Stereospecific Approach to the Synthesis of Ring-A Oxygenated Sarpagine Indole Alkaloids. Total Synthesis of the Dimeric Indole Alkaloid P-(+)-Dispegatrine and Six Other Monomeric Indole Alkaloids

Abstract

The first regio- and stereocontrolled total synthesis of the bisphenolic, bisquaternary alkaloid (+)-dispegatrine (1) has been accomplished in an overall yield of 8.3% (12 reaction vessels) from 5-methoxy-D-tryptophan ethyl ester (17). A crucial late-stage thallium(III) mediated intermolecular oxidative dehydrodimerization was employed in the formation of the C9-C9′ biaryl axis in 1. The complete stereocontrol observed in this key biaryl coupling step is due to the asymmetric induction by the natural sarpagine configuration of the monomer lochnerine (6) and was confirmed by both the Suzuki and the oxidative dehydrodimerization model studies on the tetrahydro β-carboline (35). The axial chirality of the lochnerine dimer (40) and in turn dispegatrine (1) was established by X-ray crystallography and was determined to be P(S). Additionally, the first total synthesis of the monomeric indole alkaloids (+)-spegatrine (2), (+)-10-methoxyvellosimine (5), (+)-lochnerine (6), lochvinerine (7), (+)-sarpagine (8), and (+)-lochneram (11) were also achieved via the common pentacyclic intermediate 16.

INTRODUCTION

The sarpagine-macroline group is one of the largest groups of structurally related indole alkaloids isolated principally from the Apocynaceae plant family.¹ More than 150 members of this group have been isolated to date, many of which exhibit interesting pharmacological activities.^1,2 Among them, bisindoles are of special significance because they exhibit more potent bioactivity than their monomeric counterparts.³ However, a vast majority of these alkaloids have been poorly evaluated due to paucity of material available for biological testing. The sarpagine-macroline bases are biogenetically related⁴ to the clinically important ajmaline alkaloids⁵ and common to these three classes is the azabicyclo[3.3.1]nonane core, as illustrated in Figure 1.

Figure 1.

SAR of the cycloocta[b]indole framework (I) of the sarpagine-macroline and ajmaline related indole alkaloids active against MptpB.⁷ EWG: electron withdrawing group.

In continuation of their efforts at the biology-oriented synthesis (BIOS)⁶ of natural product derived probes, Waldmann et al. targeted the sarpagine-macroline indole alkaloids which led to the cycloocta[b]indole core (I) as a starting point for library design.⁷ More than a hundred tetracyclic analogues of this core, synthesized by a stereoselective solid-phase synthesis, were investigated for their inhibitory activity in enzymatic assays with various tryrosine phosphatases. The screen yielded an unprecedented class of potent inhibitors of Mycobacterium protein tyrosine phosphatase B (MptpB) with IC₅₀ values in low micromolar range (the most potent MptpB inhibitor of the library has an IC₅₀ value of 4.71±1.14 μM). The MptpB inhibitory activity of I seemed to be greatly influenced by the `S' stereochemistry at C6 and C10 and by the presence of a β-ketoester moiety (see Figure 1). The N_b-benzyl group in I, containing: 1) an oxygen atom either within the aromatic ring system or as a phenol substituent and 2) with m,p-disubstituted electron withdrawing groups were important, to potent MptpB inhibition. MptpA and MptpB secreted by the causative organism of tuberculosis, Mycobacterium tuberculosis, are known to selectively dephosphorylate human host proteins involved in interferon-γ signaling pathways thereby preventing the initiation of host defense mechanisms.⁸ The tetracyclic core (I) of the sarpagine alkaloids thus may be considered as a promising target for the development of new drug candidates against tuberculosis.

The dried roots and leaves of Rauwolfia verticillata (Lour.) or Lou Fu Mu have been used in Chinese folk medicines for thousands of years as tranquilizers and more recently in the treatment of hypertension and hyperthyroidism.⁹ Illustrated in Figure 2 is (+)-dispegatrine (1),¹⁰ a bisquaternary, bisphenolic sarpagine alkaloid isolated from the water-soluble fraction of the root of R. verticillata (Lour.) Baill var. hainanensis Tsiang along with the monomer (+)-spegatrine (2).¹¹ Bisphenolic, bisquaternary indole alkaloids are very rare.¹² Of the 300 or so dimeric indole alkaloids isolated to date, the homodimer 1 and blumeanine¹³ (not shown) are the only two dimers belonging to this class of alkaloids. Although dispegatrine (1) and its corresponding monomer spegatrine (2) exhibit promising hypotensive activity, the affinities and activities of the dimer 1 on both α1 and α2 adrenergic receptors was about an order of magnitude greater than that of the monomer 2.^3e The quaternary alkaloids verticillatine (3)¹⁴ and macrospegatrine (4)¹⁵ illustrated in Figure 2 are also known to exhibit promising antihypertensive activity.¹⁶

Figure 2.

Ring-A oxygenated sarpagine (1–12) indole alkaloids and macroline-sarpagine (4, 13–15) bisindole alkaloids.

A number of biogenetically related ring-A oxygenated monomeric alkaloids such as (+)-10-methoxyvellosimine (5),¹⁷ (+)-lochnerine (6),¹⁸ (+)-sarpagine (7),¹⁹O-acetylsarpagine (8),²⁰ (+)-lochvinerine (9),²¹ episarpagine (10),²⁰ (+)-lochneram (11)^18c,22 and lochnerine N_b-oxide (12)²³ have also been reported (Figure 2). Lochnerine (6), which itself has no antitumor activity, when combined with vincristine or daunorubicin (not shown) at subcytotoxic concentrations induced complete inhibition of the vincristine-resistant P388 leukemic cells in vitro.²⁴ Additionally, 6 is known to exhibit promising vasorelaxant actvity.²⁵ Most recently, 6 and 7 were found to be present in some of the new macroline-sarpagine bisindoles (13–15, Figure 2) isolated from Alstonia angustifolia by Kam et al.²⁶ The promising bioactivity of some of the members of these ring-A oxygenated group of indole alkaloids combined with their complex architecture made them very attractive targets.

The total synthesis of dispegatrine (1) and four other monomeric indole alkaloids was recently reported via a general approach for the synthesis of ring-A oxygenated sarpagine alkaloids.²⁷ Such a doubly convergent route could also be employed for the potential synthesis of the complex bisindoles 4, 13–15; the biological activities of which have not yet been fully investigated. In this report, the full details of the development of this general route including the first total synthesis of alkaloids 9 and 11 are reported.

RESULTS AND DISCUSSION

Although dispegatrine (1) was isolated as a single atropodiastereomer and its structure established on the basis of spectroscopic analysis (¹H NMR, ¹³C NMR and mass spectrum comparison to monomer 2),¹⁰ the apparent axial chirality at the C9-C9′ biaryl axis in 1 was not determined. The isolation chemists also reported a semisynthesis of 1 in an almost negligible yield of 0.25% (Scheme 1), obtained by an oxidative phenolic dehydrodimerization of 2 in aqueous ammonium acetate with K₃Fe(CN)₆.¹⁰. Since only one atropodiastereomer was reportedly formed, it can be assumed that the complete atropselectivity observed in this coupling is a result of internal asymmetric induction by the natural sarpagine configuration of the monomer 2.

Scheme 1.

Partial Biomimetic Synthesis of (+)-Dispegatrine (1) by Yu et al.¹⁰

The retrosynthetic strategy was based on these reports (Scheme 2). A potentially biomimetic intermolecular biaryl coupling could be employed to construct the C9-C9′ bond in 1 via a non-phenolic coupling of 6 or by a phenolic oxidative coupling of 7 or 2. In contrast to the extremely low yield of phenolic coupling of 2,¹⁰ a non-phenolic Scholl type oxidative coupling of the methoxy analogue lochnerine (6) could be employed to this end.²⁷ By performing such a late stage biaryl coupling, one could not only avoid the potential formation of atropodiastereomeric intermediates throughout the synthesis, but also take advantage of the existing chiral centers in the sarpagine unit 6 for maximum stereoinduction. The 5-methoxy-D-(+)-tryptophan ethyl ester 17 could be employed as the starting material and the chiral transfer agent for the synthesis of the key pentacyclic framework 16 of the sarpagine alkaloids via the asymmetric Pictet-Spengler,²⁸ which on further functionalization would result in the total synthesis of the target alkaloids.

Scheme 2.

Retrosynthetic Analysis

Enantiospecific Synthesis of the 5-Methoxy-D-(+)-Tryptophan Ethyl Ester (17)

Although a number of synthetic routes to substituted tryptophans exist, principally asymmetric hydrogenation,²⁹ enzymatic synthesis,³⁰ and more recently Negishi coupling;³¹ attempts to execute these in a practical sense in the laboratory met with only limited success. Eventually, two approaches, the regiospecific bromination³² and the Larock heteroannulation previously employed³³ for the synthesis of 5-methoxy-D-tryptophan both utilizing the Schöllkopf chiral auxiliary 18 were reinvestigated.

The palladium-catalyzed heteroannulation of internal alkynes with o-iodoanilines developed by Larock et al.³⁴ has been successfully employed in this research group for the synthesis of optically active 6- and 7-methoxy tryptophans on a multihundred gram scale which in turn have resulted in the total synthesis of a number of alkoxy substituted sarpagine indole alkaloids³⁵ via the asymmetric Pictet-Spengler reaction. Attempts to employ the Larock heteroannulation for the synthesis of 17 under similar conditions, however, resulted in lower yields.^32b,33 With the success reported in the synthesis of 4-methoxytryptophan³⁶ via use of the TMS substituted propargylic chiral auxiliary 19 (Scheme 3), it was decided to first test the effect of this alkyne on the regioselectivity of the Larock heteroannulation. If successful, it would provide a shorter and more efficient synthesis of 5-methoxytryptophan 17. Since both regiospecific bromination and Larock heteroannulation employed the Schöllkopf chiral auxiliary, large scale preparation (500 g) of the Schöllkopf chiral auxiliary 18 from L-valine and glycine, based on the earlier procedure,³⁷ was undertaken. The TMS protected chiral auxiliary 19 was then prepared according to the modified procedure by Ma et al.³⁶ in 96–100% de (Scheme 3).

Scheme 3.

Synthesis of the Propargyl-Substituted Schöllkopf Chiral Auxiliary 19

Larock Heteroannulation

In order to proceed with the Larock heteroannulation, o-iodoaniline 20 was first synthesized based on a published procedure.³⁸ The amino group in p-anisidine was protected with a Boc group followed by a Snieckus ortho lithiation³⁹ with 2.2 equivalents of tert-butyllithium at −78 °C. Initially, deprotonation of the NH moiety took place, after which the next equivalent of t-butyllithium provided the ortho lithiated species stabilized by the Boc group. This process was quenched with diiodoethane to afford the desired Boc protected iodo aniline 20 in 86% yield. With the TMS-substituted alkyne 19 and the iodoaniline 20 in hand, the annulation was carried out according to the recently developed conditions of Ma et al.³⁶ Gratifyingly, in this process the starting aniline 20 was consumed entirely on continued stirring for 36 hours and the N_a-H protected indoles 21a and 21b were obtained in a combined yield of 81% (Scheme 4). Hydrolysis of the Schöllkopf chiral auxiliary in 21a (separated by chromatographic separation) with aqueous 2 N HCl in THF, accompanied by concomitant loss of the silyl group provided the optically active 5-methoxy-D-tryptophan ethyl ester (17) in a single step in 86% yield. From a practical point of view, this approach to the synthesis of 17 was disappointing. In spite of replacement of the bulkier TES-substituted alkyne by the TMS alkyne 19, the regioselectivity of this method did not improve. For this reason the earlier regiospecific bromination route³² for the preparation of N_a-H-5-methoxy-D-tryptophan ethyl ester (17) was employed. The success of this sequence rested on the ability to scale up the first few steps to multihundred gram levels.

Scheme 4.

Larock Heteroannulation for the Synthesis of 5-Methoxy-D-Tryptophan Ethyl Ester (17)

The commercially available 3-methyl-5-methoxyindole (23)⁴⁰ was prepared on a 616 g scale via the Japp-Klingmann/Fischer indole protocol developed by Abramovitch and Shapiro⁴¹ in two steps from p-anisidine (Scheme 5). Boc protection of the indole N_a-H function in 23, followed by allylic bromination^32,42 furnished the crude bromide 24 in greater than 90% yield, which was directly alkylated with the anion of the Schöllkopf chiral auxiliary (18) at −78 °C to provide the protected D-tryptophan analogue 25 in 80% yield. Earlier, the Boc group was removed in refluxing xylenes for 7 days;³² however, the much milder TMSOTf/2,6-lutidine system⁴³ was found to be much more effective. Deprotection of the Boc group in 25 was achieved on a 50 g scale at 0 °C in 12 hours to furnish the Schöllkopf analog 26 in 93% yield. Hydrolysis of 26 under aqueous acidic conditions provided the desired optically active N_a-H-5-methoxy-D-tryptophan ethyl ester (17) in 85% yield in greater than 98% ee.

Scheme 5.

Improved Regiospecfic Bromination Route for the Synthesis of 5-Methoxy Tryptophan Ethyl Ester (17) on a Large Scale

Synthesis of the 5 Methoxypentacyclic Ketone 16

Analogous to the parent system, the transformation of tryptophan 17 into the desired trans-diester 28b (see Scheme 6) should follow the well-documented trans transfer of chirality in the asymmetric Pictet-Spengler reaction⁴⁴ in a straightforward fashion; however, this was not the case. The electron rich character of the 5-methoxy indole in 17 facilitated the undesired Pictet-Spengler reaction (with the benzaldehyde imine) as observed for the N_a-methyl series;^32a moreover, this 5-methoxyindole system 28 was not stable in trifluoroacetic acid (TFA)/CH₂Cl₂ for extended periods of time. Consequently, modifications were made analogous to the work of Zhao et al.³² to carry out the Pictet-Spengler cyclization and the decarboxylation reaction to achieve optimum yields and very high diastereoselectivity. As illustrated in Scheme 6, tryptophan 17, on treatment with benzaldehyde at room temperature, followed by sodium borohydride reduction (at −10 °C) afforded the corresponding N_b-benzyltryptophan ethyl ester 27 in 93% yield. If the reaction was carried out at room temperature as in the parent system, a 1:1 mixture of 27 and the undesired Pictet-Spengler product 1-phenyl tetrahydro β-carboline (not shown) were observed as reported earlier by Zhao et al.³² However, Pictet-Spengler reaction of tryptophan derivative 27 with the aldehyde, methyl 3-formylpropanoate, in AcOH/CH₂Cl₂ afforded a mixture of trans- and cis-diesters in nearly quantitative yield. If TFA/CH₂Cl₂ was employed in this step in place of AcOH/CH₂Cl₂, decomposition of much of the N_b-benzyl tryptophan 27 was observed. However, on completion of the Pictet-Spengler reaction in acetic acid, one equivalent of TFA was added to epimerize all of the cis isomer into the desired trans-diester 28b in 93% yield. Dieckmann cyclization of 28b was followed by a base mediated hydrolysis/decarboxylation sequence to provide the optically pure tetracyclic ketone 31 (Scheme 6). Based on the work of Wang,⁴⁵ under normal basic conditions, the δ-lactam 29 was initially formed. Then, in the presence of a large excess of sodium methoxide in refluxing toluene under the same reaction conditions employed for the parent tryptophan system⁴⁵ for an extended period of time (72 hours), the desired Dieckmann product 30 was obtained in 88% yield (see Scheme 6). The key to success in the Dieckmann cyclization is the controlled stirring rate and temperature for 3 days. A base induced decarboxylation worked smoothly to provide the tetracyclic ketone 31 in 82% yield.

Scheme 6.

Synthesis of the 5-Methoxytetracyclic Ketone 31

Catalytic debenzylation of 31, under acidic conditions furnished the 10-methoxy N_b-H tetracyclic ketone 32 in 92% yield (Scheme 7). N_b-alkylation of the secondary amine 32 with (Z), 1-bromo-2-iodo-2-butene⁴⁶ under the conditions developed in Milwaukee (dry THF/K₂CO₃/reflux)^32,45 led to the formation of the desired product 33, but the reaction was very sluggish, and led to a formation of a considerable amount of baseline impurities. Since substitution by amines on bromides was clearly an S_N2 type process and it is well known that the nucleophilic strength is dependent on the solvent employed in the S_N2 reaction, it was decided to increase the nucleophilicity of the amine 32 by employing a more polar aprotic solvent. Dry acetonitrile proved to be the most suitable solvent for this process for the reaction went to completion at room temperature to give 33 in 76% yield. Attempts to increase the yield by adding more bromide were not successful. Initial attempts at executing the key enolate mediated palladium catalyzed cyclization of ketone 33 under the modified conditions⁴⁷ furnished the desired pentacyclic ketone 16, albeit in lower yields (50%). The lower yields could be due to a combination of the stronger base (t-BuONa) and the electron rich aromatic system in 33, which led to a faster E2 elimination and formation of the unwanted acetylene byproduct (not shown) before the oxidative addition of the vinyl iodide took place. This problem was circumvented by subjecting the vinyl iodide 33 to the much milder conditions [PhOK/Pd(PPh₃)₄] of Bonjoch et al.⁴⁸ to furnish the desired pentacyclic ketone 16 in 73% yield.

Scheme 7.

Synthesis of the 5-Methoxypentacyclic Ketone 16

The Regiospecific, Stereospecific Total Synthesis of (+)-Spegatrine (2), (+)-10-Methoxyvellosimine (5), (+)-Lochnerine (6), (+)-Sarpagine (7), (+)-Lochvinerine (11) and (+)-Lochneram (11)

With the key E-ethylidene intermediate 16 in hand conversion into the desired α-aldehyde present in (+)-10-methoxyvellosimine (5) was accomplished by a Wittig-hydrolysis-epimerization sequence (Scheme 8). The one carbon homologation process was achieved by a Wittig reaction, followed by acidic hydrolysis of the corresponding two steroisomeric enol methyl ethers (not shown), to afford the theromodynamically stable α-aldehyde in 5 in 90% yield. The aldehyde function of 5 was then reduced with sodium borohydride to provide (+)-lochnerine (6). As illustrated in Scheme 8, demethylation of (+)-lochnerine (6) was achieved with 5 equivalents of BBr₃ in dry CH₂Cl₂ at −78 °C for 1 hour, after which the solution was allowed to warm to room temperature and stirred for an additional 4 hours. Following work up, (+)-sarpagine (7) was obtained in 80% yield. Subsequent quaternization of the N_b-nitrogen function in 7 with excess methyl iodide provided the N_b-methiodide salt, which on stirring with silver chloride in ethanol⁴⁹ furnished (+)-spegatrine chloride (2). Lochnerine (6) on N_b-methylation under similar conditions provided (+)-lochneram (11) in 85% yield. The spectral data for the synthetic 2, 5–7 and 11 were in good agreement with that reported for the natural products^1a and resulted in the first total synthesis of these alkaloids.

Scheme 8.

Completion of the Total Synthesis of (+)-Spegatrine (2), (+)-10-Methoxyvellosimine (5), (+)-Lochnerine (6), (+)-Sarpagine (7), and (+)-Lochneram (11)

In order to synthesize the β-alcohol at C-16 in lochvinerine (9), the chemoselective hydroboration developed earlier⁵⁰ was employed. Wittig reaction of the ketone 16 was then carried out with triphenylphosphonium bromide in benzene in the presence of potassium t-butoxide to afford the diene 34 in 90% yield (Scheme 9). Analogous to earlier reports, the 9-BBN reagent was chosen as the hydroborating agent to facilitate attack from the less hindered face of the C16–C17 double bond relative to the C19–C20 site.⁵⁰ Lochvinerine (9) was obtained as the only detectable diastereomer after the hydroboration-oxidation sequence. The proton NMR spectrum of 9 was in agreement with that of the natural product²¹ although some difference was observed in chemical shifts of the reported and observed values of protons. This was mainly due to the presence of methanol in the synthetic sample. The synthetic sample retained methanol even after drying the compound under high vacuum for extended periods of time.

Scheme 9.

Completion of the Total Synthesis of (+)-Lochvinerine (9)

Direct/Intermolecular Biaryl Coupling

The biaryl core is an important subunit found in a large number of natural products such as alkaloids, coumarins, flavonoids, lignanes, polyketides tannins and terpenes. In particular, axially chiral biaryls are important as chiral ligands in asymmetric synthesis. Because of their interesting biological activity and complex architecture, natural and unnatural biaryls are considered as attractive synthetic targets.⁵¹ For this reason considerable efforts have been made in recent decades for the efficient asymmetric synthesis of biaryls both in the intramolecular and the intermolecular mode and have been the subject of many reviews.^51,52 Particularly noteworthy is the review by Bringmann et al.^51a which focusses on enantioselective methods employed for the total synthesis of complex biaryl natural products.

Within the area of intermolecular biaryl synthesis, the two most common approaches employed are: (1) oxidative coupling^52c and (2) reductive coupling such as Ullmann,^52,54 Suzuki,^52,55 Stille^52,56 or C- H activation^52,57 etc. Oxidative phenolic coupling is a powerful and economical method for the synthesis of biaryl compounds and is especially suited to natural product synthesis since many of the biosynthetic routes to biaryls involve such a coupling.⁵⁸ Although considerable progress has been made in carrying out the intramolecular oxidative couplings, in the absence of activating groups, the position of the newly formed C-C bond is determined by the electronic and steric preferences of the substrate and can lead to regioisomeric reaction products in complex substrates especially in the intermolecular mode.^52c,53c The coupling mechanism is believed to involve either one two-electron or two one-electron oxidations to form an aryl–aryl coupled dimer through the ortho and/or para positions.^52c,59 In the case of phenols bearing no ortho- and para-substituents, the ortho-ortho, ortho-para and para-para coupling reactions occur, giving rise to often unseparable mixtures. In some cases competing and/or subsequent oxidation to form quinones occurs, either from the coupled product or from the original substrate. Thus it is not always predictable which of the possible products will be formed predominantly, consequently, thorough optimization of the oxidizing agent/reaction conditions is required for successful transformations.

The Lewis- acid catalyzed direct oxidative non-phenolic coupling of two unfunctionalized arenes or the Scholl reaction⁶⁰ is one of the oldest C-C bond forming reactions and has been extensively used for the intramolecular oxidative dehydrodimerization of branched phenylene precursors to the corresponding polycyclic aromatic hydrocarbons (PAHs) such as triphenylenes, hexa-peri-hexabenzocoronenes (HBCs) etc.⁶¹ and for the synthesis of hexaalkoxytriphenylenes.⁶² Various strong acids (Brønsted acids/Lewis acids) and metal salts are generally used in combination with an oxidant (O₂, KMnO₄, RNO₂).⁶⁰ Variants of the reaction include oxidants such as: FeCl₃,⁶³ CuCl₂, or Cu(OTf)₂ with AlCl₃,⁶⁴ Tl(O₂CCF₃)₃/BF₃·OEt₂,⁶⁵ Pb(OAc)₄/BF₃·OEt₂,⁶⁶ MoCl₅,⁶⁷ SbCl₅,⁶⁸ and (CF₃CO₂)₂I^IIIC₆H₅ [phenyliodine(III) bis(trifluoroacetate): PIFA]/BF₃·OEt₂ in CH₂Cl₂.⁶⁹ Electrochemical oxidations of electron-rich aryl compounds have been reported, but are not commonly used.⁷⁰ Based on the observations of King et al.⁷¹ the outcome of the Scholl reaction of substituted substrates follows the directing group effects observed in electrophilic aromatic substitution. Alkoxy and alkyl groups are effective o,p- directors, whereas deactivating m-directors (e.g. NO₂) suppress the reaction rate. A variety of electron-rich substrates such as alkyl/alkoxy substituted aryl compounds have been successfully dimerized using the above mentioned combinations. While a majority of these C-C bond forming reactions are reported in the intramolecular mode, very few cases are reported in the intermolecular mode, albeit in substrates with simpler systems, or in substrates with substitution patterns such as to prevent side reactions.

Among the reductive coupling processes, the Suzuki reaction is the most widely used method often affording good to high yields of the coupled products with predictable regioselectivity.⁵⁵ However, the advantages of employing pre-activated aryl components can be offset by the requirement for their independent preparation in cases where multiple synthetic steps are required, especially on a large scale. Also since no standard procedures are available, each new catalyst and substrate requires time consuming optimization of the particular reaction conditions.

Reductive Coupling-Model Reaction

As per the retrosynthetic plan, both oxidative (phenolic and Scholl-type) or reductive cross coupling could be employed to form the C9-C9′ bond in 1. Although biomimetic oxidative phenolic coupling still stands as a practical method for the synthesis of many biaryls, the negligible yield obtained during the oxidative phenolic coupling of 2 by Lin et al.¹⁰ made it the weakest method of choice. In order to avoid the potential problems associated with phenolic oxidative couplings it was thus decided to employ a non-phenolic coupling reaction for the construction of the biaryl axis in 1. A non-phenolic oxidative coupling of the sarpagine alkaloid (+)-lochnerine (6), which contains all the chiral information required, could be attempted to this end. The quaternary alkaloid lochneram (11) could also serve the same purpose but could affect the isolation/purification process due to it polar nature. The non-phenolic coupling could either be a reductive cross-coupling or a Scholl type direct oxidative coupling.

Taking into consideration the pros and cons of both types of coupling reactions, it was decided to first study the biaryl coupling on a more robust model indole substrate. By doing so one would be able to compare the results of both reactions on the same substrate and then select the best method for coupling the sarpagine alkaloid 6. It would also give one a chance to study the effect of oxidative dimerization conditions on the more sensitive 5-methoxy sarpagine substrate, especially due to lack of literature precedent for such transformations. Additionally, the supposition that the natural sarpagine framework was indispensable for complete asymmetric induction in the formation of the biaryl axis could also be assessed. Tetrahydro β-carboline 35, the N_a-methyl analogue of 28b, was chosen as the model substrate for this purpose. In order to perform any type of reductive coupling reaction (such as the Ullmann, Suzuki, Stille etc.) on 35, it was necessary to first synthesize the corresponding aryl halide coupling partner.

Synthesis of the Aryl Halide Coupling Partners 36a/37a

Due to the moderate reactivity of iodine, electrophilic iodination of aromatic compounds requires the use of an appropriate oxidant for efficient transformation. Numerous methods employing iodonium donating agents have been developed over the years; many of which employ harsh reaction conditions and longer reaction times.⁷² Initially, a milder method for iodination with N-iodosuccinimide (NIS) and catalytic TFA, reported by Colobert et al.⁷³ was employed for iodination of the model substrate 35. Although the reaction proceeded at room temperature in acetonitirile, longer reaction times (16 hours) were required for complete conversion of 35 and the regiosiosmers 36a and 36b were obtained in a combined yield of 80% (Scheme 10). For synthesis of the corresponding aryl bromide, an NBS/TFA system provided the optimum yield of the desired product 37a. Recently Moorthy et al.⁷⁴ reported an expedient protocol for iodination of a variety of electron rich and electron-poor aromatic compounds using an IBX-I₂ redox couple. The high yields and very short reaction times were attributed to the rapid generation of 4 equivalents of iodonium ions per equivalent of IBX, thereby increasing the rate of the reaction even for electron poor substrates. As illustrated in Scheme 10, iodination of the model substrate 35 under these conditions resulted in a much faster (8 hours) and a cleaner reaction, with the desired product 36a formed in 82% yield. The regiochemistry of iodination in 36a was also confirmed by X-ray analysis (See SI).

Scheme 10.

Electrophilic Halogenation of 35

Attempted Ullman Coupling Reaction

With the appropriate aromatic halides 36a and 37a in hand, the Ullmann reductive coupling⁵⁴ to form the C7-C7′ bond was first investigated. Similar to other reductive coupling processes, various protocols of the Ullmann-type coupling procedures have been developed; however, very few are suitable for atropselective synthesis of biaryls mainly due to the harsh conditions (multi-hour reactions at temperatures >100 °C) required to obtain high yields of the desired biaryl. Such harsher conditions could lead to racemization of the biaryls and are thus not useful.^55e Illustrated in Table 1 are the various attempts to perform the Cu-mediated Ullmann coupling reaction of 36a and 37a. In spite of using up to 8 equivalents of activated Cu-powder at very high temperatures, either no conversion or complete decomposition of the starting materials 36a and 37a was observed (Table 1, entries 3–5). A copper(I)-thiophene-2-carboxylate (CuTC) protocol developed by Liebskind et al.⁷⁵ was also tested on both the aryl iodide 36a and bromide 37a (Table 1, entries 2 and 6) but failed to provide the desired product. After the failure of the Ullmann reaction to dimerize the model substrate 36a/37a, the Suzuki reductive coupling process⁵⁵ was next attempted. To accomplish this, the aryl boronate ester coupling partner from the corresponding aryl halide had to be prepared.

Table 1.

Attempted Copper-Mediated Ullmann Coupling Reaction of 36a and 37a

Entry	Reaction Conditions	Results
1.	Cu powder (2.5 equiv),53b DMF, 100 °C, 12 h	SM only
2.	CuTC (3.0 equiv), N-methyl 2-pyrrolidine, 70 °C, 72 h	SM only
3.	Activated Cu powder (8 equiv), DMF, reflux, 72 h	SM only
4.	Activated Cu powder (8 equiv), N-methyl 2-pyrrolidine, sealed tube, sand bath, 140 °C, 26 h	SM only
5.	Activated Cu powder (8 equiv), N-methyl 2-pyrrolidine, sealed tube, sand bath, 140 °C, 72 h	decomposition
6.	CuTC (8 equiv), N-methyl 2-pyrrolidine, sealed tube, sand bath, 140 °C, 26–72 h	decomposition

CuTC: copper(I)-thiophene-2-carboxylate; SM: Starting materials 36a or 37a.

Synthesis of the Aryl Boronic Ester Coupling Partner

The palladium catalyzed cross coupling reaction of (Bpin)₂ with haloarenes developed by Miyaura et al.^55d,e provided a direct procedure for the synthesis of aryl boronic esters from aryl halides. Application of these borylation conditions in the system under study here furnished the desired aryl boronate 39 in 55% yield, accompanied by a large amount of the hydrodehaolgenation species 35 (Table 2, entry1). Performing the reaction at a lower temperature with Pd(PPh₃)₄ as the catalyst, did decrease the amount of the byproduct 35 but also left some starting material (36a) unreacted (as observed on TLC). Based on the initial findings of Masuda et al.⁷⁶ and reports by Buchwald et al.⁷⁷ and Baudoin et al.,⁷⁸ it was found that a palladium-catalyzed coupling of pinacolborane with aryl halides in presence of a tertiary amine as a base, especially Et₃N, prevented the pinacolborane from acting as a hydride donor and thus avoided formation of the undesirable Ar-H byproduct (35 in this case). As illustrated in Table 2 (entries 3–8), a careful optimization of the reaction by varying the palladium source and the ligand, finally led to a combination of Pd(OAc)₂ with the more electron-rich and bulkier ligand 2-(dicyclohexylphosphanyl)biphenyl (DCPB) to provide 93% yield of the arylboronate 39 (Table 2, entry 8).

Table 2.

Synthesis of the Aryl Boronic Ester Coupling Partner 39

	Boron Sourcea	Pd Source	Ligandb	Basec	Solvent	Temp. (Time)	Resultsd
1	(Bpin)2 (4 equiv)	Pd(dppf)Cl2 (10 mol %)	-	KOAc	DMSO	100 °C (10 h)	39(55%) + 35 (30%)
2	(Bpin)2 (4 equiv)	Pd(PPh3)4 (10 mol %)	-	KOAc	DMSO	80 °C (15 h)	36a + 39 + 35 e
	HBpin (3 equiv)	Pd(MeCN)2Cl2 (5 mol %)	DCPB (20 mol %)	Et3N	dioxane	110 °C (3 h)	36a + 39 e
4	HBpin (3 equiv)	Pd(MeCN)2Cl2 (10 mol %)	DCPB (40 mol %)	Et3N	dioxane	110 °C (3 h)	36a +39 e
5	HBpin (3 equiv)	Pd(MeCN)2Cl2 (5 mol %)	XPhos (20 mol %)	Et3N	dioxane	100 °C (3 h)	36a + 39 + 35 e
6	HBpin (3 equiv)	Pd(OAc)2 (5 mol %)	DPEPhos (10 mol %)	Et3N	dioxane	100 °C (3 h)	39 (78%) + 35 (15%)
7	HBpin (3 equiv)	Pd(OAc)2 (5 mol %)	DCPB (20 mol %)	Et3N	dioxane	110 °C (3 h)	39 (80%) + 35 (10%)
8	HBpin (3 equiv)	Pd(OAc)2 (5 mol %)	DCPB (20 mol %)	Et3N	dioxane	100 °C (3 h)	39 (93%) + 35 (3%)

^a(Bpin)₂: Bis(pinacolato) diboron; HBpin: Pinacol borane;

^bDCPB: 2-(Dicyclohexylphosphanyl)biphenyl; DPEPhos: Bis(2-phenylphosphinophenyl)ether; XPhos: 2-Dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl;

^c3 equiv of base was employed;

^disolated yields;

^enot separated.

Suzuki -Miyaura Coupling (SMC)

With the appropriate aromatic halides (36a and 37a) and aryl boronate ester 39 in hand, the Suzuki coupling reaction of these di-ortho substituted coupling partners was next attempted. The synthesis of hindered biaryls via the Suzuki reaction, especially of substrates containing large ortho substituents, and/or ortho, ortho' substituents has been shown to be a challenging task.^73,77 The difficulty in such transformations can be increased in cases where the substrate has electron-donating groups thereby slowing down the oxidative-addition process. More recently the bulky and electron-rich, monodentate dialkylbiarylphospine ligands, synthesized by Buchwald et al.⁷⁷ were shown to improve the efficiency of such couplings. The superior activity of the catalysts derived from the biarylphosphine ligands was attributed to a combination of electronic and steric properties that enhanced the rates of oxidative addition, transmetalation, and reductive elimination steps in the catalytic cycle.

Outlined in Table 3 are the different catalyst systems that were employed to carry out the Suzuki-coupling reactions of the aryl halides 36a and 37a with the boronate ester 39. Based on the various synthetic applications⁷⁹ a general reaction system of Pd(OAc)₂/dialkyl-biarylphospine ligand in a 1:4 ratio was employed with different solvents and bases (Table 3). Initial couplings with 20 mol % of the DCPB, XPhos and DavePhos ligands, in presence of a milder base (K₃PO₄), resulted in mixtures of hydrodehaolgenation byproduct 35 and unreacted starting material 36a/37a (Table 3, entries 1–5). Employment of the more efficient, electron-rich and bulkier SPhos⁷⁹ ligand also yielded similar result (Table 3, entry 6). It was possible that a combination of basic reaction conditions at higher temperature was responsible for the extensive hydrodehaolgenation. In order to circumvent this it was decided to employ THF as the co-solvent. The low boiling point and higher dielectric constant of THF provided a more homogenous reaction medium at a lower temperature. This change reduced the excessive hydrodehaolgenation taking place and a combined yield of 30% was obtained in favor of the M(R)-atropdiastereomer (Table 3, entry 7). Stirring the reaction mixture for a longer time led to complete conversion of the starting material 36a, and a 54% combined yield of the atropodiastereomers was observed (Table 3, entry 8). Use of Pd₂(dba)₃ also provided similar results (Table 3, entry 9). At this point no further optimization was performed. Subsequently, the more direct Scholl-type oxidative dehydrodimerization of the model substrate 35 was attempted.

Table 3.

Suzuki-Miyaura Coupling (SMC) Reaction

	ArX	Pd Sourcea,b	Ligandc,d	Basee,f	Solventg	Temp. (°C)/Time (h)	Results
1	36a	Pd(OAc)2	DCPB	K3PO4	dioxane:H2O	100/24	35 + 36a
2	37a	Pd(OAc)2	DCPB	K3PO4	dioxane:H2O	100/24	35 + 37a
3	36a	Pd(OAc)2	DCPB	CsF	dioxane	100/24	35 + 36a
4	36a	Pd(OAc)2	XPhos	K3PO4	Tol:H2O	100/24	35 + 36a
5	36a	Pd(OAc)2	DavePhos	K3PO4	Tol:H2O	100/24	35 + 36a
6	36a	Pd(OAc)2	SPhos	K3PO4	Tol:H2O	100/24	35 + 36a
7	36a	Pd(OAc)2	SPhos	K3PO4	THF:H2O	50/24	38a (20%)+38b (10%) + 35 (15%) + 36a (13%)
8	36a	Pd(OAc)2	SPhos	K3PO4	THF:H2O	50/48	38a (37%) + 38b (18%) + 35 (20%)
9	36a	Pd2(dba)3	SPhos	K3PO4	THF:H2O	50/48	38a (38%) + 38b (19%) + 35 (20%)

^a5 mo % of Pd(OAc)₂;

^b2.5 mol % of Pd₂(dba)₃;

^cLigands: DCPB: 2-(Dicyclohexylphosphanyl)biphenyl; XPhos: 2-Dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl; DavePhos: 2-Dicyclohexylphosphino-2′-(N,N-dimethylamino)biphenyl; SPhos: 2-Dicyclohexylphosphino-2′6′-dimethoxybiphenyl;

^d20 mol % of ligand;

^e2 equiv. of K₃PO₄;

^f8 equiv. of CsF;

^gsolvent:H₂O/10:1.

Scholl Type Oxidative Dehdyrodimerization- Model Reaction

With samples of the atropdiastereomeric biaryls (38a and 38b) obtained from the Suzuki-Miyaura coupling on the model substrate 35 in hand, the desired products could now be identified from a potential mixture of regioisomers which could arise during the oxidative coupling of the model substrate. Since one was examining the effect of existing chiral centers on the atropselectivity, it was decided to use achiral catalysts for this process. Of the various protocols of the Scholl type oxidation discussed earlier, it was decided to carry out a hypervalent iodine(III) mediated protocol developed by Kita et al.^69,80 and a thallium(III) mediated oxidative dehydrodimerization approach developed by Taylor et al.^65,81 Both methods employ a combination of a two electron oxidant and a Lewis acid (BF₃·Et₂O) in polar aprotic solvents such as CCl₄, CH₂Cl₂ or MeCN, at temperatures ranging from −78 °C to room temperature. Although hypervalent iodine(III) has been extensively used for intramolecular coupling reactions,^69,82 there has been little progress made in the intermolecular mode.^78a,83 Thallium(III) has also been successfully employed intramolecularly for the synthesis of isoquinoline alkaloids,⁸⁴ lignans⁸⁵ and colchinol derivatives⁸⁶ as well as aporphine alkaloids.⁸⁷ Effective oxidative dimerizations of 2-substituted indoles,⁸⁸ and indolocarbazoles⁸⁹ by thallium(III) trifluoroacetate have also been reported. The intermolecular oxidative dimerization of a polysubstituted indole core by Keller et al.⁹⁰ provided a suitable precedent for which to effect the oxidative dimerization in the system under study here.

Illustrated in Table 4 are the results of the intermolecular oxidative dehydrodimerization reaction on the model tetrahydro β-carboline 35. Addition of PIFA (0.65 to 1.02 equiv) at 0 °C provided very little formation of the desired dimers 38a and 38b (Table 4, entries 1 & 2). The same trend was observed for the thallium trifluoroacetate mediated oxidation (Table 4, entry 7), although all the PIFA-oxidations seemed to produce a lot of baseline and colored impurities. A lower reaction temperature of −40 °C (both the reagents and the substrate added at the same temperature) seemed to have an immediate effect on the reaction and higher yields were obtained in both PIFA (Table 4, entry 4) and the thallium mediated oxidations (Table 4, entry 8), more so in the latter case. A decrease in the reaction temperature to −78 °C (Table 4, entry 5) or replacing PIFA with phenyliodine(III) diacetate [PIDA (−40 °C)] resulted in considerable amount of starting material 35 (Table 4, entry 6) remaining unreacted. A similar effect was observed upon the use of the milder Tl(III) acetate as the oxidant (Table 4, entry 10). Although some starting material 35 (11%) remained unreacted, a combined yield of 67% was obtained in favor of 38b, the axial chirality of which was determined to be P(S) by X-ray analysis.²⁷ Importantly, the model non-phenolic oxidative coupling reaction was completely regioselective (without any preactivation) and the combination of the milder Tl(III) acetate with BF₃·Et₂O, which provided the optimum yield (Table 4, entry 10), could now be attempted to construct the desired C9-C9' bond in 1. It was also clear that an oxidative biaryl coupling at a later stage in the synthesis, especially on the natural sarpagine framework, was essential for ensuring complete atropselectivity in the process. This was a key finding from the lack of stereospecificity in the model work.

Table 4.

Optimization of the Non-Phenolic Oxidative Coupling on the Model Substrate 35

	Oxidant (equiv)	Lewis Acid (equiv)	Temp (Time)	Solvent	Results
					% Yield	M:P
1	PIFA (0.6)	BF3·Et2O (2.5 equiv)	0 °C - rt (2 h)	DCM	nd	nd
2	PIFA (1.02)	BF3·Et2O (4 equiv)	0 °C - rt (8 h)	DCM	11	3 : 2
3	PIFA (1.02)	BF3·Et2O (4 equiv)	−0 °C - rt (1.5 h)	DCM	20	3 : 2
4	PIFA (0.8)	BF3·Et2O (3 equiv)	−40 °C (0.5 h)	DCM	30	4 : 1
5	PIFA (0.8)	BF3·Et2O (3 equiv)	−78 °C (0.5 h)	DCM	25a	4 : 1
6	PIDA (0.8)	BF3·Et2O (3 equiv)	−40 °C (2.5 h)	DCM	nd	nd
7	Tl(OCOCF3)3 (0.8)	BF3·Et2O (3 equiv)	rt (15 min)	MeCN	nd	nd
8	Tl(OCOCF3)3 (0.8)	BF3·Et2O (3 equiv)	−40 °C (20 min)	MeCN	38	2 : 3
9	Tl(OCOCF3)3 (0.5)	BF3·Et2O (2.5 equiv)	−78 °C (40 min)	MeCN	30	3 : 7
10	Tl(OCOCH3)3 (0.7)	BF3·Et2O (3 equiv)	−40 °C (1.25 h)	MeCN	67a	3 : 7

PIFA: phenyliodine(III) bis(trifluoroacetate); PIDA: Phenyliodine(III) diacetate.

^aThe yield is based on recovered starting material 36.

%Yield is based on isolation of both the diasteromers.

nd: not determined.

Thallium(III) Mediated Oxidative Coupling of (+)-Lochnerine (6)

As illustrated in Scheme 11, the monomeric sarpagine alkaloid lochnerine (6) was subjected to the modified conditions of the Tl(III) mediated oxidative dimerization. A combination of Tl(O₂CCH₃)₃ (0.65 equiv) and BF₃·Et₂O (3.0 equiv) in acetonitrile at −40 °C afforded the key C9-C9' biaryl 40 as the sole atropdiastereomer in 60% yield (based on 12% recovered starting material) with complete regioselectivity. The free indole N_a-H, highly basic N_b-nitrogen and the primary hydroxyl function at C-17 remained unaffected. The formation of the biaryl 40, accompanied by the competing electrophilic aromatic thallation byproduct 41 at C-9 is in complete agreement with the detailed mechanistic studies by Kochi et al.⁹¹ Increasing the equivalents of Tl(O₂CCH₃)₃ led to increased conversion of indole 6 to the organothallium byproduct 41 and baseline impurities. X-ray crystallographic analysis of 40 established the axial chirality at the C9-C9' bond as P(S).²⁷ The natural sarpagine configuration in indole 6 imparted complete stereocontrol in the key biaryl coupling step thereby forming a single atropdiastereomer 40. This would be in agreement with a potential biomimetic coupling in the plant since none of the other atropodiastereomer was reported during isolation and semisynthesis by Yu et al.¹⁰

Scheme 11.

Thallium(III) Mediated Oxidative Coupling of (+)-Lochnerine (6)

b.r.s.m: based on recovered starting material

The Regiospecific, Stereospecific Total Synthesis of (+)-Dispegtarine (1)

Completion of the total synthesis of (+)-dispegatrine (1) was then achieved in two more steps. As illustrated in Scheme 12, the C10-C10' methoxy groups in 40 were demethylated with 9 equivalents of BBr₃/CH₂Cl₂ at −78 °C to furnish the sarpagine dimer 42 in 80% yield. The highly zwitterionic nature of 42 was evident from the ¹H NMR of the compound which showed a large downfield shift of the C-3 and C-21 protons as is generally observed in N_b-quaternary sarpagine compounds. Due to this reason, the N_b-methylation of the highly polar dimer 42 was sluggish and very little formation of 1 was observed at room temperature. Eventually, heating the reaction mixture in a sealed tube at 40 °C led to complete conversion of the starting material 42 into the N_b-methyl salt of dispegatrine (1). A LRMS (FAB) of the sample at this stage was taken to confirm the formation of the desired product 1. Further treatment with AgCl/MeOH at room temperature completed the total synthesis of P-(+)-dispegatrine (1). The synthetic material 1 exhibited NMR spectra (¹H) that compared favorably with reported values.¹⁰ The coupling constants and splitting pattern in the ¹H NMR spectrum were in excellent agreement with the literature values,¹⁰ except for the proton chemical shifts for carbon atoms at C-3,3' and 5,5' which were observed with natural material with D₂O.²⁷ To obtain better spectroscopic data, it was decided to synthesize the bismethyl ether of 1, by subjecting the dimer 40 to N_b-quaternization first. Complete conversion to the bisquaternary salt 43 was achieved with a large excess of MeI/MeOH, both at room temperature or 40 °C. Analogous to blumeanine (isolated as its diacetate),¹³ chromatographic purification and isolation of this bisquaternary salt 43 was much easier in comparison to (+)-dispegatrine (1). The 2D NMR correlation experiments were then carried out on the dimer 43 to establish the position of the H-3,3' and H-5,5' protons.²⁷ These NMR experiments were in complete agreement with the assigned positions of H-3,3' and H-5,5'.²⁷ In the absence of an authentic sample⁹² [for circular dichroism (CD) analysis or thin layer chromatography (TLC) comparison], it is impossible to unequivocally report that synthetic 1 is identical to the natural product even though the ¹H NMR is in good agreement.⁹³ However, the fact that the biomimetic coupling by Yu et al.¹⁰ gave only the natural isomer and our oxidative coupling gave the P-atropodiastereomer from similar scaffolds strongly suggests that they are the same.

Scheme 12.

Completion of the Total Synthesis of (+)-Dispegatrine (1)

CONCLUSION

In conclusion, the regio- and diastereospecific doubly convergent first total synthesis of the P-atropodiastereomer of the dimeric indole alkaloid (+)-dispegatrine (1) has been accomplished from 5-methoxy-D-tryptophan methyl ester (17). p-Anisidine was converted into 17 on a large scale by employing the modified regiospecific bromination procedure. The stereospecific conversion of 17 into the optically active sarpagine framework 16 was achieved by the asymmetric Pictet-Spengler reaction and it was then employed to complete the first total synthesis of the monomeric 10-oxysubstituted alkaloids (+)-spegatrine (2), (+)-10-methoxyvellosimine (5), (+)-lochnerine (6), (+)-sarpagine (7), lochvinerine (9) and (+)-lochneram (11). Intermolecular non-phenolic oxidative dimerizations of highly functionalized substrates are very rare and the work described in this paper provides an efficient method to carry out such couplings, thereby providing an alternative to the phenolic oxidative couplings which often times produces complex mixtures with sensitive substrates. Both reductive cross coupling (Suzuki) and direct oxidative coupling were first studied on the electron-rich indole model substrate 35 and provided excellent insight into the regioselectivity and atropselectivity of the process. Based on the results of the model study, advantage was taken of the natural sarpagine configuration in lochnerine (6), a thallium(III)acetate mediated oxidative dimerization of which provided the atropodiastereomer P-40 exclusively. Completion of the total synthesis of 1 was then achieved in two more steps from 40. The total synthesis of the bisquaternary alkaloid 1 is thus notable for its brevity, principally because it employed lochnerine (6) as the substrate for the key biaryl coupling, inspite of the presence of the free indole N_a-H, free hydroxyl group at C-17 and a highly basic N_b-nitrogen function. In addition, the use of thallium(III) acetate as the oxidant for direct oxidative dehydrodimerization of electron-rich substrates such as 6 and 35 has expanded the scope of this oxidant in intermolecular hetero-biaryl synthesis.

EXPERIMENTAL SECTION

The experimental details for the synthesis of alkaloids 1, 2, 5–7 and compounds 16, 38a, 38b and 40–43 are contained in the SI of reference 27b. General procedures for borylation (entries 1–7, Table 2) and SMC (entries 1–8, Table 3) are analogous to the preparation for 39 and 38a,b respectively. For general experimental considerations see SI.

3-(((2R,5S)-3,6-Diethoxy-5-isopropyl-2,5-dihydropyrazin-2-yl)methyl)-5-methoxy-2-(trimethylsilyl)-1H-indole (21a) and the regioisomer 2-(((2R,5S)-3,6-diethoxy-5-isopropyl-2,5-dihydropyrazin-2-yl)methyl)-5-methoxy-1H-indole (21b)

In a round-bottom flask (2 L) equipped with a magnetic stir were added tert-butyl (2-iodo-4-methoxyphenyl)carbamate 20 (1 g, 2.86 mmol), the internal alkyne 19 (1.016 g, 3.15 mmol), palladium(II) acetate (38.5 mg, 0.171 mmol), potassium carbonate (989 mg, 7.16 mmol), lithium chloride (133 mg, 3.150 mmol) and DMF (20 mL). The reaction mixture was degassed under vacuum (argon) and then heated at 100 °C under a slow stream of argon for 36 h. The mixture was cooled to rt, and then EtOAc (400 mL) was added to the solution, after which it was then filtered through celite to remove the Pd black and inorganic salts. The solution which resulted was diluted with additional EtOAc (20 mL), and it was then washed with water (5 × 15 mL), brine (15 mL), and dried (Na₂SO₄). The solvent was removed under reduced pressure, and the residue was purified (short flash column) to give the 5-methoxy indole 21a, accompanied by the byproduct 21b. Column chromatography of the crude mixture in EtOAc/hexanes provided the desired indole 21a (873 mg, 68%) and the regioisomer 21b (140 mg, 13%).

21a: ¹H NMR (300 MHz, CDCl₃) δ 7.85 (s, 1H), 7.24 (d, 1H, J = 8.8 Hz), 7.17 (d, 1H, J = 2.3 Hz), 6.85 (dd, 1H, J = 8.8, 2.4 Hz), 4.31 – 3.97 (m, 5H), 3.91 (t, 1H, J = 3.4 Hz), 3.86 (s, 3H), 3.52 (dd, 1H, J = 14.2, 3.7 Hz), 2.87 (dd, 1H, J = 14.2, 9.4 Hz), 2.35 – 2.25 (m, 1H), 1.31 (t, 3H, J = 7.1 Hz), 1.19 (t, 3H, J = 7.1 Hz), 1.06 (d, 3H, J = 6.8 Hz), 0.70 (d, 3H, J = 6.8 Hz), 0.42 (s, 9H); ¹³C NMR (75 MHz, CDCl₃) δ 163.7 (C), 162.8 (C), 153.5 (C), 134.8 (C), 133.5 (C), 129.9 (C), 122.5 (C), 112.5 (CH), 111.1 (CH), 102.0 (CH), 60.6 (CH₂), 60.5 (CH₂), 60.4 (CH), 58.5 (CH), 55.9 (CH₃), 31.9 (CH₂), 31.5 (CH), 19.1 (CH₃), 16.5 (CH₃), 14.2 (2 × CH₃), −0.6 (3 × CH₃); EIMS (m/e, relative intensity) 443 (M^+•, 26), 232 (100), 212 (39), 190 (13), 169 (34), 73 (20); HRMS (EI-trisector) m/z: Calcd for C₂₄H₃₇N₃O₃Si 443.2604, Found 443.2590.

21b: ¹H NMR (300 MHz, CDCl₃) δ 9.08 (s, 1H), 7.18 (d, 1H, J = 8.7 Hz), 7.03 (d, 1H, J = 2.4), 6.79 (dd, 1H, J = 8.7, 2.4 Hz), 6.22 (br,s, 1H), 4.30 – 4.14 (m, 5H), 3.89 (t, 1H, J = 5.6 Hz), 3.86 (s, 3H), 3.42 (dd, 1H, J = 14.7, 3.2 Hz), 3.01 (dd, 1H, J = 14.7, 8.7 Hz), 2.30 – 2.19 (m, 1H), 1.41 – 1.34 (m, 6H), 1.02 (d, 3H, J = 6.9 Hz), 0.75 (d, 3H, J = 6.8 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 197.8 (C), 161.9 (C), 153.8 (C), 137.9 (C), 131.0 (C), 128.7 (C), 111.0 (CH), 110.8 (CH), 101.9 (CH), 100.6 (CH), 61.0 (CH), 60.9 (CH₂), 60.8 (CH₂), 55.8 (CH), 55.7 (CH₃), 32.7 (CH₂), 32.2 (CH), 18.9 (CH₃), 16.9 (CH₃), 14.3 (CH₃), 14.2 (CH₃); EIMS (m/e, relative intensity) 371 (M^+•, 32), 342 (7), 211 (36), 169 (38), 160 (100), 145 (13), 117 (26); HRMS (EI-trisector) m/z: Calcd for C₂₁H₂₉N₃O₃ 371.2209, Found 371.2200.

3-(((2R,5S)-3,6-Diethoxy-5-isopropyl-2,5-dihydropyrazin-2-yl)methyl)-5-methoxy-1H-indole (26)

To a solution of 25 (55 g, 0.116 mol) in dry CH₂Cl₂ (536 mL) under nitrogen was added 2,6-lutidine (38 g, 0.353 mol) and TMSOTf (31.4 g, 0.141 mol) dropwise at 0 °C. The solution which resulted was stirred at 0 °C for 30 min and then allowed to warm to rt (12 h). The reaction mixture was then poured into a cold saturated aq solution of NaHCO₃ (300 mL). The organic layer was separated and the combined organic layers were washed with brine (240 mL) and dried (Na₂SO₄). After removal of solvent under reduced pressure, the residue was purified by chromatography (hexanes/ethyl acetate, 6/1) to afford 26 as a colorless oil (38 g, 93%). The ¹H NMR spectra was in excellent agreement with the literature values.³²

(R)-Ethyl-2-(benzylamino)-3-(5 methoxy-1H-indol-3-yl)propanoate (27)

To a solution of tryptophan ethyl ester 17 (29 g, 110.6 mmol) in dry ethanol (500 mL) at 0 °C under nitrogen was added benzaldehyde (24.07 g, 226.89 mmol) and anhydrous Na₂SO₄ (78.6 g, 553.4 mmol). The solution was stirred at 0 °C for 5 h, cooled to −10 °C, and treated portionwise with NaBH₄ (4.44 g, 117.2 mmol) over a period of 3 h keeping the temperature below −5 °C to prevent epimerization at C-3 and the formation of unwanted tetrahydro β-carboline. After the mixture was allowed to stir for an additional 1 h, ice water (15 mL) was added and the mixture was allowed to warm to room temperature. The methanol was removed under reduced pressure and the aq residue was extracted with EtOAc (3 × 360 mL). The combined organic layers were washed with brine and dried (Na₂SO₄). After removal of the solvent under reduced pressure the residue was purified by flash chromatography (EtOAc : hexanes, 3 : 1) to afford the N_a-H, N_b-benzyl-7-methoxy-D-tryptophan ethyl ester 27 as an oil in 90% (35 g) yield. FTIR (CHCl₃) 3405, 2930, 1725 cm⁻¹; ¹H NMR (300 MHz, CDCl₃) δ 7.98 (br, 1H), 7.32 – 7.22 (m, 6H), 7.04 (dd, 2H, J = 15.1, 2.2 Hz), 6.86 (dd, 1H, J = 8.8, 2.3 Hz), 4.12 (q, 2H, J = 7.1 Hz), 3.87 (d, 1H, J = 14.6 Hz), 3.85 (s, 3H), 3.69 (dd, 2H, J = 13.4, 7.2 Hz), 3.16 (dd, 1H, J = 13.5, 6.0 Hz), 3.09 (dd, 1H, J = 13.5, 6.0 Hz), 1.95 (br, 1H), 1.18 (t, 3H, J = 7.2 Hz); ¹³C NMR (75.5 MHz, CDCl₃) δ 174.9, 154.0, 139.8, 131.4, 128.3, 128.2, 128.0, 127.0, 123.6, 112.4, 111.8, 111.2, 100.7, 61.3, 60.6, 55.9, 52.2, 29.5, 14.2; EIMS (m/e, relative intensity) 352 (M^+•, 100), 279 (50), 236 (11); HRMS (ESI-TOF) m/z: (M + H)⁺ Calcd for C₂₁H₂₅N₂O₃ 353.1865, Found 353.1856.

(1S,3R)-Ethyl-2-benzyl-6-methoxy-1-(3-methoxy-3-oxopropyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate (28b)

To a round,bottom flask (500 mL) that contained a solution of optically active N_a-H, N_b-benzyl-D-tryptophan ethyl ester 27 (21 g, 59.6 mmol) in dry CH₂Cl₂ (300 mL) was added the aldehyde, 4-oxobutyric acid methyl ester (17.23 g, 148.4 mmol), and AcOH (3.6 g, 59.9 mmol) at 0 °C. The reaction mixture which resulted was stirred at rt overnight. TFA (6.8 g, 59.6 mmol) in dry CH₂Cl₂ (75 mL) was then added at 0 °C. The reaction mixture which resulted was stirred at rt for 4 days and then cooled in an ice bath and brought to pH 8 with an aq solution of NH₄OH (14%). The aq layer was separated and extracted with CH₂Cl₂ (3 × 200 mL). The combined organic layers were washed with brine, dried (K₂CO₃) and the solvent was removed under reduced pressure. The residue which resulted was purified by flash chromatography (silica gel, EtOAc : hexanes, 1 : 4) to provide 28b (25 g, 93%) as a white crystalline solid. [α]_D²⁰ −41.08 (c 2.6, MeOH); ¹H NMR (600 MHz, CDCl₃) δ 7.92 (s, 1H), 7.38 (d, 2H, J = 7.2 Hz), 7.33 (t, 2H, J = 7.2 Hz), 7.28 (d, 1H, J = 7.8 Hz), 7.24 (d, 1H, J = 8.4 Hz), 7.02 (d, 1H, J = 2.4 Hz), 6.86 (dd, 1H, J = 9.0, 2.4 Hz), 4.33 – 4.27 (m, 1H), 4.26 – 4.20 (m, 1H), 4.01 (dd, 1H, J = 9.0, 4.8 Hz), 3.90 (s, 3H), 3.88 (d, 2H, J = 13.8 Hz), 3.59 (d, 1H, J = 13.8 Hz), 3.53 (s, 3H), 3.13 (dd, 1H, J = 15.6, 9.6 Hz), 3.02 (dd, 1H, J = 15.6, 4.8 Hz), 2.45 (dt, 1H, J = 16.8, 7.2 Hz), 2.34 (dt, 1H, J = 16.8, 6.6 Hz), 2.11 – 2.06 (m, 1H), 2.01 – 1.95 (m, 1H), 1.34 (t, 3H, J = 7.2 Hz); ¹³C NMR (150 MHz, CDCl₃) δ 174.3 (C), 173.0 (C), 154.1 (C), 139.4 (C), 135.1 (C), 131.3 (C), 129.2 (2 × CH), 128.2 (2 × CH), 127.4 (C), 127.1 (CH), 111.6 (2 × CH), 107.3 (C), 100.4 (CH), 60.8 (CH₂), 56.7 (CH), 56.0 (CH₃), 54.6 (CH), 53.3 (CH₂), 51.5 (CH₃), 29.6 (CH₂), 29.0 (CH₂), 21.1 (CH₂), 14.4 (CH₃); EIMS (m/e, relative intensity) 450 (M^+•, 59), 418 (26), 377 (51), 363 (100), 327 (50), 285 (22), 227 (13); HRMS (ESI-TOF) m/z: (M + H)⁺ Calcd for C₂₆H₃₁N₂O₅ 451.2233, Found 451.2222; Anal. Calcd for C₂₆H₃₀N₂O₅: C, 69.31; H, 6.71; N, 6.22. Found: C, 69.87; H, 6.74; N, 6.98.

(6S,10S)-Methyl-12-benzyl-9-hydroxy-2-methoxy-6,7,10,11-tetrahydro-5H-6,10-epiminocycloocta[b]indole-8-carboxylate (30)

To a solution of the trans diester 28b (10 g, 22.1 mmol) in dry toluene (400 mL), which had been predried by azeotropic removal of H₂O by a Dean Stark Trap (refluxed 6 h) under argon, was added sodium hydride (8.8 g of 60% NaH dispersion in mineral oil, 221.9 mmol) at 0 °C. Dry methanol (18.0 mL, 443.9 mmol) was added carefully to the above mixture dropwise at 0 °C (a large amount of H₂ was evolved at this point). The mixture which resulted was allowed to warm to rt for 0.5 h, and then heated to reflux for an additional 72 h (Note: The top of the flask was covered with aluminum foil to prevent carbonization of the intermediate lactam 29). The reaction mixture was then allowed to cool to rt and quenched with ice cold H₂O (200 mL). The organic layer was separated, and the aq layer was then extracted with CH₂Cl₂ (3 × 400 mL). The organic layers were combined, washed with brine, and dried (Na₂SO₄). The solvent was removed under reduced pressure and the mineral oil was separated by decantation. The residue which resulted was purified by flash chromatography (silica gel, EtOAc/hexanes, 1 : 4) to provide the N_a-H, β-ketoester 30 (7.9 g, 88%) as a yellow colored solid. FTIR (CHCl₃) 3398, 2922, 1659, 1621, 1441 cm⁻¹; ¹H NMR (600 MHz, CDCl₃) δ 12.03 (s, 1H), 7.58 (s, 1H), 7.40 (d, 2H, J = 7.2 Hz), 7.36 (t, 2H, J = 7.8 Hz), 7.32 (d, 1H, J = 7.2 Hz), 7.22 (d, 1H, J = 8.4 Hz) 6.99 (d, 1H, J = 2.4 Hz), 6.85 (dd, 1H, J = 9.0, 2.4 Hz), 4.02 (d, 1H, J = 5.4 Hz), 3.89 (s, 3H), 3.86 (d, 1H, J = 13.2 Hz), 3.81 (d, 1H, J = 6.0 Hz), 3.76 (d, 1H, J = 13.8 Hz), 3.70 (s, 3H), 3.19 (dd, 1H, J = 15.6, 6.0 Hz), 2.91 (d, 1H, J = 15.6 Hz), 2.85 (dd, 1H, J = 15.6, 5.4 Hz), 2.35 (d, 1H, J = 15 Hz); ¹³C NMR (150 MHz, CDCl₃) δ 172.6 (C), 171.7 (C). 154.2 (C), 138.3 (C), 134.3 (C), 130.7 (C), 128.8 (2 × CH), 128.5 (2 × CH), 127.5 (C), 127.3 (CH), 111.7 (CH), 111.6 (CH), 106.3 (C), 100.4 (CH), 94.4 (C), 56.0 (CH₃), 56.0 (CH₂), 55.2 (CH), 51.5 (CH₃), 49.8 (CH), 28.9 (CH₂), 22.1 (CH₂); EIMS (m/e, relative intensity) 404 (M^+•, 62), 372 (28), 289 (100), 199 (66), 156 (45); HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₂₄H₂₅N₂O₄ 405.1814, Found 405.1788.

(6S,10S)-2-Methoxy-7,8,10,11-tetrahydro-5H-6,10-epiminocycloocta[b]indol-9(6H)-one (32)

To a solution of the β-ketoester 30 (18 g, 0.045 mol) in 1,4-dioxane (350 mL) was added 33% aq KOH (350 mL). The reaction mixture which resulted was heated to reflux for 48 h. The solution was allowed to cool to rt and the 1,4-dioxane was removed under reduced pressure. The mixture that remained was extracted with CH₂Cl₂ (3 × 20 mL). The organic layer was separated, washed with brine, and dried (Na₂SO₄). The solvent was removed under reduced pressure and the residue was purified by flash chromatography (hexanes : EtOAc, 3 : 1) to afford the N_b-benzyl tetracyclic ketone 31 (12.6 g, 82%) which was taken forward to the next step without subjecting it to any analytical characterization.

To a solution of the tetracyclic ketone 31 (8.94 g, 25.8 mmol) in dry EtOH (50 mL) was added a saturated solution of EtOH/HCl(g) dropwise until the solid completely dissolved. The solvent was removed under reduced pressure to furnish an HCl salt of the N_b-benzyl tetracyclic ketone 31. Then EtOH was added to the salt and removed under reduced pressure. This process was repeated 3 times to remove excess hydrogen chloride. The HCl salt of 31 was degassed under reduced pressure at rt and back filled with argon (2 times). Dry Pd/C (10% by wt, 1.99 g, 1.54 mmol) was added to the above HCl salt followed by slow addition of dry ethanol (100 mL). The mixture was degassed under reduced pressure at rt and back filled with H₂ (3 times). The mixture which resulted was allowed to stir at rt under an atmosphere of hydrogen (1 atm) for 12 h. After analysis by TLC (silica gel plate was exposed to NH₃ vapors) indicated the absence of starting material 31, the catalyst was removed by filtration through celite and the solid washed with EtOH (3 × 15 mL). The organic layers were combined and the solvent was removed under reduced pressure to give a yellow residue, which was dissolved in a mixture of CHCl₃ (200 mL) and ice water, after which the solution was brought to pH 8 by addition of a solution of aq NH₄OH (14%). The aq layer was extracted with CHCl₃ (3 × 100 mL). The combined organic layers were washed with brine (200 mL) and dried (K₂CO₃). The solvent was removed under reduced pressure to afford the crude product, which was purified by chromatography on silica gel (EtOAc : hexanes, 5 : 1) to provide the N_b-H tetracyclic ketone 32 (6.1 g, 92% yield) as a yellow colored oil. ¹H NMR (600 MHz, CDCl₃) δ 7.87 (br, 1H), 7.29 (s, 1H), 7.23 (d, 1H, J = 9.0 Hz), 6.93 (d, 1H, J = 2.4 Hz), 6.86 (dd, 1H, J = 9.0, 2.4 Hz), 4.33 (m, 1H), 3.98 (d, 1H, J = 6.6 Hz), 3.88 (s, 3H), 3.12 (dd, 1H, J = 16.2, 6.6 Hz), 2.82 (d, 1H, J = 16.8 Hz), 2.54 – 2.44 (m, 2H), 2.21 – 2.14 (m, 2H); ¹³C NMR (150 MHz, CDCl₃) δ 211.0 (C), 154.3 (C), 134.9 (C), 130.7 (C), 127.4 (C), 112.0 (CH), 111.6 (CH), 107.5 (C), 100.3 (CH), 59.9 (CH), 55.9 (CH₃), 46.3 (CH), 35.2 (CH₂), 32.2 (CH₂), 26.0 (CH₂); HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₁₅H₁₇N₂O₂ 257.1290, Found 257.1302.

(6S,10S)-12-((Z)-2-iodobut-2-en-1-yl)-2-methoxy-7,8,10,11-tetrahydro-5H-6,10-epimino cycloocta[b]indol-9(6H)-one (33)

To a solution of the N_a-H, N_b-H tetracyclic ketone 32 (6.0 g, 23.4 mmol) and molecular sieves (5.0 g) in anhydrous acetonitrile (250 mL) under an inert atmosphere was added K₂CO₃ (12.9 g, 93.7 mmol) and Z-1-bromo-2-iodo-2-butene⁴⁶ (7.9 g, 30.4 mmol) and the mixture which resulted was stirred at rt for 8 h. Analysis by TLC (silica gel, CHCl₃ : EtOH, 4 : 1) indicated the absence of tetracyclic ketone 32. The solids were removed by filtration and washed with EtOAc (3 × 100 mL). The combined organic layers were concentrated under reduced pressure to provide a light yellow residue. Purification of the crude product by flash chromatography (silica gel, EtOAc/hexanes, 1 : 9) provided the N_b-Z-2′-iodo-2′-butenyl, tetracyclic ketone 33 (7.76 g, 76%) as a yellow colored solid. FTIR (CHCl₃) 2929, 1707 cm⁻¹; ¹H NMR (300 MHz, CDCl₃) δ 7.89 (s, 1H), 7.24 (d, 1H, J = 8.7 Hz), 6.94 (d, 1H, J = 2.3 Hz), 6.86 (dd, 1H, J = 8.7, 2.4 Hz), 5.85 (q, 1H, J = 6.3 Hz), 4.03 (d, 1H, J = 2.6 Hz), 3.88 (s, 3H), 3.72 (d, 1H, J = 6.5 Hz), 3.39 (dd, 2H, J = 17.5, 13.9 Hz), 3.11 (dd, 1H, J = 16.7, 6.6 Hz), 2.69 (d, 1H, J = 16.7 Hz), 2.56 – 2.46 (m, 2H), 2.16 – 1.99 (m, 2H), 1.82 (d, 3H, J = 6.4 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 210.2 (C), 154.2 (C), 133.0 (C), 132.7 (CH), 130.8 (C), 127.2 (C), 111.9 (CH), 111.6 (CH), 108.5 (C), 106.7 (C), 100.3 (CH), 64.1 (CH), 63.4 (CH₂), 55.9 (CH₃), 49.9 (CH), 34.5 (CH₂), 30.4 (CH₂), 21.7 (CH₃), 20.6 (CH₂); EIMS (m/e, relative intensity) 436 (M^+•, 52), 408 (6), 379 (100), 309 (10), 281 (23); HRMS (EI-trisector) m/z: Calcd for C₁₉H₂₁IN₂O₂ 436.0648, Found 436.0663.

(+)-Lochneram (11)

To a stirred solution of 6 (100 mg, 0.308 mmol) in freshly distilled MeOH (2 mL) at 0 °C was added MeI (2 mL) and the reaction was allowed to warm to rt in the dark (24 h) until disappearance of the starting material 6 (TLC, silica gel). The solvent and excess MeI was removed under reduced pressure to provide the crude N_b-methiodide salt. The solvent was removed under reduced pressure and the residue passed through a short column of activated neutral alumina using CHCl₃/MeOH (16:1) as eluant to provide (+)-lochneram (11, 122 mg) in 85% yield as a clear oil. ¹H NMR (600 MHz, CD₃OD) δ 7.31 (d, 1H, J = 9.0 Hz), 7.04 (br, s, 1H), 6.87 (d, 1H, J = 8.4 Hz), 5.68 (q, 1H, J = 6.6 Hz), 4.91 (1H, peak is embedded in CD₃OD peak), 4.46 (d, 1H, J = 15.6 Hz), 4.25 (d, 1H, J = 15.6 Hz), 3.85 (s, 3H), 3.58 (d, 3H, J = 7.8 Hz), 3.31 (d, 1H, J = 4.8 Hz, part of the peak is embedded in CD₃OD peak), 3.15 – 3.10 (m, 5H), 2.55 (t, 1H, J = 12.0 Hz), 2.21 – 2.17 (m, 2H), 1.74 (d, 3H, J = 6.6 Hz); ¹³C NMR (150 MHz) δ 154.5 (C), 132.4 (C), 131.7 (C), 127.7 (C), 126.5 (C), 120.7 (CH), 112.6 (CH), 112.0 (CH), 100.3 (C), 99.8 (CH), 65.4 (CH), 64.4 (CH₂), 62.4 (CH), 61.0 (CH₂), 54.8 (CH₃), 46.7 (CH₃), 43.6 (CH), 32.0 (CH₂), 26.0 (CH), 23.9 (CH₂), 11.6 (CH₃); HRMS (ESI-TOF) m/z: Calcd for C₂₁H₂₇N₂O₂ (M)⁺ 339.2073; Found 339.2057. The spectral data for 11 were identical to those reported in the literature.^1a,22

(+)-Lochvinerine (9)

A mixture of anhydrous potassium tert butoxide (313 mg, 0.279 mmol) and methyl-triphenylphosphonium bromide (911 mg, 0.25 mmol) in dry benzene (14 mL) was allowed to stir at rt for 1 h. The pentacyclic ketone 16 (124 mg, 0.40 mmol) in THF (5 mL) was then added into the above orange-colored solution dropwise at rt. The mixture which resulted was stirred at rt for 4 h. The mixture was diluted with EtOAc (50 mL), washed with H₂O (3 × 10 mL) as well as brine (25 mL), and dried (K₂CO₃). The solvent was removed under reduced pressure and the oil that resulted was chromatographed (silica gel, CHCl₃/MeOH; 15:1) to provide the olefin 34 (111 mg, 92%). ¹H NMR (300 MHz, CDCl₃) δ 7.81 (s, 1H), 7.18 (d, 1H, J = 8.7 Hz), 6.95 (d, 1H, J = 2.4 Hz), 6.79 (dd, 1H, J = 8.7, 2.5 Hz), 5.27 (q, 1H, J = 6.7 Hz), 4.86 – 4.84 (m, 2H), 4.15 (dd, 1H, J = 9.8, 2.2 Hz), 3.87 – 3.85 (s, 4H), 3.70 (br, s, 2H), 3.31 (d, 1H, J = 2.5 Hz), 3.13 (dd, 1H, J = 15.3, 5.4 Hz), 2.96 (dd, 1H, J = 15.3, 1.5 Hz), 2.14 (ddd, 1H, J = 12.0, 10.1, 1.7 Hz), 1.93 – 1.85 (m, 1H), 1.66 (d, 3H, J = 6.8 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 153.9 (C), 152.8 (C), 138.6 (C), 137.6 (C), 131.3 (C), 127.9 (C), 114.6 (CH), 111.4 (CH), 110.9 (CH), 105.1 (CH₂), 104.7 (C), 100.5 (CH), 56.6 (CH), 55.9 (CH₂), 55.9 (CH₃), 50.5 (CH), 36.7 (CH), 36.3 (CH₂), 26.3 (CH₂), 12.3 (CH₃); EIMS (m/e, relative intensity) 306 (M^+•, 100), 291 (16), 265 (12), 251 (10), 198 (40), 183 (16), 156 (10), 77 (10). This material was employed directly in the next step without any further characterization.

To a solution of olefin 34 (124 mg, 0.40 mmol) in THF (12 mL) was added 9-BBN (0.5 M in THF, 5 mL, 2.43 mmol) dropwise at 0 °C. The solution was allowed to warm to rt and stirred for 1.5 h. The reaction mixture was then cooled to 0 °C and NaBO₃·4H₂O (1.1g, 7.28 mmol) was added and the reaction temperature allowed to warm to rt. The mixture that resulted was stirred for 2 h at rt, diluted with CH₂Cl₂ (200 mL), washed with H₂O (3 × 50 mL) as well as brine (100 mL) and dried (K₂CO₃). The solvent was removed under reduced pressure and the residue was chromatographed (silica gel, CHCl₃/MeOH; 9:1) to provide lochvinerine 9 (98 mg, 75%) as a clear oil. ¹H NMR (300 MHz, CDCl₃) 10.2 (br, s, 1H), 7.30 (d, 1H, part of the peak is embedded in CDCl₃ peak), 6.87 (d, 1H, J = 2.2 Hz), 6.80 (dd, 1H, J = 8.8, 2.3 Hz), 5.16 (q, 1H, J = 6.8 Hz), 4.43 (d, 1H, J = 5.8 Hz), 3.88 – 3.84 (m, 4H), 3.71 (d, 1H, J = 17.6 Hz), 3.52 (dd, 1H, J₂ = 7.2 Hz, part of the peak is embedded in MeOH peak), 3.50 (1H is embedded in MeOH peak), 3.11 (d, 1H, J = 16.6 Hz), 3.36 – 3.24 (m, 2H), 2.93 (br, s, 1H), 2.35 – 2.33 (m, 1H), 2.19 (s, 1H), 2.03 – 1.95 (m, 2H), 1.63 (d, 3H, J = 6.5 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 154.0 (C), 140 – 132 (2 quaternary carbons not observed), 131.7 (C), 125.7 (C), 117.3 (CH), 112.5 (CH), 111.9 (CH), 104.2 (C), 100.2 (CH), 60.3 (CH₂), 55.7 (CH₃), 54.9 (CH₂), 53.9 (CH), 50.6 (CH), 40.9 (CH), 26.5 (CH₂), 25.7 (CH), 21.4 (CH₂), 12.7 (CH₃); HRMS (ESI-TOF) m/z: (M + H)⁺ Calcd for C₂₀H₂₅N₂O₂ 325.1916; Found 325.1920. The spectral data for 9 were identical to those reported in the literature.²¹

(1S,3R)-Ethyl-2-benzyl-5-iodo-6-methoxy-1-(3-methoxy-3-oxopropyl)-9-methyl-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate (36a)

To a solution of 35 (750 mg, 1.61 mol) in acetonitrile (20.7 mL) cooled to 0 °C, TFA (2.3 mL) was added dropwise, followed by IBX (226 mg, 0.807 mol) and I₂ (497 mg, 1.77 mol). The reaction mixture was stirred at 0 °C for 30 min and then allowed to warm to rt (8 h). The reaction mixture was diluted with ethyl acetate (30 mL), cooled in an ice bath and adjusted to pH 8 with a solution of cold aq NH₄OH (10%), followed by treatment with a saturated aq solution of Na₂S₂O₃ (35 mL) to remove excess iodine. The organic layer was separated, washed with brine (3 × 10 mL), dried (Na₂SO₄) and concentrated under reduced pressure to give a reddish brown oil. Flash column chromatography (silica gel, ethyl acetate/hexanes; 2 : 8) provided 36a (780 mg, 82%) as a white crystalline solid and the regioisomer 36b (76 mg, 8%) as a light yellow colored oil.

36a: ¹H NMR (300 MHz, CDCl₃) δ 7.38 – 7.34 (m, 3H), 7.32 (br,s, 1H), 7.30 – 7.28 (m, 1H), 7.22 (d, 1H, J = 8.7 Hz), 6.90 (d, 1H, J = 8.7 Hz), 4.40 – 4.23 (m, 2H), 4.02 (dd, 1H, J = 11.2, 4.8 Hz), 3.94 (s, 3H), 3.89 (d, 1H, J = 13.4 Hz), 3.80 (dd, 1H, J = 10.6, 3.1 Hz), 3.74 – 3.63 (m, 4H), 3.49 – 3.44 (m, 4H), 3.38 (d, 1H, J = 13.2 Hz), 2.65 (ddd, 1H, J = 17.5, 9.5, 5.4 Hz), 2.40 (dt, 1H, J = 17.5, 5.3 Hz), 2.06 – 1.95 (m, 1H), 1.93 – 1.81 (m, 1H), 1.39 (t, 3H, J = 7.1 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 173.8 (C), 172.7 (C), 152.3 (C), 139.2 (C), 137.8 (C), 133.9 (C), 129.2 (2 × CH), 128.9 (C), 128.1 (2 × CH), 126.9 (CH), 109.2 (CH), 107.9 (CH), 107.7 (C), 60.8 (CH₂), 58.3 (CH₃), 55.8 (CH), 53.3 (CH), 52.6 (CH₂), 51.2 (CH₃), 29.8 (CH₃), 29.6 (CH₂), 29.3 (CH₂), 22.2 (CH₂), 14.3 (CH₃) (One of the quaternary carbon atoms is embedded in the above carbons); EIMS (m/e, relative intensity) 590 (M^+•, 14), 517 (55), 503 (100), 377 (20), 339 (22), 303 (14); HRMS (ESI-TOF) m/z: [M + H]⁺ Calcd for C₂₇H₃₂IN₂O₅ 591.1356, Found 591.1332. The structure was confirmed by X-ray analysis (see SI).

36b: EIMS (m/e, relative intensity) 590 (M^+•, 14), 517 (55), 503 (100), 377 (20), 339 (22), 303 (14); HRMS (ESI-TOF) m/z: (M + H)⁺ Calcd for C₂₇H₃₂IN₂O₅ 591.1356, Found 591.1254. The identity of the regioisomer 36b was confirmed by comparison of the crude ¹H NMR with 36a. HRMS was also performed on the same sample. No further characterization was carried out.

(1S,3R)-Ethyl-2-benzyl-5-bromo-6-methoxy-1-(3-methoxy-3-oxopropyl)-9-methyl-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate (37a)

To a solution of indole 35 (500 mg, 1.07 mol) in acetonitrile (40 mL), cooled to 0 °C, TFA (0.12 mL, 1.61 mol) was added dropwise and this was followed by NBS (229 mg, 1.29 mol). The reaction mixture was stirred at 0 °C for 30 min and then allowed to warm to rt (18 h). The reaction mixture was diluted with ethyl acetate (30 mL), cooled in an ice bath and adjusted to pH 8 with cold aq NH₄OH solution (10%). The organic layer was separated, washed with brine (3 × 10 mL), dried (Na₂SO₄) and concentrated under reduced pressure to give a reddish brown oil. Column chromatography (silica gel, ethyl acetate/hexanes; 1 : 4) provided the 9-bromoindole 37a (380 mg, 65%) and the regioisomer 37b (68 mg, 13%).

37a: ¹H NMR (300 MHz, CDCl₃) δ 7.40 – 7.24 (m, 5H), 7.19 (d, 1H, J = 8.7 Hz), 6.94 (d, 1H, J = 8.8), 4.41 – 4.22 (m, 2H), 4.02 (dd, 1H, J = 11.2, 4.9 Hz), 3.95 (s, 3H), 3.89 (d, 1H, J = 13.2 Hz), 3.79 (dd, 1H, J = 10.8, 3.1 Hz), 3.62 (s, 3H), 3.57 (d, 1H, J = 4.9 Hz), 3.48 – 3.36 (m, 5H), 2.64 (ddd, 1H, J = 17.4, 9.5, 5.3 Hz), 2.40 (dt, 1H, J = 17.5, 5.3 Hz), 2.06 – 1.95 (m, 1H), 1.93 – 1.80 (m, 1H), 1.39 (t, 3H, J = 7.1 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 173.8 (C), 172.7 (C), 150.0 (C), 139.2 (C), 137.7 (C), 134.0 (C), 129.2 (2 × CH), 128.1 (2 × CH), 126.9 (CH), 126.3 (C), 108.9 (CH), 108.1 (CH), 107.0 (C), 103.0 (C), 60.8 (CH₂), 58.2 (CH₃), 56.0 (CH), 53.2 (CH), 52.6 (CH₂), 51.2 (CH₃), 29.8 (CH₃), 29.3 (CH₂), 27.7 (CH₂), 22.1 (CH₂), 14.3 (CH₃); EIMS (m/e, relative intensity) 544 (M^+•, 5), 542 (M^+•, 5), 471 (14), 469 (17), 457 (99), 455 (100), 377 (36); HRMS (ESI-TOF) m/z: (M + H)⁺ Calcd for C₂₇H₃₂BrN₂O₅ 543.1488, Found 543.1503.

37b: The identity of the regioisomer 37b was confirmed by comparison of the crude ¹H NMR to that of 36b. No further characterization was carried out.

(1S,3R)-Ethyl-2-benzyl-6-methoxy-1-(3-methoxy-3-oxopropyl)-9-methyl-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate (39) [Entry 8, Table 2]

General procedure for entries 1 – 7 is the same as described below.

To a resealable Schlenk tube possessing a Teflon screw valve were added 36a (118 mg, 0.2 mmol), Pd(OAc)₂ (2.2 mg, 0.01 mmol) and 2-(dicyclohexylphosphanyl)biphenyl (DCPB, 14 mg, 0.04 mmol). The Schlenk tube was capped with a rubber septum and then evacuated and backfilled with argon (this sequence was carried out a total of three times). Freshly degassed 1,4-dioxane (2.5 mL) was added via a syringe through the septum, followed by the addition of dry Et₃N (0.11 mL, 80 mg, 0.80 mmol) and pinacol borane (0.09 mL, 77 mg, 0.6 mmol). The septum was then replaced with a Teflon screw valve under a positive argon pressure and the Schlenk tube was sealed. The reaction mixture was heated to 100 °C and stirred at that temperature for 3 h. At the end of this period the reaction mixture was cooled to rt, diluted with EtOAc (10 mL) and was passed through a short pad of celite. The celite pad was further washed with EtOAc (20 mL) and the combined filtrates were washed with water (20 mL) and brine (20 mL). The organic extracts were dried (Na₂SO₄) and concentrated under reduced pressure. The crude residue thus obtained was purified by flash chromatography on a silica gel column, eluted with 4: 1 hexanes/EtOAc to afford 39 as a white solid (110 mg, 93%). mp: 182.6 - 183.8 °C; ¹H NMR (300 MHz, CDCl₃) δ 7.39 (d, 2H, J = 6.6 Hz), 7.32 (d, 2H, J = 6.6 Hz), 7.26 (m, 2H), 6.89 (d, 1H, J = 8.7 Hz), 4.29 (m, 2H), 4.06 (dd, 1H, J = 10.5, 6.0 Hz), 3.88 (s, 3H), 3.81 (d, 1H, J = 13.2 Hz), 3.75 (dd, 1H, J = 10.8, 3.0 Hz), 3.60 (s, 3H), 3.49 (s, 3H), 3.40 (d, 1H, J = 13.2 Hz), 3.15 (m, 2H), 2.62 (m, 1H), 2.42 (m, 1H), 1.97 (m, 1H), 1.84 (m, 1H), 1.49 (s, 12H), 1.37 (t, 3H, J = 7.2 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 173.9 (C), 172.9 (C), 157.8 (C), 139.4 (2 × C), 136.8 (C), 133.1 (C), 129.5 (C), 129.3 (2 × CH), 128.0 (2 × CH), 126.8 (CH), 110.4 (CH), 107.8 (CH), 106.5 (C), 83.9 (2 × C), 60.7 (CH₂), 58.3 (CH₃), 56.3 (CH), 53.2 (CH), 52.7 (CH₂), 51.2 (CH₃), 29.6 (CH₂), 29.6 (CH₃), 27.9 (CH₂), 25.1 (4 × CH₃), 21.8 (CH₂), 14.3 (CH₃); HRMS (ESI-TOF) m/z: (M + H)⁺ Calcd for C₃₃H₄₄BN₂O₇ 591.3236, Found 591.3265; Anal. Calcd for C₃₃H₄₃BN₂O₇: C, 67.12; H, 7.34; N, 4.74. Found: C, 66.91; H, 7.53; N, 4.60.

(1R,1'R,3S,3'S)-diethyl-2,2'-dibenzyl-6,6'-dimethoxy-1,1'-bis(3-methoxy-3-oxopropyl)-9,9'-dimethyl-2,2',3,3',4,4',9,9'-octahydro-1H,1'H-[5,5'-bipyrido[3,4-b]indole]-3,3'-dicarboxylate (38a) and its atropodiastereomer (1S,1'S,3R,3'R)-diethyl 2,2'-dibenzyl-6,6'-dimethoxy-1,1'-bis(3-methoxy-3-oxopropyl)-9,9'-dimethyl-2,2',3,3',4,4',9,9'-octahydro-1H,1'H-[5,5'-bipyrido[3,4-b]indole]-3,3'-dicarboxylate (38b) [Entry 9, Table 3]

General procedure for entries 1 – 8 is the same as described below.

To a resealable Schlenk tube possessing a Teflon screw valve were added 36a (11.8 mg, 0.02 mmol), 39 (17.6 mg, 0.03 mmol), Pd(OAc)₂ (0.44 mg, 0.002 mmol), S-Phos (1.6 mg, 0.004 mmol), and K₃PO₄ (8.4 mg, 0.04 mmol). The Schlenk tube was capped with a rubber septum and then evacuated and backfilled with argon (this sequence was carried out a total of three times). Freshly degassed THF (1 mL) and water (0.1 mL) were added via syringe through the septum. The septum was then replaced with a Teflon screw valve under a positive argon pressure and the Schlenk tube was sealed. The reaction mixture was heated to 50 °C and stirred at that temperature for 48 h. At the end of this period, the reaction mixture was cooled to rt and was passed through a short pad of celite. The celite pad was further washed with EtOAc (10 mL) and the combined filtrate was washed with water (5 mL) and brine (5 mL). The organic extracts were dried (Na₂SO₄) and concentrated under reduced pressure. The crude residue thus obtained was purified by flash chromatography on a silica gel column, eluted with 1: 1 hexanes/EtOAc to afford 38a as an off-white solid (6.8 mg, 37%), 38b (3.3 mg, 18%) as a light yellow solid and recovered 35 (20%).

The ¹H NMR spectra's for 38a and 38b were identical to that reported in the communication.^27b

Intermolecular oxidative coupling of the model substrate, β-carboline 35

Entry 1: To a stirred solution of the β-carboline 35 (150 mg, 0.32 mmol) in dry CH₂Cl₂ (25 mL) at 0 °C was added solid PIFA (84 mg, 0.194 mmol) and boron trifluoride diethyl etherate dropwise (114.5 mg, 0.807 mmol), after which the reaction mixture was allowed to warm to rt and stirred for 2 h. Analysis of the reaction mixture by TLC [silica gel, CHCl₃/MeOH (v/v, 9 : 1)] indicated the presence of the starting material 35, and atropdiastereomers 38a and 38b, with a considerable amount of colored impurities formed at the baseline. No further purification for the separation of the diastereomers was attempted.

Entry 2: To a stirred solution of the β-carboline 35 (100 mg, 0.215 mmol) in dry CH₂Cl₂ (2.0 mL) at 0 °C was added solid PIFA (95 mg, 0.219 mmol) and boron trifluoride diethyl etherate dropwise (122.2 mg, 0.861 mmol), after which the reaction mixture was allowed to warm to rt and stirred for 8 h. Analysis of the reaction mixture by TLC [silica gel, CHCl₃/MeOH (v/v, 9 : 1)] indicated complete conversion of the starting material 35. The reaction mixture was diluted CH₂Cl₂ (25 mL) and cooled to 0 °C, after which it was brought to pH = 8 with a cold aq solution of saturated NaHCO₃. The aq layer which resulted was extracted with CH₂Cl₂ (3 × 15 mL), and the combined organic layers were washed with a saturated aq solution of NaHSO₃ (2 × 15 mL) and dried (Na₂SO₄). The solvent was removed under reduced pressure to afford a dark oil. The crude diastereomeric mixture was purified by flash chromatography on silica gel (hexanes/ethyl acetate) to provide a combined yield of 38a + 38b: 21 mg (11%), with a diastereomeric ratio of 3 : 2 in favor of 38a.

Entry 3: To a stirred solution of the β-carboline 35 (100 mg, 0.215 mmol) in dry CH₂Cl₂ (2.0 mL) at −40 °C was added solid PIFA (95 mg, 0.219 mmol) and boron trifluoride diethyl etherate (122.2 mg, 0.861 mmol) dropwise, after which the reaction mixture was allowed to warm to rt and stirred for 1.5 h. Analysis of the reaction mixture by TLC [silica gel, CHCl₃/MeOH (v/v, 9 : 1)] indicated complete conversion of the starting material 35. Fewer baseline impurities were observed as a result of lowering the reaction temperature in this case. The workup and purification procedure were the same as entry 2: combined yield of 38a + 38b: 39 mg (20%) with a diastereomeric ratio of 3 : 2 in favor of 38a.

Entry 4: To a stirred solution of the β-carboline 35 (55 mg, 0.118 mmol) in dry CH₂Cl₂ (1.0 mL) under an inert atmosphere at −40 °C was added a solution of PIFA (40 mg, 0.094 mmol) and boron trifluoride diethyl etherate (50.41 mg, 0.355 mmol) dropwise in CH₂Cl₂ (2.0 mL), which had been precooled to −40 °C via a double ended needle transfer. The reaction mixture which resulted was stirred at −40 °C for 0.5 h. Analysis of the reaction mixture by TLC [silica gel, CHCl₃/MeOH (v/v, 9 : 1)] indicated complete conversion of the starting material 35 and a much cleaner reaction. The workup and purification procedure were the same as entry 2: combined yield of 38a + 38b: 33 mg (30%) with a diastereomeric ratio of 4 : 1 in favor of M-38a via analysis of the integration values of the ¹H NMR spectrum.

Entry 5: To a stirred solution of the β-carboline 35 (63 mg, 0.135 mmol) in dry CH₂Cl₂ (1.0 mL) under an inert atmosphere at −78 °C was added a solution of PIFA (47 mg, 0.108 mmol) and boron trifluoride diethyl etherate (57.5 mg, 0.405 mmol) dropwise in CH₂Cl₂ (2.0 mL) precooled to −78 °C via a double ended transfer needle. The reaction mixture which resulted was stirred at −78 °C for 0.5 h. Analysis of the reaction mixture by TLC [silica gel, CHCl₃/ MeOH (v/v, 9 : 1)] indicated the presence of unreacted starting material 35 along with the diastereomers 38a and 38b. The workup and purification procedure were the same as entry 2: combined yield of 38a + 38b: 31.4 mg (25%, based on recovered starting material) with a diastereomeric ratio of 4 : 1 in favor of 38a.

Entry 6: To a stirred solution of the β-carboline 35 (50 mg, 0.107 mmol) in dry CH₂Cl₂ (1.0 mL) under an inert atmosphere at −40 °C was added a solution of PIDA (27.7 mg, 0.086 mmol) and boron trifluoride diethyl etherate (45.8 mg, 0.322 mmol) dropwise in CH₂Cl₂ (2.0 mL) precooled to −40 °C via a double ended transfer needle. The reaction mixture which resulted was stirred at −40 °C for 2.5 h. Analysis of the reaction mixture by TLC [silica gel, CHCl₃/ MeOH (v/v, 9 : 1)] indicated very little formation of the diastereomers 38a and 38b with a considerable amount of starting material 35 remaining. The workup and purification procedure were the same as entry 2. No further purification for the separation of the diastereomers (38a and 38b) was attempted.

Entry 7: To a stirred solution of the β-carboline 35 (50 mg, 0.107 mmol) in dry MeCN (2.0 mL) under an inert atmosphere at rt was added solid thallium(III) trifluoroacetate (46.7 mg, 0.086 mmol) and boron trifluoride diethyl etherate (45.8 mg, 0.322 mmol) dropwise and the solution which resulted allowed to stir at rt for 15 min. Analysis of the reaction mixture by TLC [silica gel, CHCl₃/MeOH (v/v, 9 : 1)] indicated the absence of starting material 35 and formation of traces of atropdiastereomers 38a and 38b. No further purification for the separation of the diastereomers was attempted.

Note: Thallium compounds are toxic. Do not breath, ingest or get on skin: Use Caution.

Entry 8: To a stirred solution of the β-carboline 35 (50 mg, 0.108 mmol) in dry acetonitrile (2.0 mL) under an inert atmosphere at −40 °C was added a solution of thallium(III) trifluoroacetate (46.7 mg, 0.086 mmol) and boron trifluoride diethyl etherate dropwise (45.8 mg, 0.322 mmol) in MeCN (2.0 mL), which was precooled to −40 °C, via a double ended transfer needle. The reaction mixture which resulted was stirred at −40 °C for 20 min. Analysis of the reaction mixture by TLC [silica gel, CHCl₃/MeOH (v/v, 9: 1)] indicated a cleaner reaction had occurred with complete conversion of the starting material 35 and formation of the two atropdiastereomers (38a and 38b). The cold reaction mixture was diluted with CH₂Cl₂ (25 mL) and the solvent was removed under reduced pressure to give a brown residue. The residue was dissolved in fresh CH₂Cl₂ (25 mL) and cooled to 0 °C after which it was brought to pH = 8 with a cold aq solution of saturated NaHCO₃. The aq layer which resulted was extracted with CH₂Cl₂ (3 × 15 mL) and the combined organic layers were washed with brine (2 × 15 mL) and dried (Na₂SO₄). The solvent was removed under reduced pressure to afford a dark brown oil. The crude diastereomeric mixture was purified by flash chromatography on silica gel (hexanes/ethyl acetate) to provide a combined yield of 38a + 38b: 37.9 mg (38%) with a diastereomeric ratio of 2 : 3 in favor of 38b.

Entry 9: To a stirred solution of the β-carboline 35 (50 mg, 0.108 mmol) in dry acetonitrile (2.0 mL) under an inert atmosphere at −78 °C was added a solution of thallium(III) trifluoroacetate (29.2 mg, 0.054 mmol) and boron trifluoride diethyl etherate (38.2 mg. 0.269 mmol) in MeCN (2.0 mL) precooled to −78 °C via a double ended transfer needle. The reaction mixture which resulted was stirred at −78 °C for 40 min. Analysis of the reaction mixture by TLC [silica gel, CHCl₃/MeOH (v/v, 9 : 1)] indicated formation of the two atropdiastereomers (38a and 38b) and complete conversion of the starting material 35 with a considerable amount of impurities formed at the baseline. The workup and purification procedure were the same as entry 8: combined yield of 38a + 38b: 29.9 mg (30%) with a diastereomeric ratio of 3 : 7 in favor of 38b.

Entry 10: To a stirred solution of the β-carboline 35 (200 mg, 0.430 mmol) in dry acetonitrile (10 mL) under an inert atmosphere at −40 °C was added a solution of thallium(III) acetate (115.0 mg, 0.301 mmol) and boron trifluoride diethyl etherate (183.0 mg, 1.29 mmol) in MeCN (10 mL) precooled to −40 °C via a double ended transfer needle. The reaction mixture which resulted was stirred at −40 °C for 1.25 h. Analysis of the reaction mixture by TLC [silica gel, CHCl₃/MeOH (v/v, 9 : 1)] indicated the presence of starting material 37 and formation of the two atropdiastereomers (38a and 38b). The workup and purification procedure were the same as entry 8: combined yield of 38a + 38b: 227 mg (67%) with a diastereomeric ratio of 3 : 7 in favor of 38b; recovered starting material 35 (28 mg, 14%). Recrystallization of 38b from ethanol gave light brown crystals. X-ray analysis of 38b established the axial chirality (at the C9-C9') as P(S).