Letter pubs.acs.org/OrgLett
Functionalized Tetrahydropyridines by Enantioselective PhosphineCatalyzed Aza-[4 + 2] Cycloaddition of N‑Sulfonyl-1-aza-1,3-dienes with Vinyl Ketones Huamin Wang, Wei Zhou, Mengna Tao, Anjing Hu, and Junliang Zhang* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, P. R. China S Supporting Information *
ABSTRACT: The development of electron-demand disfavored [4 + 2] cycloaddition of two electron-deficient reacting partners poses a considerable challenge. An enantioselective aza-[4 + 2] cycloaddition of electron-deficient N-sulfonyl-1-aza-1,3-dienes is possible with vinyl ketones via phosphine catalysis, which provides facile access to a wide range of enantioenriched trifluoromethylated tetrahydropyridines in up to 97% yield with 97% ee and >20:1 dr. The mechanistic study indicated that this cycloaddition proceeds via a tandem intermolecular aza-Rauhut−Currier/intramolecular aza-Michael addition reaction.
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with alkyl vinyl ketones have been reported by Loh and Zhong,6a Shi,6b,c and Wu6d by utilizing the chiral phosphine catalysts derived from natural or unnatural amino acids. However, the development of more efficient new chiral phosphine catalyst to further expand the range of N-sulfonyl1-aza-1,3-butadienes is still desirable. Meanwhile, the introduction of a CF3 group at a specific site to a bioactive molecule is considered as a more and more useful strategy to improve its physicochemical, biological, and pharmaceutical properties;7 for example, a trifluoromethylated piperidine was introduced to the janus kinase inhibitors, which observed a significant enhancement of its pharmaceutical activity (Figure 1).8 However, the asymmetric synthesis of trifluoromethylated heterocycles is largely lagging due to the limited availability of trifluoromethylated substrates and robust chiral catalysts. Recently, we developed several new phosphine catalysts, such as Xiao-Phos, Wei-Phos, and Peng-Phos, which have shown good performance in enantioselective Rauhut− Currier reactions and allylation reactions.9 With these chiral phosphine catalysts in hand, we wished to examine their performance in the enantioselective aza-[4 + 2] cycladdition. Herein, we wish to report the first phosphine catalyzed asymmetric aza-[4 + 2] cycloadditions of 1-aza-1,3-butadienes derived from β-fluoroalkylated-α,β-enones with vinyl ketones, which provide a facile access to valuable chiral fluoroalkylated tetrahydropyridines (Scheme 1). Besides β-fluoroalkylated α,βenone derived imines, the chalcone-derived imines could also furnish the desired products with satisfactory diastereo- and
etrahydropyridines are an important class of sixmembered nitrogen-containing heterocycles which are widely found in a variety of natural products and biologically active compounds (Figure 1).1 Meanwhile, tetrahydropyridines
Figure 1. Bioactive molecules having tetrahydropyridine and piperidine cores.
are important organic synthons which can be readily reduced to piperidines.2 Therefore, tremendous efforts for the synthesis of tetrahydropyridines, especially in an asymmetric manner, have been undertaken, among which an organocatalytic [4 + 2] cycloaddition reaction3 has been widely recognized as one of powerful approaches. However, compared to the wellestablished Brønsted acids4a−e and proline4f−h-catalyzed enantioselective aza-[4 + 2] cycloaddition reactions, the development of other organocatalysts such as chiral phosphine5 has been less explored and unbalanced. To date, only a few examples of asymmetric phosphine-catalyzed intermolecular aza-[4 + 2] cycloadditions of N-sulfonyl-1-aza-1,3-butadienes © XXXX American Chemical Society
Received: February 17, 2017
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DOI: 10.1021/acs.orglett.7b00489 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 1. Enantioselective Phosphine-Catalyzed Aza [4 + 2] Cycloaddition
Table 1. Variation of Aza Diene and Aryl Vinyl Ketone Components
enantioselectivity. Furthermore, the mechanistic study indicated that this cycloaddition proceeds via a tandem intermolecular aza-Rauhut−Currier/intramolecular aza-Michael addition reaction. Aza diene 1a and vinyl ketone 2a were selected as model substrates for screening chiral phosphine catalysts (Figure 2).
entry
1, R1/Rf
Ar
yielda (%)
eeb (%)
1 2c 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17d 18 19 20 21e 22 23 24 25 26 27 28 29d 30 31 32d
1a, Ph/CF3 1b, 4-MeC6H4/CF3 1c, 4-MeOC6H4/CF3 1d, 4-PhC6H4/CF3 1e, 4-FC6H4/CF3 1f, 4-ClC6H4/CF3 1g, 4-BrC6H4/CF3 1h, 4-IC6H4/CF3 1i, 4-NCC6H4/CF3 1j, 4-MeO2CC6H4/CF3 1k, 4-MeO2SC6H4/CF3 1l, 2-FC6H4/CF3 1m, 2-O2NC6H4/CF3 1n, 3-ClC6H4/CF3 1o, 3-BrC6H4/CF3 1p, 2-naphthyl/CF3 1q, 2-furyl/CF3 1r, Ph/CF3 1s, Ph/CF3 1t, Ph/C2F5 1v, cyclopropyl/CF3 1c 1c 1c 1c 1c 1c 1c 1c 1c 1c 1c
2a, Ph 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2b, 4-MeC6H4 2c, 4-MeOC6H4 2d, 4-PhC6H4 2e, 4-FC6H4 2f, 4-ClC6H4 2g, 4-BrC6H4 2h, 4-O2NC6H4 2i, 4-NCC6H4 2j, 4-CF3C6H4 2k, 2-naphthyl 2l, 2-furyl
3aa, 92 3ba, 91 3ca, 84 3da, 94 3ea, 83 3fa, 88 3ga, 90 3ha, 92 3ia, 82 3ja, 88 3ka, 62 3la, 97 3ma, 96 3na, 90 3oa, 92 3pa, 92 3qa, 72 3ra, 93 3sa, 79 3ta, 65 3va, 70 3cb, 84 3cc, 90 3cd, 90 3ce, 75 3cf, 85 3cg, 84 3ch, 60 3ci, 60 3cj, 74 3ck, 95 3cl, 89
95 95 94 95 91 94 96 96 92 91 95 91 85 95 94 97 92 94 94 92 82 92 92 93 91 90 92 90 92 90 90 93
Figure 2. Phosphine catalysts employed in this study.
To our delight, chiral sulfinamide phosphines (P1 and P2) could catalyze the reaction, but low yields and ee values were obtained (Table S1, entries 1 and 2). Further screening showed that the amide moiety plays a significant effect on the enantioselectivity; the acetyl- (P3) and p-toluoyl- derived (P4) amide phosphines delivered relatively lower ee’s (Table S1, entries 3 and 4), while phosphine P5 bearing a more acidic amide moiety derived from 3,5-ditrifluoromethylbenzonic acid turned out to be a promising catalyst, furnishing the desired [4 + 2] cycloaddition product in 77% yield with 90% ee (Table S1, entry 5). The introduction of different aryl groups at the orthoposition of the phenyl ring, structured as P6 and P7, resulted in elevated enantioselectivity and diastereoselectivity (Table S1, entries 6 and 7). With the use of the best catalyst P7 as the catalyst, various solvents were then screened, and acetone was found to be the best solvent with regard to activity (Table S1, entries 8−12). Further increase of the catalyst loading to 7.5 mol % led to a slightly higher yield (Table S1, entry 13). A slightly higher ee was observed when the reaction was run at 0 °C but led to lower yield (Table S1, entry 14). With the optimal reaction conditions in hand, we then turned to explore the generality of this cycloaddition process with a variety of 1-aza-1,3-dienes with ketone 2a (Table 1). Regardless of their electronic nature and the substitution pattern of the aryl ring, high yields and ee values were furnished (Table 1, entries 1−15). The 2-naphthyl group and heterocyclic ring were also applicable, furnishing the corresponding products 3pa and 3qa in 72−92% yields with 92−97% ee’s (Table 1, entries 16 and 17). When the substituent R2 on Nsulfonylimine 1 was changed, other groups, including pmethoxyphenyl and phenyl, were all well tolerated in the reactions (Table 1, entries 18 and 19). Changing the fluoroalkyl group from a CF3 group to the C2F5 group also worked well to give the corresponding product 3ta with 92% ee in 65% yield (Table 1, entry 20). Notably, alkyl-substituted β-trifluoroalkyl α,β-enone derived imine also worked well giving rise to the fused heterocyclic 3va in acceptable yield with 82% ee (Table 1,
a
Yield of isolated trans isomer, dr >20:1; determined by 19F NMR analysis of the crude reaction mixture. bDetermined by HPLC analysis. c 10 mol % of P7 was used. d10 mol % of P7 was used at 0 °C. e10 mol % of P7 was used at −20 °C.
entry 21). Moreover, we examined the scope of the cycloaddition reaction of 1c with a series of aryl vinyl ketones 2. Aryl vinyl ketones with electron-donating groups such as 4Me (2b), 4-OMe (2c), and 4-Ph (2d) on the phenyl ring exhibited higher enantioselectivity (Table 1, entries 22−24). Halogenated substrates 2e−g were also well tolerated, affording the corresponding products 3ce−cg in 75−85% yields with 90−92% ee (Table 1, entries 25−27). Vinyl ketones 2h−j with electron-withdrawing aromatic rings also successfully afforded the corresponding products 3ch−cj with good to excellent enantioselectivity (Table 1, entries 28−30). 2-Naphthyl- and 2furyl-substituted vinyl ketones afforded the desired products in 90% and 93% ee, respectively (Table 1, entries 31 and 32). Next, we turned our attention to the challenge of enantioselective [4 + 2] cycloaddition of alkyl vinyl ketones with β-fluoroalkylated enone derived imines. For the alkyl vinyl B
DOI: 10.1021/acs.orglett.7b00489 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters ketones, catalyst P6 was identified as the best catalyst, and the substrate scope was then examined (Table S2). In general, the reaction worked quite well, delivering the corresponding products with excellent ee values and high dr values. Similar to the reaction with aryl vinyl ketone, in general, βfluoroalkylated enone derived imines with electronically and sterically different aromatic groups were also examined, and the reaction could proceed smoothly to afford the desired products (3bn−fo) with high stereoselectivity (Table S2, entries 1−13). The absolute configuration of 3dn was established by singlecrystal X-ray diffraction analysis10 (see the SI). The enantioselective cycloaddition between chalcone-derived imines (4) and methyl vinyl ketone (2n) poses considerable challenges due to their relatively lower reactivity.7a,7d Inspired by above results and with diverse catalysts in hand, we made our efforts to this cycloaddition. Gratifyingly, as shown in Table 2, the corresponding cycloadducts 5an−fn could be obtained in
Scheme 2. Gram-Scale Synthesis of 3a and Further Synthetic Transformationsa
a
Gram-scale reaction and synthetic applications. Reaction conditions: (a) P7 (5 mol %), 1a (2.8 mmol), 2a (1.5 equiv), acetone (0.1 M), rt, 88% yield, 94% ee, dr > 20:1; (b) NaBH4, 0 °C, 90% yield, 90% ee, dr > 20:1; (c) RuCl3, NaIO4, 0 °C, 75% yield, 92% ee, dr > 20:1; (d) TFA/thioanisole, rt, 70% yield, 93% ee, dr > 20:1; (e) BF3·Et2O, Et3SiH, 70 °C, 80% yield, 94% ee, dr > 20:1; (f) NaCNBH3, MeOH/ AcOH, rt, 94% ee, dr >20:1.
Table 2. [4 + 2] Cycloaddition of Methyl Vinyl Ketones with Chalcone-Derived Iminesa
entry
R1
dr
yieldb (%)
eec (%)
1 2 3 4 5 6 7
4a, Ph 4b, 4-MeoC6H4 4c, 4-FC6H4 4d, 4-ClC6H4 4e, 4-BrC6H4 4f, 4-CF3C6H4 4g, perfluorophenyl
13:1 12:1 13:1 10:1 13:1 12:1 >20:1
5an, 70 5bn, 65 5cn, 60 5dn, 60 5en, 63 5fn, 67 5gn, 75
90 90 90 90 92 90 92
idine 6d was also obtained in good yield by chemoselective reduction the olefin moiety of 3aa without any reduction of the carbonyl group. In summary, we have developed an enantioselective phosphine-catalyzed aza-[4 + 2] cycloaddition of N-sulfonyl1-aza-1,3-dienes with vinyl ketones, which provides a facile access to various functional trifluoromethylated tetrahydropyridines in moderate to good yields with high diastereo- and enantioselectivities. Mechanistic study11 indicated that this aza[4 + 2] cycloaddition proceeded via a tandem aza-Rauhut− Currier/intramolecular aza-Michael addition reaction. Furthermore, the phosphonium enolate intermediate12 plays a dual role, acting as a key intermediate for the first aza-Rauhut− Currier reaction and a real catalyst for the second aza-Michael addition step. Further efforts toward the application of these novel multifunctional phosphine catalysts for other asymmetric transformations are currently underway in our group and will be reported in due course.
a
Unless otherwise stated, reactions were performed by using 0.1 mmol of 4, 0.3 mmol of 2n, 10 mol % of P2, and 30 mol % of 2-chlorophenol in 1.0 mL of CHCl3 at 0 °C for 72 h. bYield of isolated cycloadduct. c Determined by HPLC analysis.
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60−70% yields with 90−92% ee’s regardless of the substituent, from electron-donating group MeO to weak electron-withdrawing groups such as F, Cl, and Br and even strong electronwithdrawing group CF3 with the chiral sulfinamide phosphine P2 as the catalyst, indicating that this type of cycloaddition is sensitive to the structure of the substrate and an appropriate catalyst must be correctly chosen. In addition, pentafluorosubstituted N-tosyl ketimine gave the product 5gn with 75% yield and 92% ee (Table 2, entry 7). In order to display the potential synthetic applications of this methodology, a gram-scale reaction of 1a and 2a was also carried out to furnish 1.2 g of the desired product 3aa in 88% yield and 94% ee with the use of 5 mol % of P7 as the catalyst (Scheme 2). The selective reduction of carbonyl group provided effective access to valuable chiral alcohol 6a in 90% yield as a single diastereomer (>20:1 dr) and 90% ee. The diastereoselective dihydroxylation of the olefin moiety with RuCl3/NaIO4 delivered a functionalized piperidine 6b in 75% yield with 92% ee. The deprotection of tosyl group was realized by treatment of 3aa with TFA/thioanisole, and the imine product 6c was obtained as single diastereomer in 70% yield with 93% ee and trifluoromethylated piperidine 7c was furnished in 90% yield with 94% ee by subsequent reduction with sodium cyanoborohydride. The enantioenriched piper-
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b00489. Experimental procedures; spectroscopic data for the substrates and products (PDF) X-ray data for (−)-3dn (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Junliang Zhang: 0000-0002-4636-2846 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to the 973 Program (2015CB856600), the National Natural Science Foundation of China (21425205, C
DOI: 10.1021/acs.orglett.7b00489 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
(6) (a) Shi, Z.; Yu, P.; Loh, T.-P.; Zhong, G. Angew. Chem., Int. Ed. 2012, 51, 7825. (b) Zhang, X.; Chen, G.; Dong, X.; Wei, Y.; Shi, M. Adv. Synth. Catal. 2013, 355, 3351. (c) Zhang, X. N.; Dong, X.; Wei, Y.; Shi, M. Tetrahedron 2014, 70, 2838. (d) Wang, G.; Rexiti, R.; Sha, F.; Wu, X. Tetrahedron 2015, 71, 4255. (7) For leading references, see: (a) Fujita, T.; Iwasa, J.; Hansch, C. J. Am. Chem. Soc. 1964, 86, 5175. (b) Leo, A.; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 525. (c) Katritzky, A. R.; Lobanov, V. S.; Karelson, M. Chem. Soc. Rev. 1995, 24, 279. (d) Huang, Y. Y.; Yang, X.; Chen, Z.; Verpoort, F.; Shibata, N. Chem. - Eur. J. 2015, 21, 8664. (8) For janus kinase inhibitors, see: (a) Childers, M. L.; Fuller, P.; Guerin, D.; Katz, J. D.; Pu, Q.; Scott, M. E.; Thompson, C. F.; Martinez, M.; Falcone, D.; Torres, L.; Deng, Y.; Kuruklasuriya, R.; Zeng, H.; Bai, Y.; Kong, N.; Liu, Y.; Zheng, Z. WO Patent 2014146491, A1, 2014. (b) Dinsmore, C.; Fuller, P.; Guerin, D.; Katz, J. D.; Thompson, C. F.; Falcone, D.; Deng, W.; Torres, L.; Zeng, H.; Bai, Y.; Fu, J.; Kong, N.; Liu, Y.; Zheng, Z. WO Patent 2014146493, A1, 2014. (9) (a) Su, X.; Zhou, W.; Li, Y.; Zhang, J. Angew. Chem., Int. Ed. 2015, 54, 6874. (b) Zhou, W.; Su, X.; Tao, M.; Zhu, C.; Zhao, Q.; Zhang, J. Angew. Chem., Int. Ed. 2015, 54, 14853. (c) Zhou, W.; Chen, P.; Tao, M.; Su, X.; Zhao, Q.; Zhang, J. Chem. Commun. 2016, 52, 7612. (d) Chen, P.; Yue, Z.; Zhang, J.; Lv, X.; Wang, L.; Zhang, J. Angew. Chem., Int. Ed. 2016, 55, 13316. (e) Zhou, W.; Gao, L.; Tao, M.; Su, X.; Zhao, Q.; Zhang, J. Huaxue Xuebao 2016, 74, 800. (10) The X-ray crystal structure information is available at the Cambridge Crystallographic Data Centre (CCDC) under deposition no. CCDC 1500207 ((−)-3dn). (11) Details of experiments and results are summarized in the Supporting Information. (12) Some examples for phosphonium enolate as Brønsted base: (a) Wang, H.-Y.; Zhang, K.; Zheng, C.-W.; Chai, Z.; Cao, D.-D.; Zhang, J.-X.; Zhao, G. Angew. Chem., Int. Ed. 2015, 54, 1775. (b) Lou, Y.-P.; Zheng, C.-W.; Pan, R.-M.; Jin, Q.-W.; Zhao, G.; Li, Z. Org. Lett. 2015, 17, 688. (c) Wang, H.-Y.; Zheng, C.-W.; Chai, Z.; Zhang, J.-X.; Zhao, G. Nat. Commun. 2016, 7, 12720.
21672067), and the Changjiang Scholars and Innovative Research Team in University (PCSIRT) for financial support.
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DOI: 10.1021/acs.orglett.7b00489 Org. Lett. XXXX, XXX, XXX−XXX
