according to the same reaction sequence.
The approach described herein is robust enough to provide trypargine and its derivatives in both enantiomeric forms in few steps, good overall yield and very high enantiomeric ratios from tryptamine and analogues and should be of value for the determination of the absolute configuration of others dihydro-b-carbolines such as the novel toxin isolated from the venom of the Parawixia bistriata spider endemic in the brazilian cerrado 27 and pharmacological screening of related compounds.

Experimental
Commercially available reagents and solvents were previously purified.THF was distilled from calcium hydride and redistilled from sodium/benzophenone immediately prior to use.Dichloromethane, acetonitrile and triethylamine were distilled from calcium hydride immediately prior to use.Dimethylformamide (DMF) was distilled from calcium hydride under reduced pressure and Scheme 4.

Scheme 5.
The Asymmetric Total Synthesis of (+)-and (-)-Trypargine J. Braz.Chem.Soc.1438   temperature lower than 70 o C. Methanol was distilled from magnesium containing catalytic amount of iodine.POCl 3 as distilled immediately prior to use.
The reactions were monitored with TLC plates (aluminum foils covered with silica gel) and exposed to UV radiation, followed by treatment with fosfomolibdic acid (25% ethanolic solution) or aq.KMnO 4 and heating on a hot plate.Chromatographic separations were carried out in silica gel (70-230 or 230-400 Mesh) or basic alumina (5-200 m).
Specific optical rotations were measured at 25 o C at 589 nm (sodium D line) and IR spectra were measured as film in NaCl cells and frequencies are reported as cm -1 .Melting points are not corrected.