Pd-catalyzed enantioselective access to hydrocarbazolones containing contiguous quaternary and tertiary stereocenters

Abstract

The hydrocarbazole scaffold represents the core structure of numerous monoterpenoid indole alkaloids. The development of catalytic methods that provide efficient access to enantioenriched hydrocarbazole derivatives is central for the synthesis of these bioactive alkaloids. We report here a palladium-catalyzed enantioselective formal 5-endo arylative cyclization of enaminones, facilitating the construction of hexahydrocarbazol-4-ones containing contiguous C4a-quaternary and C9a-tertiary stereocenters with high enantioselectivities (86.5 : 13.5–99 : 1 er) and diastereoselectivities (>20 : 1 dr). Notably, enaminone substrates bearing an α-allyl group undertake an arylation/Cope rearrangement cascade, offering a unique route to C1-substituted tetrahydrocarbazol-4-ones. A stereodivergent approach to all four stereoisomers of the quaternary/tertiary chiral center set is achieved by combining the catalyst with Z/E allyl substituents, yielding excellent enantioselectivity. The N-methyl group of the hydrocarbazolone products is readily removed under oxidation conditions. The utility of the method is demonstrated by the access to a variety of hydrocarbazole derivatives and the efficient syntheses of four Aspidosperma alkaloids/analogs, (+)-N-methyl aspidospermidine, (+)-C20-epi-N-methyl aspidospermidine, (+)-N-methyl fendleridine, and (+)-N-methyl limaspermidine from a hexahydrocarbazol-4-one in 3–5 steps.

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Introduction

Monoterpenoid indole alkaloids are known for their structural complexity and broad spectrum of biological activity.^1,2 Extensive research has been dedicated to their enantioselective total synthesis, particularly focusing on the alkaloids from the Aspidosperma and Kopsia families (Scheme 1a).^3–7 From a retrosynthetic perspective, the tetracyclic hydrocarbazole scaffold serves as a core structure for these complex alkaloids. Traditional synthetic strategies commonly targeted the enantioselective construction of tetrahydrocarbazol-4-ones containing a C3-all-carbon quaternary stereocenter, which corresponds to C20 of the natural products (Scheme 1b, left).⁸ A number of sophisticated total syntheses have been successfully accomplished following this synthetic logic.^9–14 Departing from these conventional strategies, alternative synthetic approaches have been developed that utilize the C4a-all-carbon quaternary stereocenter as a stereochemical linchpin. This strategy leverages hydrocarbazolone or hydrocarbazole intermediates (Scheme 1b, right), offering an efficient alternative and conceptually novel retrosynthetic disconnection pathway.^15–20 As depicted in Scheme 1c, we considered hexahydrocarbazol-4-ones, constituting a tricyclic [6.5.6] framework with sterically congested contiguous stereocenters, to be one of the ideal intermediates for concise synthesis of aspidospermidine derivatives. By taking advantage of the carbonyl and the aminoethyl group on the quaternary center, a condensation cyclization can efficiently construct the E ring, which further allows for a modular installation of various substituents and stereochemical configurations at the C20 quaternary centers. However, asymmetric catalytic assembly of the quaternary center possessed in the C4a-position within the hydrocarbazol-4-one framework presents a significant challenge.^21–26 This challenge increases in difficulty when the C ring incorporates functional groups necessary for streamlined synthetic elaboration. To our knowledge, however, there lacks catalytic methods capable of enantioselective access to hexahydrocarbazol-4-ones,^27–29 even not mentioning its application in total synthesis.

Scheme 1 Background and outline of this research.

Drawing inspiration from Overman's seminal work of 5-exo Heck cyclization reaction of dienyl aryl triflates (Scheme 1d),^30–34 we devised a formal 5-endo arylative cyclization of aryl enaminone 1 to construct hydrocarbazol-4-ones (Scheme 1e).^35–38 Using this stereochemistry-enriched hydrocarbazolone as a platform, four complex alkaloids/analogs, (+)-N-methyl aspidospermidine,³⁹ (+)-C20-epi-N-methyl aspidospermidine, (+)-N-methyl fendleridine,^40,41 and (+)-N-methyl limaspermidine,⁴⁰ were synthesized in just 3–5 isolation steps. Additionally, we developed an arylation/Cope rearrangement cascade reaction using α-allyl-substituted enaminones as the substrates, allowing access to tetrahydrocarbazol-4-ones bearing contiguous quaternary and tertiary stereocenters. Indeed, enantioselective assembly of such stereochemical dyads is especially demanding and represents the cutting edge of asymmetric catalysis.^42–44 A stereodivergent approach to all four stereoisomers was achieved by judicious choice of chiral ligands and Z/E geometry of the allyl substituents.^45–47

Results and discussion

Reaction development

Our studies began with enaminone 1a′ as the model substrate to optimize the reaction conditions (Table 1, see Tables S1–S5† for additional details and the structures of ligands). The initial investigation of chiral ligands, including bisphosphines (L1–L6),⁴⁸ phosphoramidite (L7),⁴⁹ and phosphooxazoline (L8),⁵⁰ was carried out using TMG (1,1,3,3-tetramethylguanidine) as the base (entry 1). Among them, Quinoxp* (S,S)-L4 showed the best enantioselectivity,⁵¹ albeit the yield was low (entry 2). Further optimization experiments highlighted the critical roles of the base in influencing the reactivity, and the use of Cs₂CO₃ was able to improve both the yield and enantioselectivity (entries 3–5, and Table S2†). Investigation of palladium precursors led us to find a significant enhancement in enantioselectivity using [Pd(allyl)Cl]₂ (entries 5–8 and Table S4†). During these experiments with 1a′ as the substrate, reductive deiodination was identified as the major side pathway, which limited the overall yield of the desired arylation product. This side reaction likely occurred because the rapid oxidative addition of palladium to the aryl iodide was not well-aligned with the subsequently slower cyclization step.⁵² To address this issue, we replaced the aryl iodide with an aryl bromide substrate 1a, leading to an improved yield of 70% (entry 9). Ultimately, after further optimization of the enamine reduction step (Table S5†), we found that the addition of AcOH in MeCN could dramatically facilitate the reduction efficiency, affording product 2a in 90% isolated yield with 98.5 : 1.5 er (standard conditions, entry 10).

Table 1 Selected results of condition optimizations

Entry^a	Ligand	[Pd]	Base	Sub	Reduction conditions	2a (%)	er^c of 2a
a Conducted on 0.1 mmol scale with 2 equiv. of base for 24 h. b ^1H NMR yield (isolated yield). c Determined by HPLC (Chiralcel OJ-H). d At 60 °C for 36 h.
1	L1–L3, L5–L8	Pd(OAc)2	TMG	1a′	NaBH3CN, MeOH	<56	<56 : 44
2	L4	Pd(OAc)2	TMG	1a′	NaBH3CN, MeOH	8	79.5 : 20.5
3	L4	Pd(OAc)2	TBD	1a′	NaBH3CN, MeOH	12	79.5 : 20.5
4	L4	Pd(OAc)2	Na2CO3	1a′	NaBH3CN, MeOH	12	85 : 15
5	L4	Pd(OAc)2	Cs2CO3	1a′	NaBH3CN, MeOH	31	87 : 13
6	L4	Pd(MeCN)2Cl2	Cs2CO3	1a′	NaBH3CN, MeOH	23	92 : 8
7	L4	Pd2(dba)3	Cs2CO3	1a′	NaBH3CN, MeOH	Trace	—
8	L4	[Pd(allyl)Cl]2	Cs2CO3	1a′	NaBH3CN, MeOH	33	97.5 : 2.5
9^d	L4	[Pd(allyl)Cl]2	Cs2CO3	1a	NaBH3CN, MeOH	70	98.5 : 1.5
10^d	L4	[Pd(allyl)Cl] 2	Cs 2 CO 3	1a	NaBH 3 CN, AcOH, MeCN	90	98.5 : 1.5

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Scope of the Pd-catalyzed asymmetric arylative cyclization to hexahydrocarbazol-4-ones

The substrate scope of this reaction was then explored (Scheme 2). Various substitutions on the aryl moiety of the substrates, including methyl (2b, 2e), methoxy (2c, 2f), fluoro (2d, 2g), chloro (2h), and trifluoromethyl (2i), were successfully tolerated to deliver the products in 67–90% yields with 97.5 : 2.5–99 : 1 er. A substrate with a methyl substituent ortho to the nitrogen also proved feasible, affording the corresponding product 2k in 75% yield with 99 : 1 er. Notably, the 3-methyl substituted substrate (1j) produced 2j in a lower yield and reduced enantioselectivity due to the steric hindrance.^35,38 The N-benzyl substrate 1l was also compatible, providing 2l in 82% yield with 96 : 4 er. Substituents at the α-position of the enaminone moiety were well-tolerated, furnishing ethyl, benzyl, and aminoethyl-substituted tetrahydrocarbazolones 2m–o in good yields with high enantioselectivities.

Scheme 2 Substrate scope of arylative cyclization/reduction. ^aWith 5 mol% of [Pd(allyl)Cl]₂ and 12 mol% of (S,S)-L4.

Particularly noteworthy is the product 2o, a key precursor in the total synthesis of Aspidosperma alkaloids as shown in Scheme 6. Additionally, an aryl substituted enaminone was feasible substrate, producing 2p in 76% yield with 97 : 3 er. A gem-dimethyl-substituted enaminone was also examined, affording 2q in 71% yield with 97 : 3 er.

Mechanistic consideration of the catalytic cycle

During the screening of reaction conditions, we found that no product was observed when Pd₂(dba)₃ was employed (Table 1, entry 7), which suggests that a complex directly formed from Pd(0) and L4 is not likely to be the active catalyst. Previous report by Belyk disclosed that Pd(OAc)₂ and QuinoxP* L4 in the presence of base could generate mono oxidized bisphosphine coordinated palladium(0) complex.⁵³ Inspired by this, we applied L9 in combination with Pd₂(dba)₃ as the precatalyst, the reaction showed similar reactivity and enantioselectivity (Scheme 3, top). Based on this result and the Belyk's work, it is reasonable to propose that in our system, the active catalytic Pd(0) species Pd-I is generated via the reduction of [Pd(allyl)Cl]₂ and (S,S)-L4. Next, a plausible catalytic cycle of the Pd-catalyzed arylative cyclization was provided (Scheme 3, bottom).³⁵ Oxidative addition of substrate 1a with Pd-I forms aryl palladium(II) species Pd-II, which subsequently generates iminium intermediate Pd-III. The deprotonation of iminium delivers enamine Pd-IV and followed by reductive elimination, intermediate int-I is produced while regenerating the catalytically active species Pd-I.

Scheme 3 Plausible catalytic cycle.

Scope of the Pd-catalyzed asymmetric arylative cyclization/Cope rearrangement cascade to tetrahydrocarbazol-4-ones

During the exploration of the reaction scope, a notable observation was an arylation/Cope rearrangement cascade when substrates bearing an α-allyl group were employed (Scheme 4). This reaction facilitated the formation of tetrahydrocarbazol-4-ones featuring a C1-tertiary chiral center adjacent to a bulky quaternary carbon substituent. The robustness of this method was demonstrated by the good yields and excellent enantioselectivities achieved across a range of substrates (3a–g). Remarkably, a new set of contiguous quaternary and tertiary stereocenters was assembled with perfect diastereoselectivities and enantioselectivities using tetrasubstituted allyl groups attaching two different substituents at the terminal carbon of the olefin (Scheme 4b). For instance, α-phenylbut-2-enyl enaminones (1y and 1z) led to the formation of products 3h and 3i in good yields with excellent er and >20 : 1 dr. By programming the catalyst and geometry of the allyl group, we were able to assemble all the four stereoisomers of the products (Scheme 4c). Specifically, the combination of (S,S)-L4 with (Z)-allyl substituted enaminone, (Z)-1aa, delivered (S,R)-3j in 73% yield with 99.5 : 0.5 er and >20 : 1 dr; switching to (E)-allyl substituted enaminone, (E)-1aa, produced (S,S)-3j albeit in lower yield with reduced diastereoselectivity. The (R,S)-3j and (R,R)-3j were also obtained by the combination of (R,R)-L4 with (Z)-1aa and (E)-1aa, respectively. Previous mechanistic studies suggest that the stereospecific Cope rearrangement proceeds through a rigid chair-like transition state, which allows for precise control of diastereoselectivity.^54–56 In our system, the aromatization of TS-I drives the rearrangement, enabling it to occur under relatively mild conditions compared to previously reported examples.^57–59 The mechanistic rationale of the stereospecific rearrangement based on DFT calculations was included in the ESI (Scheme S1).† The diminished yields and diastereoselectivities observed for (S,S)-3j and (R,R)-3j can be attributed to their sterically hindered and energetically demanding transition states, which lead to allyl fragmentation as a side reaction in both cases.

Scheme 4 Substrate scope of α-arylation/Cope rearrangement cascade.

Synthetic applications

The synthetic applications of the reaction were showcased (Scheme 5). A scale-up reaction produced 2a (0.78 g) in 89% yield with 97.5 : 2.5 er. Treatment of 2a with a Wittig reagent resulted in exocyclic olefin 4a in 87% yield. Under Bamford–Stevens reaction conditions,⁶⁰ tetrahydrocarbazole 4b was obtained smoothly. Moreover, both mono- and bis-bromination of 2a were successfully achieved, yielding 4c and 4d, respectively, by carefully controlling NBS loading and reaction temperature. The regioselectivity of this electrophilic substitution of 2a in accordance with that observed in the bromination of Aspidosperma alkaloids.⁶¹ Subjecting 2a to Fischer indole synthesis conditions furnished ring-fused indole 4e in 81% yield. The arched shape of 2a, with an 84.3° dihedral angle between the indoline and cyclohexanone planes caused by the contiguous stereocenters, provides an excellent stereocontrol for streamline manipulating toward the synthesis of complex molecules and natural products. Reduction of the carbonyl group with NaBH₄ delivered alcohol 4f in quantitative yield with >20 : 1 dr. Diastereoselective α-alkylation of the carbonyl was achieved in 86% yield (4g). Notably, the α-stereocenter of carbonyl 4g can be inverted through a deprotonation/protonation process at −78 °C, resulting in its diastereoisomer 4h in 89% yield.

Scheme 5 Scale-up synthesis and product derivatization.

Synthesis of Aspidosperma alkaloids and their analogs

The potential application of this method was further demonstrated by the access to complex alkaloids and their derivatives using ent-2o (99 : 1 er) as a common precursor (Scheme 6). Deprotection of the phthalimide group using hydrazine resulted in immediate condensation to form tetracyclic imine 4i in 89% yield. By modulating the sequence of alkylation, both stereochemical configurations at the C20 quaternary center were accessible. For example, treatment of 4i with 3.3 equivalents of ^tBuLi, followed by sequential addition of 10 and iodoethane, provided an intermediate 5a with the same stereochemistry at C20 as the corresponding natural products. Similarly, both intermediates 5b and 5c were obtained using the appropriate alkylating reagents in 62% and 63% yields, respectively. The exceptionally high diastereoselectivity of the alkylation of 4i might be associated with the torsional effects of planar conformational alignments of the 5–6 fused cyclohexenamine.⁶² Subsequent deprotection of the TBS group of 5a, followed by converting the resulting alcohol into a mesylate, led to spontaneous nucleophilic cyclization to from iminium ion 6a.⁶³ Without further purification, reduction of the iminium with NaBH₃CN afforded the natural product (+)-N-methyl aspidospermidine 7a in 5 total steps from ent-2o representing the shortest enantioselective total synthetic route reported to date.^64–66 Starting from 5b, reduction of the imine followed by in situ cyclization yielded (+)-C20-epi-N-methyl aspidospermidine 7b in 73% yield in 3 steps from ent-2o. Our attention was then directed to the synthesis of fendleridine derivatives using 5c as the synthetic precursor. Similar to our previous approaches, removal of the TBS group, followed by mesylation, intramolecular annulation, and C19-hemiaminal formation, resulted in the production of (+)-N-methyl fendleridine 8 in 88% yield.^67–69 Further reduction of 8 under acidic conditions provided (+)-N-methyl limaspermidine 9 in 82% yield from 5c.^15,70,71 It should be emphasized that the N-methyl group of the hexahydrocarbazolone can be removed under the conditions developed by Fukuyama⁷² to afford 4j in 46% yield, providing opportunities for the synthesis of other alkaloids of the Aspidosperma family.⁷³

Scheme 6 Applications in the synthesis of Aspidosperma alkaloids/analogs.

Conclusions

In conclusion, we have developed a palladium-catalyzed enantioselective formal 5-endo cyclization of enaminones to access hexahydrocarbazol-4-ones containing contiguous quaternary and tertiary stereocenters. Additionally, a catalytic arylation/Cope rearrangement cascade strategy has been established for α-allyl substituted enaminone substrates, achieving the stereodivergent assembly of adjacent quaternary and tertiary stereocenters. The method has proven applicable to the concise synthesis of four Aspidosperma alkaloids/analogs in just 3–5 steps from the hexahydrocarbazol-4-one ent-2o, highlighting the ability to control stereochemistry and efficiently build molecular complexity from a common intermediate accessed by our method. Further application of this reaction in the synthesis of other indole alkaloids is currently undergoing.

Data availability

All the data supporting this article have been included in the ESI.†

Author contributions

WBL conceptualized the project. HS and CLY conducted the experiments, analyzed the data, and wrote the manuscript. ZD carried out the initial optimization. PFS and YQZ performed experiments related to substrate synthesis and DFT studies. WBL and YCX revised the manuscript with input from all authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support from the NSFC (21772148, 22222111, 22371215), the National Key R&D Program of China (2022YFA1502902), the Fundamental Research Funds for the Central Universities (2042024kf0026), and Large-scale Instrument and Equipment Sharing Foundation of Wuhan University, and Dr Ran Zhang from the Core Facility of Wuhan University for the assistance of X-ray crystallographic analyses, and the Core Research Facilities of CCMS (WHU) for access to analytic equipment.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2363779 and 2407621. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc08215j

‡ H. Sun and C.-L. Yu contributed equally to this work.

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