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. Author manuscript; available in PMC: 2019 Jun 20.
Published in final edited form as: Nat Prod Rep. 2018 Jun 20;35(6):559–574. doi: 10.1039/c7np00069c

Enantioselective Palladium-Catalyzed Allylic Alkylation Reactions in the Synthesis of Aspidosperma and Structurally Related Monoterpene Indole Alkaloids

Beau P Pritchett a, Brian M Stoltz a
PMCID: PMC6013405  NIHMSID: NIHMS961707  PMID: 29658039

Abstract

Enantioselective Pd-catalyzed allylic alkylations of prochiral enolates represent a powerful tool for the construction of all-carbon quaternary stereocenters. This review describes the emergence of such reactions as strategic linchpins that enable efficient, stereocontrolled syntheses of Aspidosperma and related monoterpene indole alkaloids.

1 Introduction and statement of purpose

The structural intricacies and biological activities of monoterpene indole alkaloids have rendered these compounds attractive targets for total synthesis.1 In particular, the structurally related Aspidosperma and Kopsia classes of alkaloids have comprised some of the most frequently targeted structures in chemical synthesis over the course of more than half a century (Figure 1).2 Consequently, numerous strategically distinct total syntheses have been reported for various Aspidosperma and Kopsia family members. Without question, there are several strategies toward these targets that made profound contributions to modern organic synthesis.3 The purpose of this review, however, is to highlight completed enantioselective syntheses of Aspidosperma and Kopsia alkaloids that implement an enantioselective Pd-catalyzed allylic alkylation as the key stereodefining tactic. In our view, this strategy is distinctive in its widespread adoption by many researchers, and more importantly its prolific assembly of many functionally and topologically unique members of the Aspidosperma and Kopsia families.

Figure 1.

Figure 1

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Representative Aspidosperma and related alkaloids.

Enantioselective Pd-catalyzed allylic alkylations of prochiral enolates represent one of the most robust methods for the synthesis of α-quaternary carbonyl compounds.4 Experimental and computational studies have been conducted to elucidate the mechanism of this transformation,4c–g,5 though the precise reaction pathway varies depending on the ligand, Pd precatalyst, solvent, substrate, allyl electrophile, and additives employed. While enantioselective Pd-catalyzed allylic alkylations of prochiral enolates have been utilized in total synthesis,6 these reports largely involve the use of ketone-derived substrates. Within the past five years, such enantioselective reactions have been extended to new heterocylic substrates to construct the stereogenic all-carbon quaternary center at C20 of Aspidosperma and Kopsia alkaloids,7 a unifying feature of these classic targets (see 1, Figure 1). The C20 quaternary carbon is the stereochemical linchpin of these syntheses, and the remaining chiral centers are built with exceptional stereocontrol. The studies compiled herein, which span several research groups and substrate classes, illustrate the broad applicability of enantioselective allylic alkylation as a highly enabling disconnection for the synthesis of Aspidosperma and Kopsia alkaloids.

2 Structural overview of Aspidosperma and related monoterpene indole alkaloids

The Aspidosperma alkaloids are related through a largely conserved pentacyclic core that is most clearly visible in the sparsely functionalized namesake of the family, aspidospermidine (1).8 Two structural outliers are the nine-membered ring-containing quebrachamine (2),9 and the aminal-containing goniomitine (3).10 Vincadifformine (4)11 and tabersonine (5)12 contain additional unsaturation within the pentacyclic core. Common oxygenation patterns include primary alcohols (e.g., 6),13 N,O-ketals (e.g., 7 and 8),14,15 and oxygenation about the benzene fragment (e.g., 9–12).16–19 Leuconolam (13)20 is related to Aspidosperma alkaloids through indoline oxidative cleavage, and can undergo subsequent ring closure to furnish various aminal-containing structures (e.g., 14–16)21–23 or additional rearrangement and fragmentation to give mersicarpine (17).24 The pyrrole ring in rhazinilam (18)25 is expected to originate from a biochemical oxidation pathway similar to that of leuconolam (13).26 Kopsia alkaloids (e.g., 19–25)27–33 typically contain additional carbon-based rings, often resulting in highly caged structures.

2.1 Biosynthetic hypothesis

The biosynthesis of all monoterpene indole alkaloids is proposed to begin with an enzymatic Pictet–Spengler reaction34 between secologanin (26) and tryptamine (27) to yield strictosidine (28, Scheme 1).35 Subsequent deglycosylation and iminium condensation affords 4,21-dihydrogeissoschizine (29), which undergoes a series of skeletal rearrangements to arrive at dehydrosecodine (30). At this stage, it is envisioned that either a Diels–Alder cycloaddition, or a stepwise enamine-Michael addition/Friedel–Crafts reaction/tautomerization cascade delivers tabersonine (5), thereby enabling general entry into alkaloids of the Aspidosperma type.36 This biosynthetic hypothesis, in particular the intermediacy of achiral 30, accounts for these alkaloids being observed in both enantiomeric series.

Scheme 1.

Scheme 1

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Proposed biosynthetic pathway to Aspidosperma alkaloids

2.2 Noteworthy biological activity

In addition to their stereochemically rich polycyclic scaffolds, several Aspidosperma and Kopsia alkaloids have demonstrated promising biological activity. Vincadifformine (4) displays cytotoxicity in KB/VJ300 vincristine-resistant human oral epidermoid carcinoma cells.37 Tabersonine (5) shows cytotoxicity toward HL-60 myeloid leukemia cells at low micromolar concentration.38 Rhazinilam (18) exhibits sub-micromolar toxicities in A549 human lung adenocarcinoma and HT29 human colon adenocarcinoma cell lines,39a but is perhaps best known for its remarkable in vitro inhibition of both microtubule assembly and disassembly.39b,c

3 Enantioselective Pd-catalyzed allylic alkylations in Aspidosperma and Kopsia alkaloid syntheses

Among the many synthetic challenges facing chemists in pursuit of Aspidosperma and Kopsia alkaloids is the ubiquitous all-carbon quaternary stereocenter at C20. Since the mid 2000’s, enantioselective Pd-catalyzed allylic alkylation reactions of non-stabilized enolates have enabled access to challenging stereogenic quaternary centers.4,6 Indeed, our lab has employed this methodology to construct an array of chiral building blocks possessing an all-carbon quaternary center. In one such instance, β-amidoester 31 was exposed to a solution of Pd2(pmdba)3 (5 mol %) and (S)-(CF3)3-t-BuPHOX (L1, 12.5 mol %) in toluene to furnish α-quaternary lactam 32 in 97% yield and with 99% ee (Scheme 2). Lactam 32 can be further elaborated to chiral building blocks 33–3540 that previously required lengthy synthetic sequences to achieve high levels of enantiopurity. Consequently, the swift preparation of heterocycles 33–35 completed enantioselective formal syntheses of (–)-quebrachamine (2),41 (–)-vincadifformine (4),42 and (+)-rhazinilam (18),43 respectively. While the accessibility of small chiral heterocyclic building blocks (e.g., 33–35) is valuable in its own right, lengthy synthetic sequences would still be required to reach complex alkaloid targets. Thus, the development of new substrate classes, ideally those possessing an indole fragment, was needed in order to enable expedient total syntheses of Aspidosperma and Kopsia alkaloids.

Scheme 2.

Scheme 2

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A) Pd-catalyzed allylic alkylation; B) Chiral building blocks in the formal synthesis of Aspidosperma alkaloids

Recently, several groups have used Pd-catalyzed allylic alkylations to set the C20 all-carbon quaternary center in de novo enantioselective total syntheses. The stereochemical information at C20 can then be used to set the remaining stereocenters with high levels of selectivity. Most of the studies described herein utilize conditions developed by the Stoltz group for decarboxylative allylic alkylations, namely the combination of a palladium precatalyst (e.g., Pd2(dba)3) and a phosphinooxazoline (PHOX) ligand.4a–d While L1 is prepared from the readily available (S)-tert-leucine, the invention of a pseudoenantiomeric PHOX ligand was required to obviate the use of (R)-tert-leucine, which is prohibitively expensive.44

3.1 Lupton’s formal synthesis of (+)-Kopsihainanine A

In 2013, the Lupton group reported the enantioselective decarboxylative Pd-catalyzed allylic alkylation of Boc-protected indolone and carbazolone substrates (e.g., 36, Scheme 3).45 The authors found that the combination of Pd2(dba)3 (2.5 mol %) and (S)-t-BuPHOX (L2, 5 mol %) in toluene at 50 °C delivered the α-quaternary products (e.g., 37) in 69–98% yield and 80–94% ee. Broad functional group tolerance was observed at the α-position, and one example (37f) accommodated an electronically differentiated indole fragment.

Scheme 3.

Scheme 3

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Selected examples from Lupton's enantioselective Pd-catalyzed allylic alkylations of indolone and carbazolone

Noting the clear resemblance between their α-quaternary carbazolone products (e.g., 37d–f) and monoterpene indole alkaloids, the authors carried out a rapid enantioselective formal synthesis of (+)-kopsihainanine A (20). Allylic alkylation product 37d was treated with formic acid to effect nitrile hydration with concomitant Boc removal, and subsequent reprotection delivered N-benzyl carbazolone 38. This intermediate was previously carried through six additional steps by She and co-workers in their synthesis of (±)-kopsihainanine A (20).46

3.2 Ma’s total synthesis of methyl N-Decarbomethoxychanofruticosinate

Ma and co-workers devised an elegant total synthesis of Kopsia alkaloid (+)-methyl N-decarbomethoxychanofruticosinate (24) featuring an enantioselective Pd-catalyzed allylic alkylation of a carbazolone substrate and a late-stage intramolecular oxidative coupling reaction.47 Beginning from commercially available carbazolone 39, a four-step sequence including a Pd-catalyzed allylic alkylation using (R)-t-BuPHOX (ent-L2) delivered ent-67e (Scheme 5). Oxidative cleavage of the allyl fragment, reduction of the resulting aldehyde, and alcohol protection using TBSCl gave silyl ether 40. Reductive cyclization using nickel boride, followed by imine hydrogenation, yield tetracycle 41 with the requisite trans-fused octahydroquinoline subunit. Acylation of the secondary amine, followed by alcohol deprotection and subsequent oxidation, gave aldehyde 42. An intramolecular Reformatsky-type reaction was mediated by SmI2, and the resulting β-hydroxyamide was subjected to amide reduction and then alcohol oxidation to arrive at ketone 43. Ketone 43 was deprotonated using LHMDS, and the resulting enolate underwent smooth iodine-promoted oxidative coupling to give imine 44. Nucleophilic addition of cyanide, followed by hydrolysis and subsequent esterification provided the highly caged target, (+)-methyl N-decarbomethoxychanofruticosinate (24), in 19 steps and 5% overall yield from commercially available 39. The Ma group brilliantly combined the stereochemical control afforded by an early enantioselective Pd-catalyzed allylic alkylation, with the bond-forming capabilities of their lab’s oxidative coupling chemistry in order to access their highly sterically congested target.

Scheme 5.

Scheme 5

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Ma's total synthesis of (+)-methyl N-Decarbomethoxyfruticosinate (24)

3.3 Mukai’s total synthesis of (+)-Kopsihainanine A

Mukai and co-workers exploited the exceptional enantioselectivities achieved through the asymmetric allylic alkylation of piperidin-2-ones4b in a highly enantioselective total synthesis of (+)-kopsihainanine A (20).48 The addition of 1,3-dicarbonyl 45, available in five linear steps from indole, to a solution of Pd2(dba)3 (5 mol %) and (S)-(CF3)3-t-BuPHOX (L1, 12.5 mol %) in TBME at 40 °C furnished α-quaternary amide 46 in 82% yield and 98% ee on a one-mmol scale (Scheme 6). Following global deprotection, the key Bischler–Napieralski cyclization was performed to deliver tetracycle 48 in excellent yield as a single diastereomer bearing the desired trans-fused octahydroquinoline subunit. A further seven steps were required to advance 48 to (+)-kopsihainanine A (20). Despite multiple protecting group and redox manipulations, Mukai and co-workers completed a total synthesis of (+)-kopsihainanine A (20) in 15 steps and 3% overall yield from indole. The key primary advantages of their route were the exceptional enantioselectivity (typical of piperidin-2-one substrates), and the highly diastereoselective Bischler–Napieralski cyclization to access the trans-fused core of the natural product.

Scheme 6.

Scheme 6

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Mukai's total synthesis of (+)-Kopsihainanine A (20)

3.4 Qin’s total syntheses of multiple Kopsia alkaloids

A brilliant unified approach to multiple highly caged Kopsia alkaloids was reported by Qin and co-workers in 2017.49 Racemic carbazolone 51 underwent enantioconvergent Pd-catalyzed allylic alkylation using (S)-t-BuPHOX (L2, 13 mol %) and Pd2(dba)3 (5 mol %) in refluxing toluene to deliver the α-quaternary product (52) in 91% and 94% ee (Scheme 7). Remarkably, the crucial heteroaryl bromide motif withstood this Pd(0)-catalyzed transformation, despite somewhat forcing conditions.

Scheme 7.

Scheme 7

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Qin’s highly stereoselective core construction enabled by early enantioselective Pd-catalyzed allylic alkylation

Examples of Pd-catalyzed allylic alkylation substrates bearing a (hetero)aryl bromide moiety are surprisingly uncommon.50 Even more rare are instances where this motif is strategically leveraged in downstream chemistry, which is peculiar considering the countless modes of reactivity available to (hetero)aryl halides. Mizoroki–Heck,50b Suzuki,50c and Negishi (vide infra) cross-couplings have been achieved using Pd-catalyzed allylic alkylation products, which have enabled convergent syntheses of complex molecules. Qin and co-workers exhibited immense creativity in their use of the bromoisoxazole fragment in 52, and highlighted the synthetic power available through pairing these highly selective Pd-catalyzed alkylations with other types of reactivity.

To continue their syntheses, the terminal alkene was transformed to a primary azide (54), which could undergo an aza-Wittig reaction and ensuing hydride reduction with high diastereoselectivity. Protection of the piperidine nitrogen with TrocCl then delivered tetracycle 55. The authors masterfully unveiled a β-ketonitrile moiety through reductive N–O cleavage and concomitant bromide elimination. Diazo transfer proceeded smoothly to give α-diazoketone 56, which enabled the investigation of their key intramolecular cyclopropanation.

After screening various rhodium and copper catalysts, Qin and co-workers found that the use of 20 mol % Cu(hfacac)2 in chlorobenzene at 120 °C resulted in a 52% yield of the desired cyclopropanated indoline 57. Troc removal and cyclopropane opening was accomplished with zinc dust, and an intramolecular Mannich reaction formed the pyrrolidine subunit to give hexacycle 58. A three-step sequence of nitrogen deprotection, cyanation, and samarium-mediated acyloin condensation gave highly caged α-hydroxyketone 60. Nitrile hydration and esterification with 2-mercaptopyridine N-oxide delivered 61, which underwent radical decarboxylation to give a mixture of 62 and 63 (Scheme 7).51 Critically, intermediates 62 and 63 bear the carbocyclic cores of (–)-isokopsine (22) and (–)-kopsine (21), respectively. Heptacycle 62 was globally carbomethoxylated, and chemoselective carbonate cleavage occurred to give (–)-isokopsine (22).

Oxidative C–C scission was achieved using Pb(OAc)4 to furnish (+)-methyl chanofruticosinate (23, Scheme 8A, Left). Furthermore, (–)-isokopsine (22) could be swiftly converted to (–)-fruticosine (25) using a three-step hydride reduction/diol-cleavage/intramolecular aldol condensation sequence (Scheme 8A, Right).

Scheme 8.

Scheme 8

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Qin's divergent syntheses of multiple Kopsia alkaloids

Lastly, the authors advanced heptacycle 63 to complete the syntheses of (–)-kopsanone (19) and (–)-kopsine (21). The tertiary alcohol in 63 was converted to a xanthate ester, and ensuing radical deoxygenation gave (–)-kopsanone (19, Scheme 8B, Left). Conversely, treatment of 63 with triphosgene followed by methanolysis gave (–)-kopsine (21, Scheme 8B, Right).52

In summary, Qin and co-workers have designed and executed an impressive unified strategy toward several structurally daunting targets. Instrumental to their successes was the incorporation of a robust bromoisoxazole fragment, which remained intact during their enantioselective Pd-catalyzed allylic alkylation and underwent subsequent fragmentation to furnish an important β-ketonitrile motif. Following diazo transfer to the β-ketonitrile, a copper-catalyzed intramolecular cyclopropanation forged the indoline quaternary center. Several late-stage skeletal rearrangements were then conducted to complete syntheses of multiple highly caged Kopsia alkaloids.

3.5 Qiu’s total synthesis of (–)-Aspidophytine

Qiu and co-workers employed a Pd-catalyzed allylic alkylation as a key step in their total synthesis of (–)-aspidophytine (8).53 The authors found that upon treatment to a solution of [Pd2(dba)3]•CHCl3 and (S,S)-ANDEN-Phenyl Trost ligand (L3), β-ketoester 64 is converted to known vinylogous thioester 65 in 70% yield and with 85% ee (Scheme 9).54 Hydrolysis gave cyclohexane-1,3-dione 66, which was enriched to 97% ee through recrystallization. The dimethoxyindole fragment in (–)-aspidophytine (8) was assembled by Pd-catalyzed oxidative cyclization through the intermediacy of a vinylogous amide. Subsequent N-tosylation under phase-transfer conditions gave α-quaternary carbazolone 69. The allyl fragment in 69 was then converted to a primary azide to arrive at carbazolone 70.

Scheme 9.

Scheme 9

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Qiu's total synthesis of (–)-Aspidophytine (8)

The authors effectively used the single stereocenter in 70 to build the three remaining rings and complete their synthesis of (–)-aspidophytine (8) in a stereoselective fashion. A four-step sequence effected ketone and azide reduction, along with amine protection and cyclization to deliver cis-fused tetracycle 71. Deprotection of the Ts and Cbz groups, followed by regioselective alkylation using 2-bromoethanol furnished aminoalcohol 72. Pyrrolidine annulation and selenoxide elimination gave α,β-unsaturated imine 73, which underwent hydride reduction and reductive amination in the same pot to yield penultimate N-methyl indoline 74. This intermediate was converted to (–)-aspidophytine (8) by adapting a two-step protocol reported by Corey.55a While Qiu and co-workers successfully assembled one of the most functionally elaborate members of the Aspidosperma family, their synthesis required 21 steps and proceeded in 0.6% overall yield from known α-quaternary vinylogous thioester 65. Lessons learned in other studies described in this review would likely help cut down the step count of this route. Namely, carbazolone 68 might be available in more rapid fashion and in higher enantiomeric excess starting from carbazolone 36f, tuning the protecting group on nitrogen, and using the electron-deficient PHOX ligand L1 (cf. Scheme 3). Alternatively, the use of a dihydropyrido[1,2-a]indolone (DHPI) substrate could potentially simplify the beginning of the synthesis, as well as eliminate intermediate nitrogen protecting group manipulations (vide infra). Such improvements would elevate the efficiency of this synthesis to that of previous, more convergent reports.55

3.6 Shao’s total syntheses enabled by enantioenriched α-quaternary carbazolones

In 2013, the Shao group began a fruitful research program in the application of Pd-catalyzed allylic alkylation reactions of carbazolone substrates in the context of monoterpene alkaloid total synthesis. Their initial disclosure was published back-to-back with that of Lupton,45 and detailed the asymmetric allylic alkylation of N-benzyl carbazolone substrates (Scheme 10A).56,57 Using similar reaction conditions, a series of enantioenriched, functionalized α-quaternary carbazolone products (76) were obtained in 70–93% yield and 84–97% ee.

Scheme 10.

Scheme 10

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Shao's initial report on the Pd-catalyzed allylic alkylation of carbazolone substrates

Shao’s first application of this chemistry was in the total syntheses of (–)-aspidospermidine (1) and (+)-kopsihainanine A (20). Beginning from α-quaternary carbazolone 76a, hydration of the pendant nitrile followed by ketone reduction and acid-promoted cyclization gave lactam 77 (Scheme 10B). A three-step dehomologation protocol was used to convert 77 into tetracycle 79, which bears the desired C20 ethyl substituent. Lactam reduction and debenzylation under dissolving metal conditions furnished 80, which was subjected to a three-step sequence adapted from Heathcock and co-workers to arrive at (–)-aspidospermidine (1) in 14 steps and 10% overall yield from commercially available carbazolone 39.58 The authors found that lactam 77 could also serve as an intermediate toward (+)-kopsihainanine A (20, Scheme 10C). Hydroboration/oxidation and base-promoted cyclization of the corresponding mesylate gave pentacycle 81, which underwent α-hydroxylation and debenzylation to complete the first catalytic enantioselective synthesis of (+)-kopsihainanine (20) in short order (10 total steps), albeit in only 3.5% overall yield.

Later that same year, Shao and co-workers published an enantioselective total synthesis of (–)-limaspermidine (6, Scheme 11).59 Utilizing the same α-quaternary carbazolone (76a), a six-step sequence gave rise to silyl ether 82 in 65% overall yield. Site-selective acylation delivered α-chloroamide 83. Finkelstein displacement and subsequent silver-mediated halide abstraction occurred with concomitant annulation to furnish pentacycle 84. Global hydride reduction and desilylation completed their total synthesis of (–)-limaspermidine (6) in 14 steps and 8.9% yield from 39. The Banwell group showed that (–)-Acetylaspidoalbidine (7) can be made in two further steps.60

Scheme 11.

Scheme 11

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Shao's total synthesis of (–)Limaspermidine (6)

In a subsequent report, the Shao group expanded their investigations to access alkaloids bearing oxygenation on the arene fragment (Scheme 12).61 Allylic alkylation precursor 85 was synthesized in five steps and 41% overall yield from commercially available materials. Using their previously optimized conditions, α-quaternary carbazolone 86 was obtained in 90% yield and 91% ee. The authors then employed a familiar seven-step sequence to arrive at pentacyclic imine 87, which served as a common intermediate in their divergent total syntheses of (+)-aspidospermine (9), (–)-N-acetylcylindrocarpinol (10), (+)-cylindrocarpidine (11), and (+)-10-oxocylindrocarpidine (12). These arene-oxidized alkaloids (9–12) were synthesized in 17–18 steps and 2.2–4.6% overall yield. Through their many completed syntheses, the Shao group plainly demonstrated enantioselective Pd-catalyzed allylic alkylation as a key stereodefining feature toward Aspidosperma alkaloids, however the consistent drawback to their synthetic plans was the requirement for multi-step derivatizations of the allyl fragment to form the various C20 exocyclic substituents.

Scheme 12.

Scheme 12

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Shao's total syntheses of C12-methoxy alkaloids 9–12

3.7 Stoltz’s regiocontrolled indole-iminium cyclizations: total synthesis of (–)-Goniomitine

In 2016 the Stoltz lab disclosed the expansion of asymmetric Pd-catalyzed allylic alkylation to a novel substrate class, namely dihydropyrido[1,2-a]indolones (DHPIs, Scheme 13A).62 The α-quaternary DHPI allylic alkylation products can be isolated in consistently high yields and enantioselectivities across a variety of substituents at R1 and R2. Further, it was found that hydroamination and reduction of 89 could give rise to a critical indole intermediate bearing a C2-tethered iminium moiety (i.e., 90, Scheme 13B). By controlling the substitution at the 3-position of the indole moiety, chemoselective cyclization events occur to access isomeric tetracycles 91 and 92 (Scheme 13B). In this way, the allyl fragment in 89 is readily transformed to the propylamine fragment in monoterpene indole alkaloids 1–3 (Scheme 13C). Consequently, the C20 ethyl substituent in 1–3 could be introduced directly at the α-position (i.e., 89, R2 = Et), thereby circumventing the dehomologation required by the carbazolone substrate class (cf. Scheme 10B).

Scheme 13.

Scheme 13

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Stoltz's Pd-catalyzed allylic alkylation of DHPIs combined with chemodivergent indole-iminium cyclizations

Stoltz’s total synthesis of (–)-goniomitine (3) began from N-acylindole 93, which is available in multi-gram quantities in four steps and 47% overall yield from indole. Regioselective bromination, C-acylation, and C-alkylation proceeded in 71% yield over the three steps to afford β-amidoester 88a (Scheme 14). While C3-alkyl substrates (i.e., 88, R1 = alkyl) performed poorly in the allylic alkylation chemistry, it was found that the C3-brominated substrate 88a reacted chemoselectively when employing Pd2(pmdba)3 (5 mol %) and L1 (12.5 mol %) in TMBE at 60 °C to give α-quaternary DHPI 89a in 83% yield and with 96% ee. Ensuing Negishi arylation between 89a and organozinc chloride 94 generated cross-coupled product 95 in 98% yield, and formal anti-Markovnikov hydroamination using conditions reported by Hartwig63 yielded primary amine 96. Reduction with LiAlH4, and subsequent acetic acid quench effected the desired aminal-forming indole-iminium cyclization to complete the first catalytic enantioselective synthesis of (–)-goniomitine (3) in 11 steps and 8% overall yield from indole. To date, no other catalytic enantioselective syntheses have been reported.

Scheme 14.

Scheme 14

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Stoltz's catalytic enantioselective synthesis of (–)-Goniomitine (3)

Using the same Pd-catalyzed conditions, C3-unsubstituted α-quaternary DHPI 89b was synthesized in 71% and with 94% ee (Scheme 15). Bach and co-workers advanced 89b through six steps, including a chemoselective indole-iminium cyclization (e.g., 90 → 92, Scheme 13B), to (±)-aspidospermidine (1).64 Furthermore, hydroamination and translactamization of 89b cleanly delivered δ-lactam 98 in 66% yield over the two steps, thus intercepting Pagenkopf’s synthesis of (±)-quebrachamine (2).65 The rapid total synthesis of (–)-goniomitine (3), along with formal syntheses of (+)-aspidospermidine (1) and (–)-quebrachamine (2), demonstrated the ability of the DHPI substrate class to provide access to skeletally diverse indole alkaloids by controlling substitution at the 3-position of the indole moiety.

Scheme 15.

Scheme 15

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Asymmetric formal syntheses of (+)-Aspidospermidine (1) and (–)-Quebrachamine (2)

3.8 Stoltz’s stereocontrolled indole-iminium cyclizations: total synthesis of (+)-Limaspermidine and formal synthesis of (+)-Kopsihainanine A

Stoltz and co-workers further highlighted the synthetic versatility of the DHPI substrate class by combining enantioselective Pd-catalyzed allylic alkylations with stereo-selective indole-iminium cyclizations (Scheme 16).66 Following hydroamination of the allyl fragment in 89, a Pictet–Spengler reaction effects stereoselective C–C bond formation to furnish cis-fused octahydroquinoline 99, which is present in Aspidosperma alkaloids such as (+)-limaspermidine (6). Conversely, a Bischler–Napieralski cyclization affords access to trans-fused octahydroquinoline 100 via stereodefining C–H bond formation. This orthogonal cylization event enables synthetic entry to the trans-fused alkaloids of the Kopsia family, including (+)-kopsihainanine A (20).

Scheme 16.

Scheme 16

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Stereodivergent cyclization strategies from a common DHPI precursor

Readily available β-amidoester 88c was converted to α-quaternary DHPI 89c in 82% yield and 94% ee (Scheme 17). The authors found that hydroamination, reduction, and acid-promoted Pictet–Spengler cyclization could be achieved in one-pot, and resulted in the stereoselective formation of cis-fused tetracycle 102. Chemoselective piperidine alkylation furnished ethanolamine 103 in 62% yield over two steps. Pyrrolidine annulation and subsequent indolenine reduction gave O-benzyl limaspermidine (105), which was then debenzylated using excess BF3•Et2O to produce (+)-limaspermidine (6) in 12 steps and 14.4% overall yield from indole.

Scheme 17.

Scheme 17

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Stoltz's total synthesis of (+)-limaspermidine (6)

Having selectively constructing the cis-fused azadecalin motif present in Aspidosperma alkaloids (e.g., 6), the authors turned their attention to a stereodivergent process toward the Kopsia alkaloids (e.g., 20), which instead bear a trans-fused azadecalin substructure. Gratifyingly, pendant methyl ester 88d performed well in the Pd-catalyzed allylic alkylation chemistry, delivering α-quaternary DHPI 89d in 90% yield and with 92% ee (Scheme 18). Unfortunately, hydroamination of 89d using Schwartz’s reagent was unfruitful due to reduction of the methyl ester. A four-step sequence involving a Staudinger reduction with concomitant lactam exchange provided δ-lactam 106 in 62% overall yield. A Bischler–Napieralski reaction using conditions developed by Movassaghi and co-workers3b,67 occurred with stereoselective hydride addition to furnish trans-fused tetracycle 107 in 84% yield. The guanidine base TBD facilitated lactam formation to provide strained pentacycle 108, which intercepts Zhu’s synthesis of (±)-kopsihainanine A (20),68 thereby completing an enantioselective formal synthesis of (+)-20 in 14 steps and 12% overall yield from indole.69

Scheme 18.

Scheme 18

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Stoltz's formal synthesis of (+)-Kopsihainanine A (20)

The DHPI substrate class developed by the Stoltz group affords synthetic access to multiple monoterpene indole alkaloids bearing diverse connectivities and three-dimensional topographies. This work differs from the use of carbazolone substrates in several ways: 1) both cis- (Aspidosperma) and trans- (Kopsia) ring fusion geometries can be accessed in a highly predictable fashion, 2) the allyl group of the α-quaternary DHPI Pd-catalyzed allylic alkylation product is transformed into part of the piperidine ring found in these alkaloids, which enables 3) the installation of the C20 exocyclic substituent at an early stage obviates the need for tedious FGIs, and finally 4) the N-acyl moiety acts as a traceless protecting group for the indole nitrogen, further reducing unproductive transformations.

3.9 Zhu’s oxidation/reduction/polycyclization cascades

The Zhu group has combined Pd-catalyzed allylic alkylation with their oxidation/reduction/polycyclization cascades to complete enantioselective syntheses of a structurally unique subset of Aspidosperma alkaloids (e.g., 13–18).70 The known enantioselective Pd-catalyzed allylic alkylation of β-ketoester 109 proceeded smoothly to give α-quaternary ketone 110 in 90% yield and 92% ee (Scheme 19). The authors carried out a five-step sequence to introduce the azide and o-nitrophenyl substituents present in enone 111. Ozonolysis of 111, followed by treatment of the intermediate hydroperoxide with Ac2O and Et3N provided methyl ester 112. Hydrogenation of 112 reveals the nascent aniline and primary amine in 113, which cyclize regioselectively under the reaction conditions to give tricycle 114. The addition of KOH prompted lactamization, and sparging with oxygen provided the presumed hydroperoxide (116), which was reduced by dimethyl sulfide to deliver (–)-mersicarpine (17) in a remarkable 75% one-pot yield from azide 112. Their synthesis of (–)-17, beginning from commercially available diallyl pimelate, was completed in 10 steps and 13.5% overall yield.

Scheme 19.

Scheme 19

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Zhu's total synthesis of (–)-Mersicarpine (17)

Impressively, the authors found that hydrogenation of azide 112 in the presence of acetic anhydride resulted in acetylation of the primary amine to give acetamide 117 (Scheme 20). Aerobic oxidation of the 3-oxindole moiety followed by the addition of KOH produced lactam 118, which underwent acid-promoted cyclization to afford aminal 119. An intramolecular aldol reaction completed a total synthesis of (–)-scholarisine G (16) in 12 steps and 6.6% overall yield from diallyl pimelate. The tertiary benzylic alcohol in (–)-scholarisine G (16) was smoothly dehydrated to give (+)-melodinine E (14), which could be further elaborated to (–)-leuconolam (13) and (–)-leuconoxine (15).71–73

Scheme 20.

Scheme 20

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Zhu's unified approach to alkaloids 13–16

Following their successful elaboration of α-quaternary cyclohexanone 110 toward multiple alkaloids, Zhu and co-workers devised a clever synthesis of (–)-rhazinilam (18) beginning from five-membered β-ketoester 120 (Scheme 20).74 The known synthesis of α-quaternary cyclopentanone 121 proceeded in 87% yield and 86% ee under typical reaction conditions.75 The authors conducted a three-step sequence to access cyclopentenyl triflate 122, which was advanced through an additional three steps to give cyclopentene 123. A familiar ozonolysis with an Ac2O/Et3N workup in the presence of methanol yielded methyl ester 124, which could undergo smooth aza-Wittig cyclization to deliver imine 125. While imine 125 could be isolated, the authors were pleased to find that pyrrole formation could occur in the same pot to afford tetrahydroindolizine 126. A well-precedented reduction/hydrolysis/macrolactamization sequence completed the synthesis of (–)-rhazinilam (18) in 15 steps and 12.2% overall yield from adipic acid.

The Zhu group has elegantly leveraged the high enantioselectivities available in Pd-catalyzed allylic alkylations of cycloalkanones to prepare precursors for their reduction/oxidation/cyclization cascades. Importantly, they have demonstrated the ability to tune participating functional groups for the controlled, divergent construction of multiple Aspidosperma alkaloid scaffolds.

4 Conclusion and outlook

Monoterpene indole alkaloids of the Aspidosperma type have long inspired innovation in chemical synthesis. Over the past five years, several research groups have utilized enantioselective Pd-catalyzed allylic alkylations of prochiral enolates to synthesize a wide variety of structurally unique Aspidosperma and Kopsia alkaloids, further highlighting the widespread utility of these reactions.

Perhaps the most notable drawbacks to this strategy are the decrease in convergence, and inefficient redox manipulation of the allyl fragment (e.g., Sections 3.3 and 3.6). These issues can be mitigated by thoughtful selection of the α-substituent, and elaboration of the allyl group as a propylamine synthon (e.g., 3.4 and 3.8). Critically, in all of the aforementioned synthetic endeavors, multiple stereocenters in the respective targets were constructed with high diastereoselectivity by leveraging the all-carbon quaternary center formed in the Pd-catalyzed allylic alkylation event.

Future studies to reduce the loadings of precious-metal catalysts, develop non-precious-metal-catalyzed allylic alkylation reactions, and to combine this reactivity with emerging synthetic methods will be of paramount importance not only in future work toward these classic alkaloid targets, but more broadly for the continued advancement of chemical synthesis.

Scheme 4.

Scheme 4

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Enantioselective formal synthesis of (+)-Kopsihainanine A (20)

Scheme 21.

Scheme 21

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Zhu's total synthesis of (–)-Rhazinilam (18)

Acknowledgments

The authors wish to thank NIH-NIGMS (R01GM080269), Amgen, the Gordon and Betty Moore Foundation, and Caltech for financial support. B. P. P. thanks the NSF for a predoctoral fellowship (Grant DGE-1144469). Dr. Robert Allen Craig, II, is thanked for editorial assistance.

Footnotes

Conflicts of interest

The authors declare no conflicts of interest.

Notes and references

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