Catalyst-Controlled Regioselective Acylation of β‑Ketoesters with α

Letter Cite This: Org. Lett. 2018, 20, 7278−7282

pubs.acs.org/OrgLett

Catalyst-Controlled Regioselective Acylation of β‑Ketoesters with α‑Diazo Ketones Induced by Visible Light Dan Liu,†,§ Wei Ding,†,§ Quan-Quan Zhou,† Yi Wei,† Liang-Qiu Lu,*,† and Wen-Jing Xiao*,†,‡ †

Hubei International Scientific and Technological Cooperation Base of Pesticide and Green Synthesis, Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, 152 Luoyu Road, Wuhan, Hubei 430079, P. R. China ‡ State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China

Org. Lett. 2018.20:7278-7282. Downloaded from pubs.acs.org by UNIV OF LEICESTER on 11/16/18. For personal use only.

S Supporting Information *

ABSTRACT: A catalyst-controlled acylation reaction of βketoesters was developed for the first time by combining visiblelight photoactivation with Lewis acid or base catalysis. By employing a NiCl2·glyme complex with a bis(oxazoline) ligand as the Lewis acid catalyst, C-acylation products are exclusively achieved, while utilizing pyridine or DABCO as the Lewis base catalyst affords O-acylation products with complete regioselectivity. A range of β-ketoesters with satisfactory structural diversity were suitable for this transformation, demonstrating the functional group compatibility of the method, which was attributed to the mild reaction conditions. This success is heavily built upon the visible-light-induced Wolff rearrangement and the unique catalytic activation modes, and thus, this work significantly expands the applications of ketene chemistry. regiocontrollable acylation reaction of β-ketoesters that is built upon the classic Wolff rearrangement. Notably, this reaction, which can be used to generate reactive ketenes from readily available α-diazo ketones, has been known since 1902;8 however, its synthetic potential in conjunction with visiblelight irradiation at room temperature is underexplored.9 Attracted by the green and mild conditions of this transformation, we have recently realized a Wolff rearrangement/ decarboxylative [4 + 2] cycloaddition cascade of vinyl carbamates with α-diazo ketones by merging visible-light photoactivation with palladium catalysis.7d One of the most important inspirations for this method is the possible tolerance of both α-diazo ketones and the generated ketenes to transition-metal catalysts, which are believed to promote the facile decomposition of these two species.10,11 Thus, as depicted in Scheme 1b, we envisioned that LA-chelated enolate intermediate B would react with ketene A to smoothly deliver the C-acylation product. However, the utilization of a LB catalyst would result in the nucleophilic addition to ketene A, and generated zwitterionic immediate C could abstract a proton from a β-ketoester.12 Resulting transient acylated species D could react with the generated enolate intermediate E to provide the enol ester product through a selective Oacylation process. Building upon our recent experience with the transformation of β-ketoesters7c,e,13 through synergetic visible-light photocatalysis and nickel catalysis, we began our study of C-acylation

he acetylation of β-ketoesters is undoubtedly a powerful tool for installing carbon chains and increasing molecular complexity in organic chemistry, and this technique has been widely used in the synthesis of natural products, pharmaceuticals, and agrochemical agents.1 The intrinsic problem of this transformation is the challenging regioselectivity, i.e., controlling C-acylation over O-acylation of the enolate intermediate.2 Generally, enol esters are the major product when β-ketoesters are treated with acyl chlorides and stoichiometric organic bases, such as NEt3 or pyridine (Py) (Scheme 1a, left).3 In contrast, to switch the regioselectivity, stoichiometric Na or Mg metals,4a,b sodium hydride,4c magnesium ethylate,4d or a combination of pyridine and MgCl24e are typically used because alkaline earth metal ions are believed to help improve the selectivity for C-acylation via chelation between the metal ions and enolates.2b Despite the effectiveness of the existing protocols, from the point of view of green synthetic chemistry, the search for new methods that avoid the use of stoichiometric bases with concomitantly substantial salt waste is highly desirable. Herein, we describe a catalyst-controlled, regio-divergent acylation of β-ketoesters with α-diazo ketones by combining visible-light photoactivation with Lewis acid or base catalysis. Both the C-acylation and O-acylation processes can be realized in high efficiency and selectivity under mild and environmentally friendly conditions (Scheme 1b). Recently, visible-light photoactivation has received increasing attention from the synthetic community due to the advantages of mild reaction conditions and good compatibility with metal and organocatalysis.5,6 With our focus on organic photochemical syntheses,7 in this work, we devised a

T

© 2018 American Chemical Society

Received: October 6, 2018 Published: November 2, 2018 7278

DOI: 10.1021/acs.orglett.8b03189 Org. Lett. 2018, 20, 7278−7282

Letter

Organic Letters Scheme 1. Regioselective Acylation Reactions of βKetoesters

Table 1. Condition Optimization for the LA-Catalyzed CAcylation Reactiona

entry

deviation from “standard conditions”

yieldb (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

none no NiCl2·glyme no L1 in the dark ZnCl2, instead of NiCl2·glyme CuCl2, instead of NiCl2·glyme CoCl2, instead of NiCl2·glyme FeCl3, instead of NiCl2·glyme MeCN, instead of CHCl3 THF, instead of CHCl3 toluene, instead of CHCl3 L2, instead of L1 L3, instead of L1 L4, instead of L1 2,2-diphenylacetyl chloride and NEt3, instead of 2a, in the dark

99 (97)c 3 8 22 22 0 85 89 51 11 39 54 0 16 10

Standard conditions A: 1a (0.1 mmol), 2a (0.2 mmol), NiCl2·glyme (0.01 mmol, 10 mol %), L1 (0.01 mmol, 10 mol %), and CHCl3 (1.0 mL) at room temperature under an argon atmosphere and then irradiation of 2 × 3 W blue LEDs. bDetermined by 1H NMR analysis using dimethyl sulfone as an internal standard. cIsolated yield in parentheses. a

with the developed nickel/oxazoline system. Initially, we performed this reaction using 1-indanone-derived β-ketoester 1a and 1,2-diphenyl α-diazo ketone 2a as the model substrates in the presence of NiCl2·glyme (10 mol %) and bis(oxazoline) ligand L1 (10 mol %). Indeed, we found that the desired Cacylation product 3aa was generated in an excellent yield in CHCl3 under the irradiation of 2 × 3 W blue LEDs at room temperature (Table 1, entry 1, 99% yield). The results of the control experiments show that the nickel catalyst, ligand, and visible light are all essential for this transformation (Table 1, entries 2−4). Replacing NiCl2·glyme with other Lewis acids (except CuCl2) can also provide the desired product, albeit in various yields (Table 1, entries 5−8).14 When CH3CN, THF, and toluene were employed as the reaction media instead of CHCl3, notable decreases in the yield were observed (Table 1, entries 9−11). In addition, other nitrogen-containing ligands, such as pybox ligand L2, bipyridine ligand L3, and phenanthroline L4, were evaluated, but they were less effective than ligand L1 (Table 1, entries 12−14). On the other hand, although acyl chlorides are an efficient acyl source for the Cacylation of β-ketoesters under the assistance of stoichiometric base and alkaline earth metal ions, when the reaction was performed with diphenylacetyl chloride, it provided the desired product in an extremely low yield in our nickel-catalyzed system (Table 1, entry 15). Having determined the optimal conditions for the visiblelight-induced, Ni-catalyzed C-acylation reaction of β-ketoesters, we began to probe the generality of the present method. As highlighted in Scheme 2, a wide range of β-ketoesters bearing electron-withdrawing (3ba−da and 3ga) or electron-donating groups (3ea and 3fa) on the benzene ring were suitable for this transformation. The substituent position had no obvious effect on the reaction efficiency, and the corresponding acylation products were accessed in high yields (3da−ga, 91−99% yields). The disubstituted substrate (3ha) and the 2,3-

naphthaline-derived substrate (3ia) were also tolerated by the current conditions, and the desired products were afforded in good yields. Moreover, the C-acylation reaction of a heteroaryl-substituted β-ketoester and an aryl-substituted βketoamide proceeded smoothly and provided the corresponding products in 99% yield (3ja and 3ka). The replacement of the methyl ester with a sterically bulky 1-adamantyl ester was examined, and the substrates were converted into the corresponding acylation products in quantitative yields (3la, 3ma, and 3na, 99% yield). Further studies indicated that this catalytic acylation reaction showed a satisfactory functional group compatibility attributable to the mild conditions. For instance, the introduction of alkynyl and vinyl groups, which are versatile for further derivatization, into the β-ketoester substrates had no impact on the reaction efficiency (3ma−oa, 90%−99% yields). Aliphatic cyclic β-ketoesters and exocyclic alkenyl-substituted β-ketoesters could be employed as effective substrates, producing the desired products in high yields (3pa and 3qa, 99% yield and 88% yield). Next, the scope of other reaction partners, α-diazoketones, was investigated under the standard conditions (Scheme 3). First, in addition to biphenyl-substituted α-diazoketones, a variety of monoaryl-substituted substrates were examined. The reaction efficiency was not substantially influenced by variations in either the electronic demands or the substituent position on the benzene ring, and the desired products were delivered in good yields (3ab−al, 60−82% yields). To expand the scope of this reaction further, more challenging substrates, 7279

DOI: 10.1021/acs.orglett.8b03189 Org. Lett. 2018, 20, 7278−7282

Letter

Organic Letters Scheme 2. Scope of β-Ketoesters for the LA-Catalyzed CAcylationab

reactive to be purified, and an in situ procedure starting from acyl chlorides and a stoichiometric base is usually employed to prepare these materials. Obviously, the O-acylation reaction would be a strongly competitive process if this thermal method is adopted.15 As exemplified in Scheme 3, the methyl- or tertbutyl-substituted substrates can successfully participate in this reaction, affording the corresponding products in good yields (3am, 87% yield; 3an, 72% yield). Furthermore, the success of the visible-light photoactivation/nickel catalysis strategy can also be extended to monosubstituted α-diazoketones bearing an alkenyl group as well as methyl- and phenyl-substituted αdiazoketones (3ao, 67% yield; 3ap, 62% yield, 1:1 dr). Subsequently, we turned our attention to investigating the related O-acylation reaction of β-ketoesters with α-diazo ketones. Simply replacing the nickel catalyst with a Lewis base catalyst, such as pyridine or DABCO, provided the optimal conditions for this process. As highlighted in Scheme 4, representative substrates were proven effective under the Scheme 4. Representative Examples of the LB-Catalyzed OAcylationab

a Standard conditions A: 1a (0.1 mmol), 2a (0.2 mmol), NiCl2·glyme (0.01 mmol, 10 mol %), L1 (0.01 mmol, 10 mol %), and CHCl3 (1.0 mL) at room temperature under an argon atmosphere and then irradiation of 2 × 3 W blue LEDs. bIsolated yield.

Scheme 3. Scope of α-Diazoketones for the LA-Catalyzed CAcylationab

a

Standard conditions B: 1a (0.2 mmol), 2a (0.4 mmol), pyridine (0.01 mmol, 10 mol %), CHCl3 (2.0 mL) at room temperature under an argon atmosphere and then irradiation of 2 × 3 W blue LEDs. b Isolated yield. cUsing DABCO as the catalyst. DABCO: 1,4diazabicyclo(2.2.2) octane.

reaction conditions involving visible-light irradiation and LB catalysis. For example, with respect to the aromatic βketoesters, the electronic properties and the position of the substituents on the benzene ring did not substantially affect the reaction efficiency, and the desired O-acylation products were achieved in good yields (4aa, 4da, 4fa, and 4ha, 76−89% yields). In addition, the heteroaryl-incorporated β-ketoester (4ja) and aryl-substituted β-ketoamide (4ka), as well as the aliphatic β-ketoester (4pa and 4sa) and exocyclic alkenylsubstituted β-ketoester (4qa), could be successfully utilized as effective substrates, affording the corresponding products in 67−92% yields. Additionally, many other α-diazo ketones were found to be feasible for the O-acylation. For example, α-diazo ketones having monoaryl, monoalkyl, and monoalkenyl substituents or both aryl and alkyl substituents were well

Standard conditions A: 1a (0.1 mmol), 2a (0.2 mmol), NiCl2·glyme (0.01 mmol, 10 mol %), L1 (0.01 mmol, 10 mol %), CHCl3 (1.0 mL) at room temperature under argon atmosphere and then irradiation of 2 × 3 W blue LEDs. bIsolated yield. cThe dr value was 1:1 determined by 1H NMR. a

including monoalkyl-substituted α-diazoketones, were tested. Notably, the corresponding ketene intermediates were too 7280

DOI: 10.1021/acs.orglett.8b03189 Org. Lett. 2018, 20, 7278−7282

Organic Letters

■

tolerated by the optimal reaction conditions and provided the corresponding enol ester products in good yields (4ab, 4am, 4ao, and 4ap, 62−87% yields). Furthermore, we performed two gram-scale acylation reactions to demonstrate the synthetic utility of the method. Subjecting β-ketoester 1a and α-diazo ketone 2a to standard conditions A or B successfully provided the C-acylation and Oacylation products in high yields and with complete regioselectivities (Scheme 5a). Additionally, we subjected

Letter

AUTHOR INFORMATION

Corresponding Authors

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

Liang-Qiu Lu: 0000-0003-2177-4729 Wen-Jing Xiao: 0000-0002-9318-6021 Author Contributions §

D.L. and W.D. contributed equally to this work.

Scheme 5. Synthetic Utility of This Method

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful to the National Science Foundation of China (No. 21822103, 21820102003, 21772052, 21772053, 21572074, and 21472057), the Program of Introducing Talents of Discipline to Universities of China (111 Program, B17019), the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (NO. 201422), and the Natural Science Foundation of Hubei Province (2017AHB047) for support of this research.

■

(1) (a) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, Part B: Reactions and Synthesis, 5th ed.; Springer, 2007. (b) Warren, S.; Wyatt, P. Organic Synthesis: The Disconnection Approach The Disconnection Approach, 2nd ed.; Wiley-Blackwell, 2008. (2) (a) Ferris, J. P.; Sullivan, C. E.; Wright, B. G. J. Org. Chem. 1964, 29, 87. (b) Ferris, J. P.; Wright, B. G.; Crawford, C. C. J. Org. Chem. 1965, 30, 2367. (c) Rhoads, S. J.; Hasbrouck, R. W. Tetrahedron 1966, 22, 3557. (3) For selected examples, see: (a) Rozzell, J. D.; Benner, S. A. J. Am. Chem. Soc. 1984, 106, 4937. (b) Mohrig, J. R.; Dabora, S. L.; Foster, T. F.; Schultz, S. C. J. Org. Chem. 1984, 49, 5179. (c) Mansour, T. S.; Evans, C. A. Synth. Commun. 1990, 20, 773. (d) Panda, N.; Mishra, P.; Mattan, I. J. Org. Chem. 2016, 81, 1047. (4) For selected examples, see: (a) De Zoysa, G. H.; Cameron, A. J.; Hegde, V. V.; Raghothama, S.; Sarojini, V. J. Med. Chem. 2015, 58, 625. (b) Spassow, A. Org. Synth. 1941, 21, 46. (c) Meyer, W. L.; Brannon, M. J.; Burgos, C. da G.; Goodwin, T. E.; Howard, R. W. J. Org. Chem. 1985, 50, 438. (d) Krueckert, K.; Flachsbarth, B.; Schulz, S.; Hentschel, U.; Weldon, P. J. J. Nat. Prod. 2006, 69, 863. (e) Neufeind, S.; Huelsken, N.; Neudoerfl, J.-M.; Schloerer, N.; Schmalz, H.-G. Chem. - Eur. J. 2011, 17, 2633. (5) Selected reviews on visible light photocatalysis, see: (a) Narayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40, 102. (b) Xuan, J.; Xiao, W.-J. Angew. Chem., Int. Ed. 2012, 51, 6828. (c) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322. (d) Schultz, D. M.; Yoon, T. P. Science 2014, 343, 1239176. (e) Ravelli, D.; Protti, S.; Fagnoni, M. Chem. Rev. 2016, 116, 9850. (f) Liu, Q.; Wu, L.-Z. Natl. Sci. Rev. 2017, 4, 359. (g) Marzo, L.; Pagire, S.; Reiser, O.; König, B. Angew. Chem., Int. Ed. 2018, 57, 10034. (6) Selected reviews on the dual catalysis merging visible-light photocatalysis with metal or organocatalysis; see: (a) Hopkinson, M. N.; Sahoo, B.; Li, J.-L.; Glorius, F. Chem. - Eur. J. 2014, 20, 3874. (b) Skubi, K. L.; Blum, T. R.; Yoon, T. P. Chem. Rev. 2016, 116, 10035. (c) Twilton, J.; Le, C. C.; Zhang, P.; Shaw, M. H.; Evans, R. W.; MacMillan, D. W. C. Nat. Rev. Chem. 2017, 1, 52. (7) Recent work from our group on visible-light photocatalysis: (a) Chen, J.-R.; Hu, X.-Q.; Lu, L.-Q.; Xiao, W.-J. Acc. Chem. Res. 2016, 49, 1911. (b) Liu, Y.-Y.; Yu, X.-Y.; Chen, J.-R.; Qiao, M.-M.; Qi, X.-T.; Shi, D.-Q.; Xiao, W.-J. Angew. Chem., Int. Ed. 2017, 56, 9527. (c) Ding, W.; Lu, L.-Q.; Zhou, Q.-Q.; Wei, Y.; Chen, J.-R.; Xiao, W.-J.

structurally complex β-ketoester 1r, which was derived from the pharmaceutical agent estrone 3-methyl ether,16 to the standard acylation conditions; impressively, the corresponding products 3ra and 4ra were obtained in 89% and 78% yields, respectively (Scheme 5b). These two transformations provide new protocols for the structural modification of complex molecules, which lay the foundation for a search for new compounds with potential biological activities. In conclusion, we have successfully developed a visible-lightinduced, regio-divergent acylation reaction of β-ketoesters with α-diazoketones. In particular, employing Lewis acid catalysts to activate the β-ketoesters or Lewis base catalysts to activate the photogenerated ketene intermediates promotes selective Cacylation and O-acylation processes with high reaction efficiency and controllable regioselectivity. The satisfactory product diversity and functional group compatibility, mild and green conditions, and user-friendly operation highlight the significance and practicality of the present protocols. Explorations of additional applications of reactive ketenes through visible light-induced Wolff rearrangements are in progress in our laboratory.

■

REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03189. Detailed experimental procedures and full spectroscopic data for all compounds (PDF) 7281

DOI: 10.1021/acs.orglett.8b03189 Org. Lett. 2018, 20, 7278−7282

Letter

Organic Letters J. Am. Chem. Soc. 2017, 139, 63. (d) Li, M.-M.; Wei, Y.; Liu, J.; Chen, H.-W.; Lu, L.-Q.; Xiao, W.-J. J. Am. Chem. Soc. 2017, 139, 14707. (e) Liu, J.; Ding, W.; Zhou, Q.-Q.; Liu, D.; Lu, L.-Q.; Xiao, W.-J. Org. Lett. 2018, 20, 461. (f) Yu, X.-Y.; Chen, J.-R.; Wang, P.-Z.; Yang, M.N.; Liang, D.; Xiao, W.-J. Angew. Chem., Int. Ed. 2018, 57, 738. (8) For the pioneering work, see: (a) Horner, L.; Spietschka, E. Chem. Ber. 1952, 85, 225. For selected reviews and books, see: (b) Kirmse, W. Eur. J. Org. Chem. 2002, 2002, 2193. (c) Kurti, L.; Czako, B. Strategic Applications of Named Reactions in Organic Synthesis; Elsevier: Amsterdam, 2005; pp 494−495. (d) Coquerel, Y.; Rodriguez, J. The Wolff Rearrangement: Tactics, Strategies and Recent Applications in Organic Synthesis. In Molecular Rearrangements in Organic Synthesis; Rojas, C. M., Ed.; Wiley, 2015; Chapter 3, pp 59−84. (9) For the recent mechanism study on photoinduced Wolff rearrangement, see: (a) Chiang, Y.; Gaplovsky, M.; Kresge, A. J.; Leung, K. H.; Ley, C.; Mac, M.; Persy, G.; Phillips, D. L.; Popik, V. V.; Roedig, C.; Wirz, J.; Zhu, Y. J. Am. Chem. Soc. 2003, 125, 12872. (b) Burdzinski, G. T.; Wang, J.; Gustafson, T. L.; Platz, M. S. J. Am. Chem. Soc. 2008, 130, 3746. (c) Wang, J.; Burdzinski, G.; Kubicki, J.; Platz, M. S. J. Am. Chem. Soc. 2008, 130, 11195 For recent applications of visible-light-induced Wolff rearrangement, see: . (d) Vaske, Y. S. M.; Mahoney, M. E.; Konopelski, J. P.; Rogow, D. L.; McDonald, W. J. J. Am. Chem. Soc. 2010, 132, 11379. (e) Knowles, J. P.; Elliott, L. D.; Booker-Milburn, K. I. Beilstein J. Org. Chem. 2012, 8, 2025. (f) Pinho, V. D.; Gutmann, B.; Kappe, C. O. RSC Adv. 2014, 4, 37419. (g) Bernardim, B.; Hardman-Baldwin, A. M.; Burtoloso, A. C. B. RSC Adv. 2015, 5, 13311. See also ref 7d. (10) For selected examples, see: (a) Xiao, Q.; Zhang, Y.; Wang, J.-B. Acc. Chem. Res. 2013, 46, 236. (b) Xia, Y.; Qiu, D.; Wang, J.-B. Chem. Rev. 2017, 117, 13810. (c) Pang, Y.; He, Q.; Li, Z.-Q.; Yang, J.-M.; Yu, J.-H.; Zhu, S.-F.; Zhou, Q.-L. J. Am. Chem. Soc. 2018, 140, 10663. (d) Liu, Z.-H.; Sivaguru, P.; Zanoni, G.; Anderson, E. A.; Bi, X.-H. Angew. Chem., Int. Ed. 2018, 57, 8927. (11) For selected examples, see: (a) Geoffroy, G. L.; Bassner, S. L. Adv. Organomet. Chem. 1988, 28, 1. (b) Wack, H.; Drury, W. J.; Taggi, A. E.; Ferraris, D.; Lectka, T. Org. Lett. 1999, 1, 1985. (c) Kumar, P.; Troast, D. M.; Cella, R.; Louie, J. J. Am. Chem. Soc. 2011, 133, 7719. (12) For selected examples, see: (a) Taggi, A. E.; Hafez, A. M.; Wack, H.; Young, B.; Drury, W. J.; Lectka, T. J. Am. Chem. Soc. 2000, 122, 7831. (b) Hodous, B. L.; Fu, G. C. J. Am. Chem. Soc. 2002, 124, 1578. (c) Huang, X.-L.; He, L.; Shao, P.-L.; Ye, S. Angew. Chem., Int. Ed. 2009, 48, 192. (d) Ibrahim, A. A.; Wei, P.-H.; Harzmann, G. D.; Kerrigan, N. J. J. Org. Chem. 2010, 75, 7901. (13) For recent work on the transformations of ketoesters, see: (a) Govender, T.; Arvidsson, P. I.; Maguire, G. E. M.; Kruger, H. G.; Naicker, T. Chem. Rev. 2016, 116, 9375. (b) Wozniak, L.; Murphy, J. J.; Melchiorre, P. J. Am. Chem. Soc. 2015, 137, 5678. (c) Zhu, Y.-B.; Zhang, L.; Luo, S.-Z. J. Am. Chem. Soc. 2014, 136, 14642. (d) Deng, Q.-H.; Wadepohl, H.; Gade, L. H. J. Am. Chem. Soc. 2012, 134, 10769. (14) See the Supporting Information for more details of condition optimizations. (15) Treatment of the diphenylacetyl chlorides or propionyl chloride with β-ketoester 1a under the standard conditions A in the presence of 2.0 equiv of NEt3 provided the enol esters as the major products. See the details in the Supporting Information. (16) Paquette, L. A.; Dahnke, K.; Doyon, J.; He, W.; Wyant, K.; Friedrich, D. J. Org. Chem. 1991, 56, 6199.

7282

DOI: 10.1021/acs.orglett.8b03189 Org. Lett. 2018, 20, 7278−7282