Cycloaddition of Vinylindoles with Vinyldiazoacetates

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

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Rhodium-Catalyzed Asymmetric Dearomative [4 + 3]-Cycloaddition of Vinylindoles with Vinyldiazoacetates: Access to Cyclohepta[b]indoles Guangyang Xu, Long Chen, and Jiangtao Sun* Jiangsu Key Laboratory of Advanced Catalytic Materials & Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, P. R. China S Supporting Information *

ABSTRACT: A rhodium-catalyzed enantioselective formal [4 + 3]-cycloaddition of vinylindoles with vinyldiazoacetates has been developed, affording the dearomative cyclization products containing a newly formed seven-membered ring in up to 99% ee. Rh2(S-DOSP)4 has been proven to be the best catalyst for the cycloaddition of 3-vinylindoles with vinyldiazoacetates, whereas Rh2(S-TCPTTL)4 has enhanced the enantioselectivity for 2-vinylindoles.

T

cyclohepta[b]indole. This approach works well, but still needs multistep synthesis. Thus, to develop a more efficient protocol for the rapid construction of such chiral cyclohepta[b]indoles is highly desired. In the 1990s, Davies and co-workers made a seminal work for the enantioselective synthesis of seven-membered carbon rings by rhodium-catalyzed formal [4 + 3]-annulation of vinyldiazoacetates with dienes.11a This reaction proceeded via a tandem asymmetric cyclopropanation/Cope rearrangement12 to deliver chiral 1,4-cycloheptadienes. Later, they expanded this reaction to a range of chiral transformations.11 Inspired by the reports of Davies, we envisioned that the vinylindoles might behave as formal dienes to react with vinyldiazoacetates in the presence of a chiral rhodium catalyst. If possible, the in situ formed chiral vinylcyclopropane intermediates might undergo Cope rearrangement to afford the cyclization products (Scheme 1, eq 2).13 However, metal-catalyzed reaction of indole derivatives with diazo compounds often took on a variety of reaction patterns (Scheme 1, eq 3), including alkylation,14 cyclopropanation,15 dearomative olefination,16 [3 + 2]-cycloaddition,17 cyclopropanation for 3-/2-vinyl indoles,18 and others,19 which might result in various background reactions and give rise to different side products. To overcome this challenge, and in continuation with our research interests in carbene chemistry, we herein wish to report a dearomative strategy20 to synthesize chiral cyclohepta[b]indoles in one step. We commenced our studies by performing the reaction between 3-vinylindole 1a and phenyl vinyldiazoacetate 2a in dichloromethane at −10 °C (Table 1). The initial evaluation on

he indoles/indolines fused with a seven-membered carbon ring (cyclohepta[b]indoles) are important motifs found in a variety of natural products and pharmaceuticals with diverse biological activities, such as actinophyllic acid,1 kopsifoline D,2 ambiguine P,3 ervitsine,4 ajmaline,5 SIRTI-inhibitor IV,6 antitubercular agent,7 and the inhibitor of adipocyte fattyacid-binding protein (A-FABP) (Figure 1).8

Figure 1. Examples of biologically active compounds that contain the cyclohepta[b]indole core.

Given their great importance, methods for the direct preparation of cyclohepta[b]indoles have attracted much attention, and a variety of protocols have been established.9 However, enantioselective syntheses of such tricyclic scaffolds are rare.10 Recently, Gaich and co-workers reported an asymmetric synthesis of cyclohepta[b]indoles via a tandem reaction sequence (Scheme 1a).10a Namely, the initial asymmetric Simmons−Smith reaction introduced the chirality of the product. Subsequent oxidation/Wittig reaction produced chiral vinyl cyclopropane, which would undergo [3,3]sigmatropic rearrangement to deliver the target chiral © XXXX American Chemical Society

Received: April 28, 2018

A

DOI: 10.1021/acs.orglett.8b01353 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Optimization of the Reaction Conditionsa

Scheme 1. Previous Reports and Our Strategy

entry 1 2 3 4 5

a variety of chiral dirhodium tetracarboxylate catalysts were conducted. It was found that phthalimido catalysts21 gave the cycloaddition adducts in good yields but lower levels of enantioselectivity (Table 1, entries 1−4). The use of Rh2(STCPTTL)4 provided 3a in 78% yield and 38% ee with inverse configuration (Table 1, entry 5).22 Next, proline-derived dirhodium tetracarboxylate catalysts were examined (Table 1, entries 6−9), and Rh2(S-DOSP)423 was proven to be the best one. As a result, the desired product 3a was obtained with 83% yield and 58% ee (Table 1, entry 8). Solvent screening was then performed using Rh2(S-DOSP)4 as the ideal catalyst (Table 1, entries 10−15). Gratifyingly, the enantioselectivity was improved for all of six solvents screened, albeit with slightly decreased yield. To further improve the enantioselectivity, mixed solvents were examined (Table 1, entries 16−19). To our delight, the use of hexane/CCl4 in a 4:1 ratio furnished 3a in 80% yield and 82% ee (Table 1, entry 19). Furthermore, when the reaction was carried out at −35 °C for 12 h, the ee value of 3a increased to 95% (Table 1, entry 20). However, high reaction temperature resulted in higher yield but lower enantioselectivity (Table 1, entry 21). Reducing the catalyst loading to 0.5 mol % led to lower yield (Table 1, entry 22). Notably, without the addition of molecular sieves, the yield dropped dramatically (Table 1, entry 23). With the established optimal reaction conditions in hand, we next examined the substrate scope for 3-vinylindoles and vinyldiazoacetates (Scheme 2). Various substituents on the phenyl ring of the aryl vinyldiazoacetates were well tolerated, including electron-donating and electron-withdrawing groups, as well as the para-, meta-, and ortho-substitution patterns (3b−f). The use of thienyl vinyldiazoacetate furnished the desired product 3g in 59% yield and 95% ee within 24 h. Under higher temperature, alkyl vinyldiazoacetate was also amenable to the reaction, and the target product 3h was obtained in 42%

cat.

6 7 8 9 10 11 12 13 14 15 16

Rh2(S-PTTL)4 Rh2(S-PTPG)4 Rh2(S-PTPA)4 Rh2(S-TFPTTL)4 Rh2(STCPTTL)4 Rh2(S-MSP)4 Rh2(S-BSP)4 Rh2(S-DOSP)4 Rh2(S-BP)4 Rh2(S-DOSP)4 Rh2(S-DOSP)4 Rh2(S-DOSP)4 Rh2(S-DOSP)4 Rh2(S-DOSP)4 Rh2(S-DOSP)4 Rh2(S-DOSP)4

17

Rh2(S-DOSP)4

18

Rh2(S-DOSP)4

19

Rh2(S-DOSP)4

20d

Rh2(S-DOSP)4

21

Rh2(S-DOSP)4

22e

Rh2(S-DOSP)4

23f

Rh2(S-DOSP)4

T (°C)

yieldb (%)

eec (%)

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2

−10 −10 −10 −10 −10

81 77 73 80 78

−43 −4 −30 −27 38

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 DCE CHCl3 CCl4 THF hexane toluene hexane/CCl4 (1:1) hexane/CCl4 (1:4) hexane/CCl4 (2:1) hexane/CCl4 (4:1) hexane/CCl4 (4:1) hexane/CCl4 (4:1) hexane/CCl4 (4:1) hexane/CCl4 (4:1)

−10 −10 −10 −10 −10 −10 −10 −10 −10 −10 −10

75 80 83 65 78 76 70 41 72 59 81

28 52 58 9 59 61 76 74 79 74 79

−10

67

77

−10

81

80

−10

80

82

−35

78

95

0

85

74

−35

59

95

−35

51

95

solvent

a

To a mixture of 1a (0.2 mmol), [Rh] (1 mol %), and 4 Å MS (200 mg) in solvent (4 mL) was added 2a (0.24 mmol) in solvent (2 mL) via a syringe pump over 1 h. The mixture was stirred for another 1 h. b Isolated yields. cDetermined by chiral HPLC. dReaction time 12 h. e 0.5 mol % of catalyst was used. fWithout 4 Å MS.

yield and 91% ee. Next, the scope of 3-vinylindole compounds was evaluated. The use of 4-CN-3-vinylindole provided 3i in 92% yield and 93% ee. Both electron-donating and electronwithdrawing C5/C6-substituted indole derivatives were tolerated, providing the corresponding products in good yields and good to excellent ee values (3j−m). The configuration of 3m was determined by X-ray diffraction. The reaction of 7methyl-3-vinyl indole with 2a afforded 3n in 84% yield and 85% ee. Finally, the 3-vinylindoles bearing different substitution patterns at the C2 position were employed, and the desired products were isolated in moderate yields and good ee values (3o−q). It should be noted that all of the products were obtained in excellent dr values (>20:1). B

DOI: 10.1021/acs.orglett.8b01353 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 2. Substrate Scopea−c

Scheme 3. Cycloaddition of 2-Vinylindoles with Vinyldiazoacetatesa−c

a To a mixture of 4 (0.2 mmol), Rh2(S-TCPTTL)4 (1 mol %), and 4 Å MS (200 mg) in n-hexane/CCl4 (4 mL, 4:1) was added 2 (0.24 mmol) in n-hexane/CCl4 (2 mL, 4:1) via a syringe pump over 1 h at −35 °C. The mixture was stirred for another 12−36 h. bIsolated yields. The dr values were determined by crude H NMR analysis. cDetermined by chiral HPLC. a

To a mixture of 1 (0.2 mmol), Rh2(S-DOSP)4 (1 mol %), and 4 Å MS (200 mg) in n-hexane/CCl4 (4 mL, 4:1) was added 2 (0.24 mmol) in 2 mL of solvent via a syringe pump over 1 h at −35 °C. The mixture was stirred for another 6−24 h. bIsolated yields. The dr values were determined by crude H NMR analysis. cDetermined by chiral HPLC.

yield and 86% ee (Scheme 4, eq 1).24 Second, the reaction of 2vinylindole 4a with 6 gave cyclopropanation product 8 in 70% Scheme 4. Control Experiments and Further Exploration

We then turned our attention to the cycloaddition of 2vinylindoles with vinyldiazoacetates (Scheme 3). However, the results were not satisfactory when Rh2(S-DOSP)4 was used as the catalyst. After extra optimization, we found Rh2(STCPTTL)4 was the best catalyst for this annulation (see the Supporting Information for detailed optimization). The reaction of 2-vinylindoles with aryl vinyldiazoacetates bearing either electron-donating or electron-withdrawing substituents at the phenyl ring proceeded smoothly to afford the cyclization products in moderate to good yields and good to excellent enantioselectivities (5a−e). Thienyl vinyldiazoacetate was also tolerated to give 5f in 63% yield and 95% ee. However, alkyl vinyldiazoacetate was not very suitable for this reaction; for example, 5g was obtained in 50% yield and 36% ee. The variation of 2-vinylindole substrates was also examined, and the corresponding products were isolated in moderate to good yields and moderate to excellent ee values (5h−l). The absolute configuration of 5k was confirmed by X-ray diffraction. Again, excellent diastereoselectivity (>20:1) was observed for all of the cycloaddition products. Control experiments were conducted to verify the reaction process. First, the reaction of 3-vinylindole 1a with phenyl diazoacetate 6 provided cyclopropanation product 7 in 72%

yield and 49% ee catalyzed by Rh2(S-TCPTTL)4 (Scheme 4, eq 2). These results provide strong evidence for our initial hypothesis. Namely, the formal [4 + 3]-cycloaddition of vinylindoles with vinyldiazoacetates involved sequential asymmetric cyclopropanation/Cope rearrangement. Next, further transformations were carried out (Scheme 4, eqs 3 and 4). Compounds 3a and 5a can easily isomerize to the C

DOI: 10.1021/acs.orglett.8b01353 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

(9) For selected examples, see: (a) Han, X.; Li, H.; Hughes, R. P.; Wu, J. Angew. Chem., Int. Ed. 2012, 51, 10390. (b) Shu, D.; Song, W.; Li, X.; Tang, W. Angew. Chem., Int. Ed. 2013, 52, 3237. (c) Kusama, H.; Sogo, H.; Saito, K.; Suga, T.; Iwasawa, N. Synlett 2013, 24, 1364. (d) Mei, G.; Yuan, H.; Gu, Y.; Chen, W.; Chung, L. W.; Li, C.-C. Angew. Chem., Int. Ed. 2014, 53, 11051. (e) Li, Y.; Zhu, C.-Z.; Zhang, J. Eur. J. Org. Chem. 2017, 2017, 6609. (f) Wang, Z.; Addepalli, Y.; He, Y. Org. Lett. 2018, 20, 644. (g) Kroc, M. A.; Prajapati, A.; Wink, D. J.; Anderson, L. L. J. Org. Chem. 2018, 83, 1085. (10) (a) Gritsch, P. J.; Stempel, E.; Gaich, T. Org. Lett. 2013, 15, 5472. (b) Loh, C. C. J.; Badorrek, J.; Raabe, G.; Enders, D. Chem. Eur. J. 2011, 17, 13409. (11) (a) Davies, H. M. L.; Stafford, D. G.; Doan, B. D.; Houser, J. H. J. Am. Chem. Soc. 1998, 120, 3326. (b) Davies, H. M. L.; Doan, B. D. J. Org. Chem. 1999, 64, 8501. (c) Deng, L.; Giessert, A. J.; Gerlitz, O. O.; Dai, X.; Diver, S. T.; Davies, H. M. L. J. Am. Chem. Soc. 2005, 127, 1342. (d) Schwartz, B. D.; Denton, J. R.; Lian, Y.; Davies, H. M. L.; Williams, C. M. J. Am. Chem. Soc. 2009, 131, 8329. (e) Guzmán, P. E.; Lian, Y.; Davies, H. M. L. Angew. Chem., Int. Ed. 2014, 53, 13083. (f) Olson, J. P.; Davies, H. M. L. Org. Lett. 2008, 10, 573. (12) For selected reviews on cyclopropanation/Cope rearrangement, see: (a) Davies, H. M. L. Tetrahedron 1993, 49, 5203. (b) Davies, H. M. L.; Denton, J. R. Chem. Soc. Rev. 2009, 38, 3061 For selected examples, see:. (c) Davies, H. M. L.; Smith, H. D.; Korkor, O. Tetrahedron Lett. 1987, 28, 1853. (d) Davies, H. M. L.; McAfee, M. J.; Oldenburg, C. E. M. J. Org. Chem. 1989, 54, 930. (e) Reddy, R. P.; Davies, H. M. L. J. Am. Chem. Soc. 2007, 129, 10312. (f) Davies, H. M. L.; Lian, Y. Acc. Chem. Res. 2012, 45, 923. (13) For selected examples on [4 + 3]-cycloaddition of vinyldiazoacetates with dienes, see: (a) Davies, H. M. L.; Ahmed, G.; Churchill, M. R. J. Am. Chem. Soc. 1996, 118, 10774. (b) Davies, H. M. L.; Hodges, L. M. J. Org. Chem. 2002, 67, 5683. (c) Lian, Y.; Miller, L. C.; Born, S.; Sarpong, R.; Davies, H. M. L. J. Am. Chem. Soc. 2010, 132, 12422. (14) (a) Gibe, R.; Kerr, M. A. J. Org. Chem. 2002, 67, 6247. (b) Chan, W.-W.; Yeung, S.-H.; Zhou, Z.; Chan, A. S. C.; Yu, W.-Y. Org. Lett. 2010, 12, 604. (c) Lian, Y.; Davies, H. M. L. Org. Lett. 2010, 12, 924. (d) Johansen, M. B.; Kerr, M. A. Org. Lett. 2010, 12, 4956. (e) DeAngelis, A.; Shurtleff, V. W.; Dmitrenko, O.; Fox, J. M. J. Am. Chem. Soc. 2011, 133, 1650. (f) Cai, Y.; Zhu, S.-F.; Wang, G.-P.; Zhou, Q.-L. Adv. Synth. Catal. 2011, 353, 2939. (g) Lian, Y.; Davies, H. M. L. Org. Lett. 2012, 14, 1934. (h) Xi, Y.; Su, Y.; Yu, Z.; Dong, B.; McClain, E. J.; Lan, Y.; Shi, X. Angew. Chem., Int. Ed. 2014, 53, 9817. (i) Gao, X.; Wu, B.; Huang, W.-X.; Chen, M.-W.; Zhou, Y.-G. Angew. Chem., Int. Ed. 2015, 54, 11956. (15) (a) Gnad, F.; Poleschak, M.; Reiser, O. Tetrahedron Lett. 2004, 45, 4277. (b) Hedley, S. J.; Ventura, D. L.; Dominiak, P. M.; Nygren, C. L.; Davies, H. M. L. J. Org. Chem. 2006, 71, 5349. (c) DelgadoRebollo, M.; Prieto, A.; Pérez, P. J. ChemCatChem 2014, 6, 2047. (d) Lehner, V.; Davies, H. M. L.; Reiser, O. Org. Lett. 2017, 19, 4722. (16) (a) Liu, K.; Xu, G.; Sun, J. Chem. Sci. 2018, 9, 634. (b) Xu, G.; Liu, K.; Sun, J. Org. Lett. 2018, 20, 72. (17) (a) Lian, Y.; Davies, H. M. L. J. Am. Chem. Soc. 2010, 132, 440. (b) Jing, C.; Cheng, Q.-Q.; Deng, Y.; Arman, H.; Doyle, M. P. Org. Lett. 2016, 18, 4550. (18) (a) Marcin, L. R.; Denhart, D. J.; Mattson, R. J. Org. Lett. 2005, 7, 2651. (b) Pirovano, V.; Brambilla, E.; Tseberlidis, G. Org. Lett. 2018, 20, 405. (19) (a) Hong, X.; France, S.; Mejía-Oneto, J. M.; Padwa, A. Org. Lett. 2006, 8, 5141. (b) Shimada, N.; Oohara, T.; Krishnamurthi, J.; Nambu, H.; Hashimoto, S. Org. Lett. 2011, 13, 6284. (c) Jiang, L.; Jin, W.; Hu, W. ACS Catal. 2016, 6, 6146. (20) For a recent review on catalytic asymmetric dearomatization, see: Zheng, C.; You, S.-L. Chem. 2016, 1, 830. (21) (a) Kitagaki, S.; Anada, M.; Kataoka, O.; Matsuno, K.; Umeda, C.; Watanabe, N.; Hashimoto, S. J. Am. Chem. Soc. 1999, 121, 1417. (b) Saito, H.; Oishi, H.; Kitagaki, S.; Nakamura, S.; Anada, M.; Hashimoto, S. Org. Lett. 2002, 4, 3887. (c) Yamawaki, M.; Tsutsui, H.;

corresponding rearrangement products 9 and 10 in good yields without significant ee erosion. In summary, we have developed a rhodium-catalyzed asymmetric [4 + 3]-annulation of vinylindole compounds with vinyldiazoacetates, affording tricyclic compounds containing a newly formed seven-membered ring in moderate to good yields and moderate to excellent enantioselectivities. Meanwhile, Rh2(S-DOSP)4 was proven to be the best catalyst for 3vinylindole subtrates, whereas Rh2(S-TCPTTL)4 was the most efficient one for 2-vinylindoles.

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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.8b01353. Experimental procedures along with characterizing data and copies of NMR spectra (PDF) Accession Codes

CCDC 1838779 (3m) and 1838780 (5k) 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], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected] ORCID

Jiangtao Sun: 0000-0003-2516-3466 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (No. 21572024), the Natural Science Foundation of Jiangsu Province (BK20151184), Jiangsu Key Laboratory of Advanced Catalytic Materials & Technology (BM2012110), and the Green Manufacturing Collaborative Innovation Center for their financial support.

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REFERENCES

(1) For selected examples, see: Yoshii, Y.; Tokuyama, H.; Chen, D. Y.-K. Angew. Chem., Int. Ed. 2017, 56, 12277. (2) Lee, K.; Boger, D. L. J. Am. Chem. Soc. 2014, 136, 3312 and references cited therein. (3) (a) Raveh, A.; Carmeli, S. J. Nat. Prod. 2007, 70, 196. (b) Mo, S.; Krunic, A.; Chlipala, G.; Orjala, J. J. Nat. Prod. 2009, 72, 894. (4) Andriantsiferana, M.; Besselièvre, R.; Riche, C.; Husson, H.-P. Tetrahedron Lett. 1977, 18, 2587. (5) (a) Siddiqui, S.; Siddiqui, R. H. J. Indian Chem. Soc. 1931, 8, 667. (b) Woodward, R. B. Angew. Chem. 1956, 68, 13. (6) Zhao, X.; Allison, D.; Condon, B.; Zhang, F.; Gheyi, T.; Zhang, A.; Ashok, S.; Russell, M.; MacEwan, I.; Qian, Y.; Jamison, J. A.; Luz, J. G. J. Med. Chem. 2013, 56, 963. (7) Yamuna, E.; Kumar, R. A.; Zeller, M.; Prasad, K. J. R. Eur. J. Med. Chem. 2012, 47, 228. (8) Barf, T.; Lehmann, F.; Hammer, K.; Haile, S.; Axen, E.; Medina, C.; Uppenberg, J.; Svensson, S.; Rondahl, L.; Lundbäck, T. Bioorg. Med. Chem. Lett. 2009, 19, 1745. D

DOI: 10.1021/acs.orglett.8b01353 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Kitagaki, S.; Anada, M.; Hashimoto, S. Tetrahedron Lett. 2002, 43, 9561. (22) For an explanation for the inverse configuration, see: Liao, K.; Pickel, T. C.; Boyarskikh, V.; Bacsa, J.; Musaev, D. G.; Davies, H. M. L. Nature 2017, 551, 609. (23) Davies, H. M. L.; Bruzinski, P. R.; Lake, D. H.; Kong, N.; Fall, M. J. J. Am. Chem. Soc. 1996, 118, 6897. (24) For Davies’ asymmetric cyclopropanation of aryl olefins with vinyldiazoacetates, see: (a) Davies, H. M. L.; Huby, N. J. S.; Cantrell, W. R., Jr.; Olive, J. L. J. Am. Chem. Soc. 1993, 115, 9468. (b) Davies, H. M. L.; Walji, A. M.; Nagashima, T. J. Am. Chem. Soc. 2004, 126, 4271. (c) Qin, C.; Boyarskikh, V.; Hansen, J. H.; Hardcastle, K. I.; Musaev, D. G.; Davies, H. M. L. J. Am. Chem. Soc. 2011, 133, 19198. (d) Negretti, S.; Cohen, C. M.; Chang, J. J.; Guptill, D. M.; Davies, H. M. L. Tetrahedron 2015, 71, 7415.

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DOI: 10.1021/acs.orglett.8b01353 Org. Lett. XXXX, XXX, XXX−XXX