Letter pubs.acs.org/OrgLett
Palladium-Catalyzed [5 + 2] Cycloaddition of Vinyloxiranes with Sulfamate-Derived Cyclic Imines To Construct 1,3-Oxazepine Heterocycles Yang Wu, Chunhao Yuan, Chang Wang, Biming Mao, Hao Jia, Xing Gao, Jianning Liao, Feng Jiang, Leijie Zhou, Qijun Wang, and Hongchao Guo* Department of Applied Chemistry, China Agricultural University, Beijing 100193, P. R. China S Supporting Information *
ABSTRACT: Palladium-catalyzed [5 + 2] cycloaddition of 2-aryl-2vinyloxiranes with sulfamate-derived cyclic imines is described. The zwitterionic allylpalladium intermediates act as five-membered synthon to react with sulfamate-derived cyclic imines to furnish [5 + 2] cycloaddition, giving 1,3-oxazepine derivatives in moderate to excellent yields with excellent regioselectivities. heterocumulenes, activated olefins, and imines to afford fivemembered heterocyclic compounds (Scheme 1a).6 The
O
xazepines are seven-membered heterocyclic compounds incorporating nitrogen and oxygen in the ring. The 1,3oxazepines are a branch of oxazepines, which have been extensively studied due to their biological activity and their great importance in natural products.1 A considerable number of methods toward the synthesis of 1,3-oxazepine ring have been reported in recent years.2 However, the reported methods often suffer from drawbacks such as complicated substrate preparation, non-one-pot reaction, and frequently low yields. Therefore, new, simple, and efficient protocols for constructing oxazepine rings are still in great demand. Transition-metal-catalyzed cycloadditions are powerful tools for the convergent synthesis of diverse carbo- and heterocycles.3 As a type of readily accessible and versatile synthons, due to high reactivity of strained oxirane, vinyloxiranes were often employed as reaction partners in various metal-catalyzed cycloadditions for synthesis of heterocycles with new skeletons or structural features.4 These vinyloxiranes can work as two-,5 three-,6 four-,7 or five-membered8 synthons when reacting with a variety of unsaturated systems to form two bonds and achieve a cycloaddition reaction.4 In most cases, vinyloxiranes acted as a three-membered synthon to furnish a [3 + 2] cycloaddition to give five-membered heterocyclic compounds.6 The examples with the use of vinyl oxiranes as an oxygen-containing five-atom partner in a cycloaddition reaction are very limited.4 Zhang has made a great contribution in this area and reported elegant examples involving a rhodium-catalyzed [5 + 2] cycloaddition reaction of vinylic oxiranes with alkynes for diverse synthesis of a cyclic compound.8 Among the metal catalytic systems used for the cycloaddition reactions of vinylic oxiranes, palladium catalysis has been established as a reliable tool for the efficient construction of diverse cyclic compounds.9 Under palladium catalysis conditions, π-allylpalladium intermediates generated from vinyloxiranes exhibit diverse reactivity toward unsaturated systems.4 Generally, the π-allylpalladium intermediates worked as a threemembered synthon in a [3 + 2] cycloaddition with © 2017 American Chemical Society
Scheme 1. Palladium-Catalyzed Cycloaddition Reactions via Zwitterionic Allylpalladium Intermediates
reactions in which zwitterionic palladium intermediates served as five-membered synthons are rare. Essentially, zwitterionic palladium intermediates from vinyloxiranes are the same as those from vinylethylene carbonates.10 Most recently, with the use of zwitterionic palladium intermediates from vinylethylene carbonates as five-membered synthons, Zhao developed a palladium-catalyzed [5 + 4] cycloaddition of N-tosyl azadienes with substituted vinylethylene carbonates to produce ninemembered heterocycles.11 Herein, we describe palladiumcatalyzed [5 + 2] cycloadditions of vinyloxiranes with cyclic aldimines to give 1,3-oxazepine derivatives (Scheme 1b). Initially, the reaction of sulfamate-derived cyclic aldimine 1a and vinyloxirane 2a was chosen as model reaction for screening reaction conditions (Table 1). In the presence of 2.5 mol % of Pd2(dba)3·CHCl3 and 7.5 mol % of PPh3, the reaction was carried out in dichloromethane (DCM) at room temperature for 48 h. Unfortunately, no desired product was observed (entry 1). Various racemic bisphosphine ligands were next screened. To our delight, the [5 + 2] cycloaddition product could be obtained in every case when bisphosphine was used as a ligand (entries 2−8).12a Among these ligands, flexible Received: August 30, 2017 Published: November 20, 2017 6268
DOI: 10.1021/acs.orglett.7b02704 Org. Lett. 2017, 19, 6268−6271
Letter
Organic Letters Table 1. Optimization of Reaction Conditionsa
entry 1 2 3 4 5 6 7 8 9 10 11 12d
ligand PPh3 dppe dppp dppb dppbz dppf DPEphos Xantphos DPEphos DPEphos DPEphos DPEphos
solvent DCM DCM DCM DCM DCM DCM DCM DCM DCE THF toluene DCM
t (h) 48 72 72 48 72 48 24 48 24 24 24 24
yield (%)b e
NR 10 44 74 20 28 85 39 82 66 30 96
Table 2. Scope of the [5 + 2] Cycloaddition with Respect to Sulfamate-Derived Cyclic Imines 1a
[5 + 2]/[3 + 2]c − 5:2 5:3 5:6 10:1 25:1 25:1 20:1 25:1 20:1 20:1 25:1
a All reactions were carried out with 1a (0.1 mmol), 2a (0.16 mmol), Pd2(dba)3·CHCl3 (2.5 mol %), and ligand (7.5 mol %) in 1 mL of solvent at rt under indicated reaction conditions. bIsolated yield. c Determined by 1H NMR. d10 mol % TBAB (Tetrabutylammonium bromide) was added. eNo reaction occurred.
diphosphine DPEphos gave better catalytic results in terms of yield and regioselectivity (entry 7). The diphosphine Xantphos with a more rigid structure showed less efficiency in comparison with DPEphos (entry 8). With the use of DPEphos as ligand, other solvents such as 1,2-dichloroethane (DCE), tetrahydrofuran (THF), and toluene were also examined, but led to inferior results (entry 7 vs entries 9−11). Additionally, when the structurally related vinylethylene carbonate instead of 2a was used as the substrate under otherwise identical conditions, the corresponding product 3aa was obtained in 84% yield.12b Since Ooi reported that a phase transfer catalyst is beneficial to Pd-catalyzed cycloaddition reactions,10k to further improve the yield, 10 mol % tetrabutylammonium bromide (TBAB) was used and, interestingly, the yield was increased to 96% (entry 12). The attempt to develop an asymmetric variant of this reaction was not satisfactory, and up to 39% ee was obtained (see Supporting Information for details). With the optimized reaction conditions in hand, we applied this newly developed cycloaddition reaction to differently substituted sulfamate-derived cyclic imines (Table 2). As shown in Table 2, those substrates 1b−1k with electron-rich substituents on the aryl moiety performed the reaction well, giving the corresponding products 3ba−3ka in moderate to excellent yields with excellent regioselectivities (entries 2−11). Strangely, 6-MeO-substituted cyclic imine 1e displayed lower reactivity, giving the desired product 3ea in 54% yield with 20:1 regioselectivity (entry 5). The sulfamate-derived cyclic imines with electron-deficient substituents were also compatible substrates, undergoing the reaction to afford the products in moderate to high yields (entries 12−18). However, compared with those substrates with electron-rich substituents, the sulfamate-derived cyclic imines with electron-deficient sub-
a
All reactions were carried out with 1a (0.2 mmol), 2a (0.32 mmol), Pd2(dba)3·CHCl3 (2.5 mol %), DPEphos (7.5 mol %), and 10 mol % TBAB in 2 mL of CH2Cl2 at rt for corresponding time. bIsolated yield. c Determined by 1H NMR.
stituents gave the 1,3-oxazepine derivative in relatively lower yields (entries 2−11 vs 12−18). Two special substrates 1s and 1t could also carry out the reaction to afford the 1,3-oxazepine derivative 3sa and 3ta in 87% and 68% yield with excellent regioselectivities, respectively (entries 19, 20). The structure of the products was unequivocally determined through X-ray crystallographic data of the product 3ba.13 Attention was then turned to variation in substituents on the aromatic group of vinyloxiranes 2 (Table 3). Whether with an electron-donating or -withdrawing group, vinyloxiranes could be converted to the corresponding products (entries 1−6). Unfortunately, when the ortho-fluoro-substituted substrate 2c and meta-chloro-substituted substrate 2e were applied to the protocol, the reaction was quite sluggish and worked for 48 h to give the seven-membered cycloaddition products 3ac and 3ae in low yields. To explore the influence of the steric hindrance in the Pd-πallyl intermediate, unsubstituted vinyloxirane 4, which lacks an aryl group at the 2-position of the epoxide ring, was employed (Table 4). A quick screening (see Supporting Information) revealed that the standard reaction conditions established for aryl-substituted vinyloxirane were applicable to vinyloxirane 4. The reaction of the cyclic imine 1a and vinyloxirane 4 proceeded smoothly in the presence of a catalytic amount of 6269
DOI: 10.1021/acs.orglett.7b02704 Org. Lett. 2017, 19, 6268−6271
Letter
Organic Letters
excellent diastereoselectivity (>20:1) albeit with a 37% yield (entry 10). The relative configuration of the [3 + 2] cycloaddtion products was confirmed by single crystal X-ray analysis of the product 7b.13 We also performed a Pd-catalyzed [5 + 2] cycloaddition on a 4 mmol scale (0.72 g). As shown in Scheme 2, the reaction
Table 3. Scope of the [5 + 2] Cycloaddition with Respect to Vinyloxiranes 2a
entry
Ar
t (h)
3
yield (%)b
[5 + 2]/[3 + 2]c
1 2 3 4 5 6
3-MeOC6H4 (2b) 2-FC6H4 (2c) 4-FC6H4 (2d) 3-ClC6H4 (2e) 4-ClC6H4 (2f) 4-BrC6H4 (2g)
24 48 24 48 24 72
3ab 3ac 3ad 3ae 3af 3ag
70 25 75 48 71 61
20:1 14:1 25:1 20:1 25:1 25:1
Scheme 2. Synthesis on the Gram Scale and Further Transformation of the Cycloadduct
a
All reactions were carried out with 1a (0.2 mmol), 2a (0.32 mmol), Pd2(dba)3·CHCl3 (2.5 mol %), DPEphos (7.5 mol %), and 10 mol % TBAB in 2 mL of CH2Cl2 at rt for corresponding time. bIsolated yield. c Determined by 1H NMR.
Table 4. [3 + 2] Cycloaddition of Sulfamate-Derived Cyclic Imines 1 with Vinyloxiranesa
worked well under the standard reaction conditions to produce the product 3aa in 76% yield with excellent regiostereoselectivity, demonstrating the potential of scaling up the protocol. Furthermore, oxidation of the [5 + 2] products 3aa and 3ba with mCPBA under mild conditions gave the epoxides in excellent yields, albeit with moderate diastereoselectivities. The relative configuration of the epoxides was determined by X-ray crystallographic analysis of the epoxides 8ba and 8ba′.13 In summary, we have developed a Pd-catalyzed [5 + 2] cycloaddition between vinyloxiranes and sulfamate-derived cyclic imines, providing biologically interesting sulfamatefused 1,3-oxazepine derivatives in moderate to excellent yields with excellent regioselectivities. The cycloaddition reaction shows good functional group compatibility. Additionally, the catalyst system is commercially available, the reaction conditions are mild, and the reaction is operational on the gram scale, indicating that the current [5 + 2] cycloaddition could be a practical method in the synthesis of 1,3-oxazepine derivatives. When the unsubstituted vinyloxirane and alkylsubstituted vinyloxirane were used as a cycloaddition partner, [3 + 2] cycloaddition occurred to give oxazolidine derivatives in moderate to excellent yields.
a
All reactions were carried out with 1 (0.2 mmol), 4, 5, or 6 (0.30 mmol), Pd2(dba)3·CHCl3 (2.5 mol %), DPEphos (7.5 mol %), and 5 mol % TBAB in 2 mL of CH2Cl2 at rt for the corresponding time. b Isolated yield. cThe dr is >20:1, determined by 1H NMR analysis. d Entry 9, dr was 5:3.
Pd/DPEphos and TBAB in DCM at room temperature for 6 h to give the [3 + 2] cycloadduct 7a, not [5 + 2] cycloadduct, in 96% yield with excellent diastereoselectivity (entry 1). The [5 + 2] cycloadduct has not been observed in this reaction. About the regioselectivity, a plausible mechanism has been proposed in Supporting Information. In the [3 + 2] cycloaddition of vinyloxiranes, as illustrated in Table 4, both electron-rich and -deficient groups on the aryl ring of the cyclic imines 1 were well tolerated, the products 7b−7h were obtained in good to excellent yields with excellent regio- and diastereoselectities (entries 2−7). The substrate 1s bearing a naphthyl moiety also underwent the [3 + 2] cycloaddtion to give the product 7h without loss of efficiency and selectivity (entry 8). The reaction of methyl substituted vinyloxirane 5 afforded the product 7i in 92% yield with moderate diastereoselectivity (entry 9). The bulky cyclohexyl substituted vinyloxirane (6) underwent the cycloaddition reaction to give the [3 + 2] cycloadduct 7j with
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02704. Experimental procedure, characterization data, NMR spectra, X-ray crystallographic data (PDF) Crystallographic data for compound 3ba (CIF) Crystallographic data for compound 7b (CIF) Crystallographic data for compound 8ba (CIF) Crystallographic data for compound 8ba′ (CIF) 6270
DOI: 10.1021/acs.orglett.7b02704 Org. Lett. 2017, 19, 6268−6271
Letter
Organic Letters
■
Lett. 2011, 13, 864. (q) Coletti, A.; Whiteoak, C. J.; Conte, V.; Kleij, A. W. ChemCatChem 2012, 4, 1190. (r) Whiteoak, C. J.; Martin, E.; Belmonte, M. M.; Benet-Buchholz, J.; Kleij, A. W. Adv. Synth. Catal. 2012, 354, 469. (s) Anselmo, D.; Bocokić, V.; Decortes, A.; EscuderoAdán, E. C.; Benet-Buchholz, J.; Reek, J. N. H.; Kleij, A. W. Polyhedron 2012, 32, 49. (t) Yang, Y.; Hayashi, Y.; Fujii, Y.; Nagano, T.; Kita, Y.; Ohshima, T.; Okuda, J.; Mashima, K. Catal. Sci. Technol. 2012, 2, 509. (u) Ema, T.; Miyazaki, Y.; Koyama, S.; Yano, Y.; Sakai, T. Chem. Commun. 2012, 48, 4489. (v) Liu, Z.; Feng, X.; Du, H. Org. Lett. 2012, 14, 3154. (w) Wu, W.-Q.; Ding, C.-H.; Hou, X.-L. Synlett 2012, 23, 1035. (x) Lo, B.; Lam, S.; Wong, W.-T.; Chiu, P. Angew. Chem., Int. Ed. 2012, 51, 12120. (y) Lam, S.; Lo, B.; Wong, W.-T.; Chiu, P. Asian J. Org. Chem. 2012, 1, 30. (z) Ma, C.; Huang, Y.; Zhao, Y. ACS Catal. 2016, 6, 6408. (7) Liu, L. L.; Chiu, P. Chem. Commun. 2011, 47, 3416. (8) (a) Feng, J.-J.; Zhang, J. J. Am. Chem. Soc. 2011, 133, 7304. (b) Feng, J.-J.; Zhang, J. ACS Catal. 2017, 7, 1533. (9) (a) Muniz, K.; Martinez, C. J. Org. Chem. 2013, 78, 2168. (b) Arai, S.; Nishida, A. Synlett 2012, 23, 2880. (c) Guo, L.-N.; Duan, X.-H.; Liang, Y.-M. Acc. Chem. Res. 2011, 44, 111. (d) Larock, R. C. Top. Organomet. Chem. 2005, 14, 147. (e) de Meijere, A.; von Zezschwitz, P.; Brase, S. Acc. Chem. Res. 2005, 38, 413. (f) Guitian, E.; Perez, D.; Pena, D. Top. Organomet. Chem. 2005, 14, 109. (g) Tietze, L. F.; Ila, H.; Bell, H. P. Chem. Rev. 2004, 104, 3453. (10) For a review, see: (a) Khan, A.; Zhang, Y. J. Synlett 2015, 26, 853 For examples, see:. (b) Khan, A.; Zheng, R.; Kan, Y.; Ye, J.; Xing, J.; Zhang, Y. J. Angew. Chem., Int. Ed. 2014, 53, 6439. (c) Khan, A.; Yang, L.; Xu, J.; Jin, L. Y.; Zhang, Y. J. Angew. Chem., Int. Ed. 2014, 53, 11257. (d) Khan, A.; Xing, J.; Zhao, J.; Kan, Y.; Zhang, W.; Zhang, Y. J. Chem. - Eur. J. 2015, 21, 120. (e) Yang, L.; Khan, A.; Zheng, R. F.; Jin, L. Y.; Zhang, Y. J. Org. Lett. 2015, 17, 6230. For selected examples on zwitterionic allylpalladium intermediates from other substrates, see: (f) Guo, C.; Janssen-Müller, D.; Fleige, M.; Lerchen, A.; Daniliuc, C. G.; Glorius, F. J. Am. Chem. Soc. 2017, 139, 4443. (g) Guo, C.; Janssen-Müller, D.; Fleige, M.; Lerchen, A.; Daniliuc, C. G.; Glorius, F. J. Am. Chem. Soc. 2017, 139, 4443. (h) Guo, C.; Fleige, M.; JanssenMüller, D.; Daniliuc, C. G.; Glorius, F. J. Am. Chem. Soc. 2016, 138, 7840. (i) Wei, Y.; Lu, L.-Q.; Li, T.-R.; Feng, B.; Wang, Q.; Xiao, W.-J.; Alper, H. Angew. Chem., Int. Ed. 2016, 55, 2200. (j) Xu, C.-F.; Zheng, B.-H.; Suo, J.-J.; Ding, C.-H.; Hou, X.-L. Angew. Chem., Int. Ed. 2015, 54, 1604. (k) Ohmatsu, K.; Imagawa, N.; Ooi, T. Nat. Chem. 2013, 6, 47. (l) Ohmatsu, K.; Kawai, S.; Imagawa, N.; Ooi, T. ACS Catal. 2014, 4, 4304. (m) Li, T.-R.; Tan, F.; Lu, L.-Q.; Wei, Y.; Wang, Y.-N.; Liu, Y.-Y.; Yang, Q.-Q.; Chen, J.-R.; Shi, D.-Q.; Xiao, W.-J. Nat. Commun. 2014, 5, 5500. (n) Lowe, M. A.; Ostovar, M.; Ferrini, S.; Chen, C. C.; Lawrence, P. G.; Fontana, F.; Calabrese, A. A.; Aggarwal, V. K. Angew. Chem., Int. Ed. 2011, 50, 6370. (o) Fontana, F.; Chen, C. C.; Aggarwal, V. K. Org. Lett. 2011, 13, 3454. (p) Fontana, F.; Tron, G. C.; Barbero, N.; Ferrini, S.; Thomas, S. P.; Aggarwal, V. K. Chem. Commun. 2010, 46, 267. (q) Wang, C.; Tunge, J. A. J. Am. Chem. Soc. 2008, 130, 8118. (r) Shintani, R.; Murakami, M.; Hayashi, T. J. Am. Chem. Soc. 2007, 129, 12356. (s) Shintani, R.; Hayashi, T. J. Am. Chem. Soc. 2006, 128, 6330. (t) Wang, C.; Tunge, J. A. Org. Lett. 2006, 8, 3211. (u) Trost, B. M.; Fandrick, D. R. J. Am. Chem. Soc. 2003, 125, 11836. (11) Yang, L.-C.; Rong, Z.-Q.; Wang, Y.-N.; Tan, Z. Y.; Wang, M.; Zhao, Y. Angew. Chem., Int. Ed. 2017, 56, 2927. (12) (a) In ref 10e, Zhang reported that Pd-catalyzed cycloaddition of vinylethylene carbonates with sulfonylimines gave [3 + 2] cycloadducts. (b) In consideration of atom economy, we prefer to choose vinyloxiranes rather than vinylethylene carbonates as a substrate. (13) Crystallographic data for 3ba, 7b, 8ba, and 8ba′ have been deposited with the Cambridge Crystallographic Data Centre as deposition numbers CCDC 1569469, 1569468, 1569470, and 1569472, respectively.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hongchao Guo: 0000-0002-7356-4283 Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS This work is supported by the NSFC (21372256 and 21572264). REFERENCES
(1) For selected examples, see: (a) Abdel-Hafez, A. A.; Abdel-Wahab, B. A. Bioorg. Med. Chem. 2008, 16, 7983. (b) Mueller, R.; Rodriguez, A. L.; Dawson, E. S.; Butkiewicz, M.; Nguyen, T. T.; Oleszkiewicz, S.; Bleckmann, A.; Weaver, C. D.; Lindsley, C. W.; Conn, P. J.; Meiler, J. ACS Chem. Neurosci. 2010, 1, 288. (c) Martínez, W. R.; Militão, G. C. G.; da Silva, T. G.; Silva, R. O.; Menezes, P. H. RSC Adv. 2014, 4, 14715. (d) Martinez-Farina, C. F.; Jakeman, D. L. Chem. Commun. 2015, 51, 14617. (2) For selected examples, see: (a) Praly, J. P.; Di Stèfano, C. D.; Somsák, L. Tetrahedron: Asymmetry 2000, 11, 533. (b) Ma, C.; Liu, S.J.; Xin, L.; Falck, J. R.; Shin, D.-S. Tetrahedron 2006, 62, 9002. (c) Chiou, W.-H.; Mizutani, N.; Ojima, I. J. Org. Chem. 2007, 72, 1871. (d) Sridharan, V.; Maiti, S.; Menendez, J. C. J. Org. Chem. 2009, 74, 9365. (e) Pouliot, M.-F.; Angers, L.; Hamel, J.-D.; Paquin, J.-F. Tetrahedron Lett. 2012, 53, 4121. (f) Salih, N.; Salimon, J.; Yousif, E.; Hamed, A. Asian J. Chem. 2013, 25, 6748. (g) Abood, Z. H.; Hussein, M. M.; Shaheed, I. M. Asian J. Chem. 2015, 27, 3074. (h) Skvorcova, M.; Grigorjeva, L.; Jirgensons, A. Org. Lett. 2015, 17, 2902. (i) Abood, Z. H. Asian J. Chem. 2016, 28, 2582. (j) Mollo, M. C.; Orelli, L. R. Org. Lett. 2016, 18, 6116. (k) Shen, W. B.; Xiao, X. Y.; Sun, Q.; Zhou, B.; Zhu, X. Q.; Yan, J. Z.; Lu, X.; Ye, L. W. Angew. Chem., Int. Ed. 2017, 56, 605. (3) (a) Lautens, M.; Klute, W.; Tam, W. Chem. Rev. 1996, 96, 49. (b) Yet, L. Chem. Rev. 2000, 100, 2963. (c) Aubert, C.; Buisine, O.; Malacria, M. Chem. Rev. 2002, 102, 813. (d) Nakamura, I.; Yamamoto, Y. Chem. Rev. 2004, 104, 2127. (e) Hashimoto, T.; Maruoka, K. In Handbook of Cyclization Reactions; Ma, S., Ed.; Wiley-VCH: Weinheim, Germany, 2009; Chapter 3, pp 87−168. (4) (a) Olofsson, B.; Somfai, P. In Aziridines and Epoxides in Organic Synthesis; Yudin, A. K., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2006; p 315. (b) He, J.; Ling, J.; Chiu, P. Chem. Rev. 2014, 114, 8037. (c) Ilardi, E. A.; Njardarson, J. T. J. Org. Chem. 2013, 78, 9533. (5) (a) Pale, P.; Bouquant, J.; Chuche, J.; Carrupt, P. A.; Vogel, P. Tetrahedron 1994, 50, 8035. (b) Yıldırım, M.; Dürüst, Y. Tetrahedron 2011, 67, 3209. (c) Liu, H.; Liu, G.; Qiu, G.; Pu, S.; Wu, J. Tetrahedron 2013, 69, 1476. (6) (a) Trost, B. M.; Angle, S. R. J. Am. Chem. Soc. 1985, 107, 6123. (b) Trost, B. M.; Sudhakar, A. R. J. Am. Chem. Soc. 1987, 109, 3792. (c) Trost, B. M.; Sudhakar, A. R. J. Am. Chem. Soc. 1988, 110, 7933. (d) Ohno, M.; Mori, K.; Hattori, T.; Eguchi, S. J. Org. Chem. 1990, 55, 6086. (e) Larksarp, C.; Alper, H. J. Am. Chem. Soc. 1997, 119, 3709. (f) Shim, J.-G.; Yamamoto, Y. J. Org. Chem. 1998, 63, 3067. (g) Larksarp, C.; Alper, H. J. Org. Chem. 1998, 63, 6229. (h) Trost, B. M.; McEachern, E. J. J. Am. Chem. Soc. 1999, 121, 8649. (i) Shim, J.G.; Yamamoto, Y. Tetrahedron Lett. 1999, 40, 1053. (j) Shim, J.-G.; Yamamoto, Y. Heterocycles 2000, 52, 885. (k) Raghunath, M.; Zhang, X. Tetrahedron Lett. 2005, 46, 8213. (l) Davoust, M.; Cantagrel, F.; Metzner, P.; Brière, J.-F. Org. Biomol. Chem. 2008, 6, 1981. (m) Decortes, A.; Belmonte, M. M.; Benet-Buchholz, J.; Kleij, A. W. Chem. Commun. 2010, 46, 4580. (n) Sako, S.; Kurahashi, T.; Matsubara, S. Chem. Lett. 2011, 40, 808. (o) Shaghafi, M. B.; Grote, R. E.; Jarvo, E. R. Org. Lett. 2011, 13, 5188. (p) Lo, B.; Chiu, P. Org. 6271
DOI: 10.1021/acs.orglett.7b02704 Org. Lett. 2017, 19, 6268−6271
