Catalytic Enantioselective Synthesis of 3,4-Polyfused Oxindoles with

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Catalytic Enantioselective Synthesis of 3,4-Polyfused Oxindoles with Quaternary All-Carbon Stereocenters: A Rh-Catalyzed C−C Activation Approach Bo Qiu,† Xiao-Tong Li,† Jian-Yu Zhang,† Jun-Ling Zhan,† Shuang-Ping Huang,‡ and Tao Xu*,†,§

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†

Key Laboratory of Marine Drugs, Ministry of Education; School of Medicine and Pharmacy, Ocean University of China, and Laboratory for Marine Drugs and Bioproducts of Qingdao National Laboratory for Marine Science and Technology, Qingdao 266003, China ‡ Department of Chemistry & Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China § Open Studio for Druggability Research of Marine Natural Products, Qingdao National Laboratory for Marine Science and Technology, 1 Wenhai Road, Aoshanwei, Jimo, Qingdao 266237, China S Supporting Information *

ABSTRACT: The first Rh-catalyzed enantioselective synthesis of a 3,4-polyfused oxindole ring system enabled by carboacylation of acrylic amides based on C−C activation is reported. This transformation provides a new entry to access 3,4-polyfused oxindoles bearing quaternary stereocenters. Trito pentacyclic 3,4-fused oxindoles were asymmetrically generated in good yields (up to 95%) with good to excellent enantioselectivity (88%−97% ee). Application in the first total synthesis of xylanigripones A was completed in 6 steps with a 14% overall yield. fficiency in complex molecule syntheses is greatly dominated by strategies in building the key structural motifs. In particular, fused rings are extremely common skeletons in natural products and drugs, and their access via enantioselective and atom-economic methods is of significant importance.1 3,4-Polyfused oxindole represents a core structural motif in numerous biologically important natural products such as welwetindolinone,2a,b ergot-type alkaloids (xylanigripones,2c ergoline-8-amine2d), among which a C3 allcarbon quaternary stereocenter (e.g., communesins2e,f) exacerbates its synthetic challenge (Figure 1). Despite various stepwise and elegant cycloaddition methods for building oxindole spiro/fused-ring systems starting from indoles and/ or oxindoles,3 we are particularly intrigued by transition metal catalyzed enantioselective de novo oxindole synthesis bearing a C3 quaternary stereocenter.4 Pioneering work in asymmetric Heck or Heck-type reaction has been successfully applied in forging C3-disubstituted or spirocyclic oxindoles (Figure 1A).5 A double C−H activation/Heck-type annulation cascade was also conceived to access 3,4-fused oxindoles (Figure 1B) but in a racemic fashion from Zhu and co-workers.6 Carboacylation of alkenes enabled by C−C activation of benzocyclobutenone has emerged as a new strategy in forging fused-ring skeletons bearing a quaternary stereocenter. Pioneering work has been accomplished by Dong and co-workers (Figure 1C), who also showed its application in building indoline-type skeletons in a highly enantioselective fashion.7 We have been keenly pursuing an enantioselective catalytic C−C bond activation approach, which also represents a new entry to build enantioenriched 3,4-

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© XXXX American Chemical Society

polyfused oxindole building blocks featuring an all-carbon quaternary stereocenter (Figure 1C). Carbon−carbon bond activation8 has emerged as a unique booming area in modern transition metal catalysis although asymmetric cases and application in natural product synthesis are still rare.7,9 To the best of our knowledge, only three examples of asymmetric metal-catalyzed carboacylation of olefins through C−C cleavage of four-membered ketones have been reported,7,10 in which simple nonpolar olefins were employed as coupling partners. In this letter, we describe the first enantioselective Rh-catalyzed carboacylation of acrylamides via metal insertion of benzocyclobutenone C−C bonds,10,11 and also show our efforts toward the first total synthesis of new ergot-type natural product xylanigripones A. The key precursor 3-MeNH benzocyclobutenone S1 was readily prepared in gram scale in one batch (see Supporting Information (SI) for detail). We started investigating the reaction with 1a, which was quickly prepared from S1 and purchased acrylic chloride through amide formation. We first tested the reaction with 5 mol % [Rh(COD)Cl]2 and 12 mol % DPPF, and a very inspiring 30% yield was obtained for 2a with full conversion (entry 1, Table 1). The feasibility with bidentate DPPF provided a very rich reservoir for chiral ligands. To our disappointment, the most commonly used (S,S)-DIOP and spirocyclic (S)-SDP only provided a low yield with low enantioselectivity (entries 2 and 3, Table 1). In Received: October 26, 2018

A

DOI: 10.1021/acs.orglett.8b03412 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Table 1. Selected Optimization Conditions

Figure 1. Representative strategies for de novo synthesizing C3quaternary oxindoles and bioactive molecules containing a 3,4-fused oxindole core. a

All reaction were run with 10 mol % rhodium complex (rh based) and 12 mol % ligand on a 0.1 mmol scale in dioxane at 150 °C for 12 h unless otherwise noted. bConversions were determined based on recycled starting material after isolation. cIsolated yield. dEe was determined by chiral HPLC. eToluene was used as solvent. f2.5 mol % [Rh(COD)Cl]2 and 6 mol % ligand were used. gThe reaction was heated at 85 °C for 48 h; dppf = 1,1′-Bis(diphenylphosphino)ferrocene.

contrast, use of (S)-BINAP as the ligand increased both yield (67%) and ee (61% ee, entry 4). A continuing survey of 4carbon separated bidentate phosphine ligand (S)-MeOBIPHEP led to an even better yield (73%) but almost same ee (62% ee, entry 5), manifesting the privilege of axially chiral electron-rich ligands for asymmetric induction. A survey of other ligands with axial chirality revealed that good enantioselectivity (75% ee) was obtained with (S)-SYNPHOS albeit with a 19% yield (entry 6). The yield (57%) and ee (80%) were further improved by using (S)-SEGPHOS (entry 7). We hypothesized that the bulkier electron-rich ligands would likely help to enhance the catalyst reactivity due to the increased δ-donating effect to the RhIII center and thus better ligation with acrylamides through π-back-donation. Indeed, by using more electron-rich (S)-DTBM-SEGPHOS ligands (entry 8), the yield was further improved to 73% and ee to 90%. Encouraged by this result, a number of Rh precatalysts were examined (entries 9−11). [Rh(COD)Cl]2 proved to be the most efficient. Aiming to further enhance the functional compatibility of this transformation, a 2.5 mol % Rh-salt and 6 mol % of ligand were tested, showing no decrease in efficacy (72% yield, 90% ee) albeit a longer reaction time is needed. Gratifyingly, when the temperature was lowered to 85 °C, an enhanced ee value (95% ee) was obtained with no decrease in efficacy (87% yield) as well. To the best of our knowledge, this represents the lowest temperature for enantioselective carboacylation of acrylamides via Rh-catalyzed C−C activation

of benzocyclobutenones. The structure and absolute configuration of 3,4-fused tricyclic oxindole 2a were determined through X-ray crystallography (CCDC 1873018; 2b, CCDC 1873019). With the optimal conditions in hand, we continued to explore the substrate scope (Table 2). Broad acrylamide coupled benzocyclobutenone substrates with different steric and electronic properties have been examined. Delightedly, high to excellent enantioselectivity was obtained. It was found that changing the steric bulkiness of alkyl substituents on 2position of acrylamides (1a−1d) does not affect the enantioselectivity, and 96−98% ee’s are observed (entries 1− 4), albeit with slightly decreasing yields (52∼87%) due primarily to the increasing steric bulkiness. It was gratifying to find that the electron- and sterically alternative phenyl group (1e, entry 5) can also be tolerated in this transformation featuring excellent yield (95%) and ee (97%). To our surprise, a substrate bearing a distal olefin (1f) affords a good yield B

DOI: 10.1021/acs.orglett.8b03412 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Table 2. Substrate Scope

a All reaction were run with 2.5 mol % [Rh(COD)Cl]2 and 6 mol % (S)-DTBM-SEGPHOS on a 0.1 mmol scale in dioxane at 85 °C for 48 h unless otherwise noted. bIsolation yield. cEe was determined by chiral HPLC. d5 mol % [Rh(COD)Cl]2 and 12 mol % (S)-DTBM-SEGPHOS were used. e 5 mol % [Rh(COD)Cl]2 and 12 mol % (S)-SEGPHOS were used, and the reaction was heated at 110 °C for 36 h instead. f2.5 mol % [Rh(COD)2]BF4 and 3 mol % (S)-DTBM-SEGPHOS in dioxane at 85 °C for 12 h.

(79%) but lower ee (88%) compared to those of 1e, presumably due to loss of binding ability between the chiral ligand and Rh center caused by competitive coordination from a second distal olefin (entry 6). A vivid example demonstrating the chemoselectivity of this methodology is shown by 2g (entry 7) that the more strained cyclopropane subunit was preserved, while a high yield (84%) and enantioselectivity (96% ee) were observed. Electronic variation by introducing Cl (1h) on benzocyclobutenone led to similar if not better enantioselectivity (97% ee) with moderate isolated 44% yields

(entries 8) caused by partial decomposition of starting material. The N-Et substrate 1i underwent smooth carboacylation with similar efficiency (87% yield) and slightly decreasing 93% ee, showing steric sensitivity on nitrogen substituents (entry 10). A grand challenge hampering the “cut and sew” methodology7 applied in complex natural product synthesis is the unviability of sterically hindered coupling partners, to be specific the tri- and tetra-substituted olefins, presumably due to their low binding affinity.12 We hypothesized that trisubC

DOI: 10.1021/acs.orglett.8b03412 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

provided compound 6 (52% yield for two steps), which served as a precursor for a Pd-catalyzed C−N bond coupling15 reaction and Boc deprotection sequence. The naphthylamine 7 was obtained in 82% yield over two steps. The sulfuric acid promoted annulation took place and removed the benzyl protecting group in one pot furnishing natural product xylanigripones A (1) in a 50% yield (Scheme 2). The spectra of synthetic sample match well with the reported data.

stituted acrylamides would make a viable substrate together with a cationic Rh-catalyst based on the following rationales: (1) the stabilized/polarized acrylamides tend to bind the Rhcenter through enhanced π-back-donation; (2) a cationic Rh precatalyst would leave one more coordination vacancy that would increase the possibility for steric olefins to coordinate the transition metal center. So we tested the trisubstituted olefins as well as cyclic olefins (Table 2, entries 10−16). Cinnamic amide-type derivatives with electronically varied substituents on phenyl ring (1j−1m) were prepared and tested. It was found that (S)-SEGPHOS instead provided better enantioselectivity control (95%−98% ee), although moderate yields were obtained (entries 10−13). No obvious trend based on para-substitution was observed, and the moderate yield was attributed to sterically challenging trisubstituted olefins, as an elevated reaction temperature (110 °C) is needed. It is noteworthy that the DHP-containing acrylamide 1n underwent this transformation with only 5 mol % Rh-catalyst at 85 °C affording rather complex tetracyclic 3,4polyfused oxindole 2n in 75% yield and 98% ee (entry 14). Encouraged by this success, dehydrochroman- and indenecontaining substrates (1o and 1p) were subjected to the reaction conditions, and an even higher efficiency (90% yield for 2o and 84% yield for 2p) and asymmetric control (95% ee for 2o and 97% ee for 2p) were obtained (entries 15−16). The absolute stereochemistry of 2j and 2n were established through X-ray crystallography (CCDC 1873021 and 1873022, respectively). With highly diverse 3,4-tricyclic oxindoles in hand, we set forth to seek their application in complex alkaloid natural product synthesis, to be specific, xylargripones A, an ergot-type natural product. It was first isolated2c from Xylaria nigripes, which is a well-known medicinal fungus for treating insomnia13 and depression.14 Our retrosynthesis toward xylanigripones A (1) consisted of pyridine annulation from compound 7 and the key Rh-catalyzed carboacylation of acrylamides 4 via C−C activation toward tricyclic oxindole 5 (Scheme 1).

Scheme 2. Total Synthesis of Xylargripones A (1)

In conclusion, we have developed the first enantioselective Rh-catalyzed carboacylation of acrylamides via C−C bond activation. A very diverse substrate scope was demonstrated obtaining synthetically valuable 3,4-polyfused oxindoles in good to excellent enantioselectivity (88−98% ee) with good yields featuring an chiral all-carbon quaternary center at the C3-position. The reaction can be performed at 85 °C showing compatibility with a series of functional groups. It displays general efficacy and enantioselectivity, especially toward the challenging trisubstituted/cyclic acrylamides. To illustrate the practicality of this transformation, the first total synthesis of xylanigripones A was completed in 6 steps (from S2). We believe this transformation would find further application in 3,4-polyfused oxindole-containing natural product synthesis.

Scheme 1. Retrosynthesis of Xylanigripones A (1)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03412. Experimental procedures; spectral data (PDF) Accession Codes

CCDC 1873018−1873019 and 1873021−1873022 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Our synthesis commenced with benzocyclobutenone S2, which was swiftly prepared similarly as S1 (see SI for detail). Amide formation between S2 and the commercially available acryloyl chloride 3 afforded key reaction precursor 4 in 68% yield. The C−C activation reaction proceeded smoothly at even 80 °C with only 2.5 mol % Rh salt/3 mol % DTBMSegphos as catalyst, and a satisfyingly 96% yield was obtained for tricyclic oxindole 5. The highly effective key reaction paved the way for natural product synthesis. An oxidative aromatization mediated by DDQ followed by a triflation

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tao Xu: 0000-0001-5868-4407 D

DOI: 10.1021/acs.orglett.8b03412 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Notes

(8) For selected reviews on C−C activation, see: (a) Jones, W. D. Nature 1993, 364, 676−677. (b) Murakami, M.; Ito, Y. Top. Organomet. Chem. 1999, 3, 97−129. (c) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870−883. (d) Jun, C.-H. Chem. Soc. Rev. 2004, 33, 610−618. (e) Satoh, T.; Miura, M. Top. Organomet. Chem. 2005, 14, 1−20. (f) Necas, D.; Kotora, M. Curr. Org. Chem. 2007, 11, 1566−1591. (g) Crabtree, R. H. Chem. Rev. 1985, 85, 245−269. (h) Ruhland, K. Eur. J. Org. Chem. 2012, 2012, 2683−2706. (i) Korotvicka, A.; Necas, D.; Kotora, M. Curr. Org. Chem. 2012, 16, 1170−1214. (j) Seiser, T.; Saget, T.; Tran, D. N.; Cramer, N. Angew. Chem., Int. Ed. 2011, 50, 7740−7752. (k) Murakami, M.; Matsuda, T. Chem. Commun. 2011, 47, 1100− 1105. (l) Dermenci, A.; Coe, P. W.; Dong, G. Org. Chem. Front. 2014, 1, 567−581. (m) C−C Bond Activation. Topics in Current Chemistry; Dong, G., Ed.; Springer-Verlag: Berlin, 2014. (n) Chen, F.; Wang, T.; Jiao, N. Chem. Rev. 2014, 114, 8613−8661. (o) Souillart, L.; Cramer, N. Chem. Rev. 2015, 115, 9410−9464. (p) Shaw, M. H.; Bower, J. F. Chem. Commun. 2016, 52, 10817−10829. (q) Fumagalli, G.; Stanton, S.; Bower, J. F. Chem. Rev. 2017, 117, 9404−9432. (r) Chen, P.-H.; Billett, B. A.; Tsukamoto, T.; Dong, G. ACS Catal. 2017, 7, 1340− 1360. (9) To the best of our knowledge, the only examples involving application of carboacylation via catalytic C−C activation in total synthesis are reported by: (a) Xu, T.; Dong, G. Angew. Chem., Int. Ed. 2014, 53, 10733−10736. For recent total synthesis involving βcarbon elimination of a pinene, see: (c) Kerschgens, I.; Rovira, A. R.; Sarpong, R. J. Am. Chem. Soc. 2018, 140, 9810−9813. (d) Masarwa, A.; Weber, M.; Sarpong, R. J. Am. Chem. Soc. 2015, 137, 6327−6334. (10) For only three examples of enantioselective carboacylation of olefins through C−C bond cleavage of four-membered ketones, see: (a) Xu, T.; Ko, H. M.; Savage, N. A.; Dong, G. J. Am. Chem. Soc. 2012, 134, 20005−20008. (b) Liu, L.; Ishida, N.; Murakami, M. Angew. Chem., Int. Ed. 2012, 51, 2485−2488. (c) Souillart, L.; Parker, E.; Cramer, N. Angew. Chem., Int. Ed. 2014, 53, 3001−3005. For other selected catalytic enantioselective C−C activation through oxidative addition, see: (d) Parker, E.; Cramer, N. Organometallics 2014, 33, 780−787. (e) Souillart, L.; Cramer, N. Angew. Chem., Int. Ed. 2014, 53, 9640−9644. (f) Zhou, X.; Dong, G. J. Am. Chem. Soc. 2015, 137, 13715−13721. (g) Deng, L.; Xu, T.; Li, H.; Dong, G. J. Am. Chem. Soc. 2016, 138, 369−374. (11) For asymmetric C−C activation reactions via β-carbon elimination, see: (a) Matsuda, T.; Shigeno, M.; Makino, M.; Murakami, M. Org. Lett. 2006, 8, 3379−3381. (b) Seiser, T.; Cramer, N. Angew. Chem., Int. Ed. 2008, 47, 9294−9297. (c) Seiser, T.; Roth, O. A.; Cramer, N. Angew. Chem., Int. Ed. 2009, 48, 6320− 6323. (d) Shigeno, M.; Yamamoto, T.; Murakami, M. Chem. - Eur. J. 2009, 15, 12929−12931. (e) Seiser, T.; Cramer, N. Chem. - Eur. J. 2010, 16, 3383−3391. (f) Seiser, T.; Cramer, N. Angew. Chem., Int. Ed. 2010, 49, 10163−10167. (g) Zhou, X.; Dong, G. Angew. Chem., Int. Ed. 2016, 55, 15091−15095. (12) (a) Sun, T.; Zhang, Y.; Qiu, B.; Wang, Y.; Qin, Y.; Dong, G.; Xu, T. Angew. Chem., Int. Ed. 2018, 57, 2859−2863 and references cited therein . For selected examples involving discussing olefins binding to Rh, see: (b) Xu, T.; Dong, G. Angew. Chem., Int. Ed. 2012, 51, 7567−7571. (c) Xu, T.; Savage, N. A.; Dong, G. Angew. Chem., Int. Ed. 2014, 53, 1891−1895. (13) Ma, Z. Z.; Zuo, P. P.; Chen, W. R.; Tang, L. J.; Shen, Q.; Sun, X.-H.; Shen, Y. Studies on the Sedative and Sleeping Effects of Wuling mycelia and Its Pharmacological Mechanism. Chin. Pharm. J. 1999, 34, 374−377. (14) Li, D.; Zheng, J.; Wang, M.; Feng, L.; Liu, Y.; Yang, N.; Zuo, P. J. Ethnopharmacol. 2016, 186, 181−188. (15) For selected reviews on Pd-catalyzed C−N coupling reactions, see: (a) Ruiz-Castillo, P.; Buchwald, S. L. Chem. Rev. 2016, 116, 12564−12649. (b) Hartwig, J. F. Nature 2008, 455, 314−322.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank “1000 Talents Plan for Young Professionals” and OUC for a startup fund, NSFC (Nos. 81502913, U1606403, and U1706213) and the pilot QNLMST (Nos. 2015ASTPES14 and 2018SDKJ0403) for research grants. T.X. is a Taishan Youth Scholar. Prof. Chuang-Chuang Li and Prof. Jing Xu of SUST and Dr. Lili Shi of SZPKU are acknowledged for help with X-ray crystallography.

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DEDICATION Dedicated to Professor Hua-Shi Guan on the occasion of his 80th birthday

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