Letter Cite This: Org. Lett. 2018, 20, 1070−1073
pubs.acs.org/OrgLett
Asymmetric Construction of Fluoroalkyl Tertiary Alcohols through a Three-Component Reaction of (Bpin)2, 1,3-Enynes, and Fluoroalkyl Ketones Catalyzed by a Copper(I) Complex Xu-Cheng Gan,† Qi Zhang,†,‡ Xue-Shun Jia,*,‡ and Liang Yin*,† †
CAS Key Laboratory of Synthetic Chemistry of Natural Substances, Centre for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China ‡ Department of Chemistry, Shanghai University, 99 Shangda Road, Shanghai, 200444, China S Supporting Information *
ABSTRACT: An asymmetric three-component reaction of Bis(pinacolato)diboron ((BPin)2), 1,3-enynes, and fluoroalkyl ketones was carried out by using a copper(I)-Ph-BPE complex as the catalyst, which afforded a series of chiral fluoroalkyl diols in good to high yield, good to high diastereoselectivity, and high enantioselectivity after an oxidative workup. The reaction exhibits advantages that include a broad substrate scope, high functional group compatibility, high stereoselectivity, and an easy reaction protocol. The synthetic utility of the reaction was showcased by several transformations. value of the α-protons of trifluoromethyl alkyl ketones or the γprotons of α,β-unsaturated ones lower, which leads to the generation of enolates or dienolates and occurrence of side reactions and deactivation of the basic catalyst. Therefore, many reported catalytic asymmetric reactions involving trifluoromethyl ketones suffered from moderate enantioselectivity.13 1,3-Enynes may react through several reaction pathways in the presence of nucleophiles,16 because both the triple bond and the double bond can engage a nucleophile. Recently, 1,3-enynes were employed in three-component reactions to construct complex molecules by means of the reactivity of the conjugated double bond.17 Hoveyda and co-workers successfully developed a three-component diastereo- and enantioselective reaction of 1,3-enynes, bis(pinacolato)diboron and aldehydes catalyzed by the complex derived from CuCl and a novel bis-phosphine (Scheme 1).18 The Buchwald group also disclosed an elegant copper(I)-catalyzed three-component reaction, consisting of 1,3enynes, (MeO)2MeSiH, and ketones, which led to the preparation of an array of chiral homopropargyl alcohols.19 To the best of our knowledge, there are no reports on the three-component reaction of trifluoromethyl ketones with 1,3enynes, possibly due to the above-mentioned difficulties encountered in employing these electrophiles in asymmetric catalysis. Herein, we describe a catalytic asymmetric threecomponent reaction of 1,3-enynes, bis(pinacolato)diboron, and trifluoromethyl ketones, as well as perfluoroalkyl ketones. This reaction exhibits advantages that include a broad substrate scope,
ntroduction of fluorine atoms or fluorinated substituents to organic molecules is currently a routine strategy to modify the physical, chemical, and biological properties of chemical leads.1 It is reported that in most recent years about 30% of pharmaceuticals contain at least one fluorine atom.2 Among the various fluorinated substituents,3 the trifluoromethyl group (CF3) has attracted the most attention from the chemical community due to its ubiquitous properties, such as high electronegativity, high electron density, steric hindrance, hydrophobic character, and metabolic stability. CF3-containing compounds have found widespread application in biological,1 medicinal,2 agrochemical,4 and material chemistry.5 In view of the prevalence of chiral trifluoromethylated stereogenic carbon centers in biologically active compounds, there is continuing interest in the development of highly efficient methods for the asymmetric synthesis of chiral CF3-containing molecules.6 Essentially there are two pathways to construct chiral CF3containing molecules. One is to introduce a trifluoromethyl group into molecules via asymmetric nucleophilic, electrophilic, or radical trifluoromethylation.7−9 The other is to transform CF3containing molecules to chiral ones.10−12 Due to their ready availability, one of the most popular strategies is to employ trifluoromethyl ketones as a prochiral reaction partner.13,14 However, three reasons make such catalytic asymmetric reactions challenging.15 First, the steric hindrance of CF3 is similar to an isopropyl group,6a which renders the enantiotopic facial differentiation nontrivial. Second, trifluoromethyl ketones are activated toward addition reactions, which makes it challenging to achieve high enantioselectivity in catalytic asymmetric reactions, because uncatalyzed and nonenantioselective pathways can occur with competitive rates. Third, the high electronegativity of the trifluoromethyl group makes the pKa
I
© 2018 American Chemical Society
Received: December 27, 2017 Published: February 8, 2018 1070
DOI: 10.1021/acs.orglett.7b04039 Org. Lett. 2018, 20, 1070−1073
Letter
Organic Letters
phenone with various substituents, KOMe was employed as the choice of base in the following substrate investigation. Having established the optimal reaction conditions, the substrate scope of trifluoromethyl ketones was evaluated (Scheme 2). As for the trifluoroacetophenone, the position of
Scheme 1. Catalytic Asymmetric Three-Component Reactions with 1,3-Enynes
Scheme 2. Substrate Scope of Trifluoromethyl Ketonesa
high functional group compatibility, an easy reaction protocol, and high stereoselectivity. We commenced investigation of the three-component reaction of 1,3-enyne (1a), bis(pinacolato)diboron (2), and trifluoroacetophenone (3a) in the presence of a copper(I) catalyst (Table 1). The BPin adduct was transformed to the Table 1. Optimization of Reaction Conditionsa
entry
ligand
base
total yield (%)b
drc
ee (%)c
1 2 3 4 5 6 7 8 9 10 11 12 13 14d 15d,e
(R)-BINAP (R)-BINAP (R)-TOL-BIANP (R)-SEGPHOS (R)-DM-SEGPHOS (R)-DIFLUORPHOS (R)-DTBM-SEGPHOS (R,R)-QUINOXP* (R)-(S)-JOSIPHOS (R,RP)-TANIAPHOS (R,R)-Ph-BPE (R,R)-Ph-BPE (R,R)-Ph-BPE (R,R)-Ph-BPE (R,R)-Ph-BPE
LiOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu KOtBu LiOMe NaOMe KOMe
85 85 86 87 86 84 88 78 71 89 87 87 84 87 91
8/1 7/1 6/1 9/1 10/1 8/1 12/1 6/1 6/1 11/1 8/1 7/1 10/1 14/1 12/1
0 6 18 13 44 9 −55 −13 0 −46 81 86 96 99 98
a
1a (0.2 mmol), 2 (0.3 mmol), 3 (0.3 mmol). Isolated yield reported. Diastereo- and enantioselectivity determined by chiral-stationary-phase HPLC analysis. b10 mol % KOMe used, and 200 mol % MeOH added.
methyl on the phenyl ring did not have a significant effect on the yield and enantioselectivity (entries 2−4). However, orthomethyl trifluoroacetophenone afforded the product in lower diastereoselectivity. Both electron-donating groups (including methyl, methoxyl, and dimethyl amino) and electron-withdrawing groups (including halide, trifluoromethyl, and ester) did not affect the reaction results (entries 2−12). Heteroaromatic trifluoromethyl ketones, as well as naphthyl trifluoromethyl ketones, were also competent substrates to give the diols in excellent yield, good to excellent diastereoselectivity, and excellent enantioselectivity (entries 13−16). It is noteworthy that an α,β-unsaturated trifluoromethyl ketone (3q, 3r) was also a suitable reaction partner to generate the product without compromising both yield and stereoselectivity (entries 17−18). The preexisting chiral center did not disturb both diastereo- and enantioselectivity, albeit the product 4s was obtained in moderate yield, possibly due to the presence of acidic γ-protons (entry 19). However, in the reaction of phenylethynyl trifluoromethyl ketone (3t), the corresponding product 4t was not obtained. Decreasing the amount of KOMe to 10 mol % and adding 200 mol % MeOH as an additive led to the formation of 4t in 60% yield, 10/1 dr, and 91% ee. The absolute configurations of two newly formed chiral centers were unambiguously assigned to S (trisubstituted carbon center) and S (tetrasubstituted carbon center) by X-ray crystallography
a
1a (0.1 mmol), 2 (0.12 mmol), 3a (0.15 mmol). bIsolated yield. Determined by chiral stationary-phase HPLC analysis. d5 mol % Cu(CH3CN)4PF6, 6 mol % (R,R)-Ph-BPE. e1.5 equiv of 2 and 1.5 equiv of base employed.
c
corresponding diol 4a by NaBO3·4H2O mediated oxidation for ease of handling. With NaOtBu as base, the reaction catalyzed by the complex derived from Cu(CH3CN)4PF6 and (R)-BINAP afforded the diol in 85% yield, 7/1 dr, and 6% ee. Detailed screening of commercially available chiral bis-phosphines identified (R,R)-Ph-BPE as the best, as the diol 4a was obtained in 87% yield, 8/1 dr, and 81% ee. Since the base was reported to have a profound effect on the enantioselectivity in the reactions of bis(pinacolato)diboron,20 the study of base was performed. NaOMe and KOMe outperformed to give the diol 4a in ≥98% ee and >10/1 dr. Further efforts to increase the diastereoselectivity through screening the solvents were fruitless. Moreover, a screening of copper sources proved nonproductive. Due to the inferior performance of NaOMe in the cases of trifluoroaceto1071
DOI: 10.1021/acs.orglett.7b04039 Org. Lett. 2018, 20, 1070−1073
Letter
Organic Letters
of chiral perfluoroalkyl diols 7a−7d decreased, possibly due to the decreasing reactivity caused by the increasing steric hindrance. Fortunately, the enantioselectivity of products was maintained at as high as ≥97% ee. As for diastereoselectivity, no evident tendency was observed. The BPin product, generated in the copper(I)-catalyzed threecomponent reaction, was employed as a nucleophile of Suzuki− Miyaura coupling,21 which coupled with bromobenzene to give product 8 in 70% yield (for two steps) in the presence of a Pd(II)-RuPhos complex (Scheme 5). Furthermore, the synthetic
(see the Supporting Information). The absolute configurations in other products were tentatively assigned by analogy. Next, the scope of 1,3-enynes was studied under the optimal reaction conditions as shown in Scheme 3. As for the 1,3-enynes Scheme 3. Substrate Scope of 1,3-Enynesa
Scheme 5. Transformations of Products
versatility of the diols (4 and 5), generated through the oxidation of the BPin products, was demonstrated by several transformations of product 4a (Scheme 5). By utilizing a reported method,22 the acetylene moiety was reduced by sodium borohydride to cis-olefin 9 in 81% yield in the presence of NiCl2. trans-Olefin 10 was also accessed through reduction of the acetylene with lithium aluminum hydride. Full saturation of the acetylene led to 11 in 97% yield by Pd/C-catalyzed hydrogenation. Moreover, a cyclization was achieved to afford a highly functionalized oxetane 12 in 88% yield under Mitsunobu reaction conditions. Then, the acetylene moiety in 12 reacted with NaN3 to form NH-1,2,3-triazole 13 in 80% yield through an oxidative “click reaction”.23 In summary, we have developed a catalytic asymmetric threecomponent reaction of bis(pinacolato)diboron, 1,3-enynes, and fluoroalkyl ketones by using a copper(I) complex. The method uses mild conditions; employs commercially available catalyst precursors; enables the formation of an array of chiral fluoroalkyl diols in good to high yield, good to high diastereoselectivity, and high enantioselectivity; and tolerates many functional groups. The synthetic utility of the present reaction was showcased by product derivatization.
a
1a (0.2 mmol), 2 (0.3 mmol), 3 (0.3 mmol). Isolated yield reported. Diastereo- and enantioselectivity determined by chiral-stationary-phase HPLC analysis.
with an aromatic substituent, both electron-donating (methoxyl group) and electron-withdrawing groups (chloride, fluoride, bromide, and trifluoromethyl) were well tolerated (entries 2−7). Both diastereo- and enantioselectivity were not sensitive to the substitution pattern on the phenyl ring, as 5b, 5c, and 5e were isolated in uniformly high diastereo- and enantioselectivity. Furthermore, a heteroaromatic group, such as 3-thienyl, did not have an unfavorable effect on the reaction outcome (entry 8). In addition to aromatic substituents, aliphatic substituents were also compatible (entries 9−16). Various functional groups, including silyl ether, benzyl ether, benzoyl ester, tosylate, sulfamide, alkyl chloride, and cyclopropyl groups, were well accepted under the present catalytic system. Although the diastereoselectivity was moderate, the enantioselectivity was generally excellent (≥98% ee). It would be more challenging to make use of perfluoroalkyl ketones in asymmetric catalysis due to the minimal difference in steric hindrance between perfluoroalkyl and phenyl groups. Several perfluoroalkyl phenyl ketones were evaluated under the present catalytic system (Scheme 4). From pentylfluoroethyl, heptafluoropropyl, nonafluorobutyl to tridecafluorohexyl, yields
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b04039. Experimental procedures, X-ray diffraction, spectroscopic data for all new compounds including 1H, 13C, and 19F NMR spectra (PDF)
Scheme 4. Catalytic Asymmetric Three-Component Reactions with Perfluoroalkyl Phenyl Ketones
Accession Codes
CCDC 1811805 contains 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 data_
[email protected], or by contacting The Cambridge 1072
DOI: 10.1021/acs.orglett.7b04039 Org. Lett. 2018, 20, 1070−1073
Letter
Organic Letters
K.; Kumagai, N.; Shibasaki, M. J. Am. Chem. Soc. 2015, 137, 15929− 15939. (c) Saito, A.; Kumagai, N.; Shibasaki, M. Angew. Chem., Int. Ed. 2017, 56, 5551−5555. (11) For selected catalytic asymmetric examples with α/β-CF3-α,βunsaturated compounds, see: (a) Kawai, H.; Okusu, S.; Tokunaga, E.; Sato, H.; Shiro, M.; Shibata, N. Angew. Chem., Int. Ed. 2012, 51, 4959− 4962. (b) Zhang, W.; Tan, D.; Lee, R.; Tong, G.; Chen, W.; Qi, B.; Huang, K.-W.; Tan, C.-H.; Jiang, Z. Angew. Chem., Int. Ed. 2012, 51, 10069−10073. (c) Kawai, H.; Yuan, Z.; Kitayama, T.; Tokunaga, E.; Shibata, N. Angew. Chem., Int. Ed. 2013, 52, 5575−5579. (d) Gao, J.-R.; Wu, H.; Xiang, B.; Yu, W.-B.; Han, L.; Jia, Y.-X. J. Am. Chem. Soc. 2013, 135, 2983−2986. (e) Wen, L.; Yin, L.; Shen, Q.; Lu, L. ACS Catal. 2013, 3, 502−506. (f) Xu, B.; Zhang, Z.-M.; Xu, S.; Liu, B.; Xiao, Y.; Zhang, J. ACS Catal. 2017, 7, 210−214. (12) For selected catalytic asymmetric examples with other than αCF3-enolates and α/β-CF3-α,β-unsaturated compounds, see: (a) Wu, Y.; Hu, L.; Li, Z.; Deng, L. Nature 2015, 523, 445−450. (b) Tsuchida, K.; Senda, Y.; Nakajima, K.; Nishibayashi, Y. Angew. Chem., Int. Ed. 2016, 55, 9728−9732. (c) Zhou, X.; Wu, Y.; Deng, L. J. Am. Chem. Soc. 2016, 138, 12297−12302. (d) Hu, L.; Wu, Y.; Li, Z.; Deng, L. J. Am. Chem. Soc. 2016, 138, 15817−15820. (e) Holmes, M.; Nguyen, K. D.; Schwartz, L. A.; Luong, T.; Krische, M. J. J. Am. Chem. Soc. 2017, 139, 8114−8117. (13) For selected examples, in which the enantioselectivity needs further improvements, see: (a) Zhang, G.-W.; Meng, W.; Ma, H.; Nie, J.; Zhang, W.-Q.; Ma, J.-A. Angew. Chem., Int. Ed. 2011, 50, 3538−3542. (b) Kokotos, C. G. J. Org. Chem. 2012, 77, 1131−1135. (c) Czerwiński, P.; Molga, E.; Cavallo, L.; Poater, A.; Michalak, M. Chem. - Eur. J. 2016, 22, 8089−8094. (14) For selected examples with high enantioselectivity, see: (a) Noda, H.; Amemiya, F.; Weidner, K.; Kumagai, N.; Shibasaki, M. Chem. Sci. 2017, 8, 3260−3269. (b) Zheng, Y.; Tan, Y.; Harms, K.; Marsch, M.; Riedel, R.; Zhang, L.; Meggers, E. J. Am. Chem. Soc. 2017, 139, 4322− 4325. (c) van der Mei, F. W.; Qin, C.; Morrison, R. J.; Hoveyda, A. H. J. Am. Chem. Soc. 2017, 139, 9053−9065. (d) Mszar, N. W.; Mikus, M. S.; Torker, S.; Haeffner, F.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2017, 56, 8736−8741. (15) Lee, K.; Silverio, D. L.; Torker, S.; Robbins, D. W.; Haeffner, F.; van der Mei, F. W.; Hoveyda, A. H. Nat. Chem. 2016, 8, 768−777. (16) Sasaki, Y.; Horita, Y.; Zhong, C.; Sawamura, M.; Ito, H. Angew. Chem., Int. Ed. 2011, 50, 2778−2782. (17) For selected examples of three-component reactions involving 1,3-enynes, see: (a) Kong, J.-R.; Ngai, M.-Y.; Krische, M. J. J. Am. Chem. Soc. 2006, 128, 718−719. (b) Komanduri, V.; Krische, M. J. J. Am. Chem. Soc. 2006, 128, 16448−16449. (c) Patman, R. L.; Williams, V. M.; Bower, J. F.; Krische, M. J. Angew. Chem., Int. Ed. 2008, 47, 5220−5223. (d) Geary, L. M.; Woo, S. K.; Leung, J. C.; Krische, M. J. Angew. Chem., Int. Ed. 2012, 51, 2972−2976. (e) Geary, L. M.; Leung, J. C.; Krische, M. J. Chem. - Eur. J. 2012, 18, 16823−16827. (f) Cheng, J.-K.; Loh, T.-P. J. Am. Chem. Soc. 2015, 137, 42−45. (g) Mori, Y.; Kawabata, T.; Onodera, G.; Kimura, M. Synthesis 2016, 48, 2385−2395. (h) Nguyen, K. D.; Herkommer, D.; Krische, M. J. J. Am. Chem. Soc. 2016, 138, 5238−5241. (18) Meng, F.; Haeffner, F.; Hoveyda, A. H. J. Am. Chem. Soc. 2014, 136, 11304−11307. (19) Yang, Y.; Perry, I. B.; Lu, G.; Liu, P.; Buchwald, S. L. Science 2016, 353, 144−150. (20) (a) Jiang, L.; Cao, P.; Wang, M.; Chen, B.; Wang, B.; Liao, J. Angew. Chem., Int. Ed. 2016, 55, 13854−13858. (b) Chen, L.; Zou, X.; Zhao, H.; Xu, S. Org. Lett. 2017, 19, 3676−3679. (21) Doucet, H. Eur. J. Org. Chem. 2008, 2008, 2013−2030. (22) Wen, X.; Shi, X.; Qiao, X.; Wu, Z.; Bai, G. Chem. Commun. 2017, 53, 5372−5375. (23) Hu, L.; Mück-Lichtenfeld, C.; Wang, T.; He, G.; Gao, M.; Zhao, J. Chem. - Eur. J. 2016, 22, 911−915.
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Liang Yin: 0000-0001-9604-5198 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the “Thousand Youth Talents Plan”, the National Natural Sciences Foundation of China (No. 21672235), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB20000000), the “Shanghai Rising-Star Plan” (No. 15QA1404600), CAS Key Laboratory of Synthetic Chemistry of Natural Substances and Shanghai Institute of Organic Chemistry.
■
REFERENCES
(1) (a) Salwiczek, M.; Nyakatura, E. K.; Gerling, U. I. M.; Ye, S.; Koksch, B. Chem. Soc. Rev. 2012, 41, 2135−2171. (b) Bos, M.; Poisson, T.; Pannecoucke, X.; Charette, A. B.; Jubault, P. Chem. - Eur. J. 2017, 23, 4950−4961. (2) (a) Gillis, E. P.; Eastman, K. J.; Hill, M. D.; Donnelly, D. J.; Meanwell, N. A. J. Med. Chem. 2015, 58, 8315−8359. (b) Zhou, Y.; Wang, J.; Gu, Z.; Wang, S.; Zhu, W.; Aceña, J. L.; Soloshonok, V. A.; Izawa, K.; Liu, H. Chem. Rev. 2016, 116, 422−518. (3) For selected reviews on fluoroalkylation, see: (a) Ni, C.; Hu, M.; Hu, J. Chem. Rev. 2015, 115, 765−825. (b) Ni, C.; Hu, J. Chem. Soc. Rev. 2016, 45, 5441−5454. (4) (a) Banks, R. E.; Smart, B. E.; Tatlow, J. C. Organofluorine Chemistry: Principles and Commercial Applications; Plenum Press: New York, 1994. (b) Smart, B. E. J. Fluorine Chem. 2001, 109, 3−11. (5) Hiyama, T. Organofluorine Compounds: Chemistry and Application; Springer-Verlag: Berlin Heidelberg, 2000. (6) (a) Nie, J.; Guo, H.-C.; Cahard, D.; Ma, J.-A. Chem. Rev. 2011, 111, 455−529. (b) Mikami, K.; Fustero, S.; Sánchez-Roselló, M.; Aceña, J. L.; Soloshonok, V. Synthesis 2011, 2011, 3045−3079. (c) Yang, X.; Wu, T.; Phipps, R. J.; Toste, F. D. Chem. Rev. 2015, 115, 826−870. (7) For selected examples of catalytic asymmetric nucleophilic trifluoromethylation: (a) Kawai, H.; Kusuda, A.; Nakamura, S.; Shiro, M.; Shibata, N. Angew. Chem., Int. Ed. 2009, 48, 6324−6327. (b) Okusu, S.; Hirano, K.; Yasuda, Y.; Tanaka, J.; Tokunaga, E.; Fukaya, H.; Shibata, N. Org. Lett. 2016, 18, 5568−5571. (c) Okusu, S.; Hirano, K.; Yasuda, Y.; Tokunaga, E.; Shibata, N. RSC Adv. 2016, 6, 82716−82720. (8) For selected examples of catalytic asymmetric electrophilic trifluoromethylation: (a) Allen, A. E.; MacMillan, D. W. C. J. Am. Chem. Soc. 2010, 132, 4986−4987. (b) Deng, Q.-H.; Wadepohl, H.; Gade, L. H. J. Am. Chem. Soc. 2012, 134, 10769−10772. (9) For selected examples of catalytic asymmetric radical trifluoromethylation: (a) Zhu, R.; Buchwald, S. L. J. Am. Chem. Soc. 2015, 137, 8069−8077. (b) Lin, J.-S.; Dong, X.-Y.; Li, T.-T.; Jiang, N.-C.; Tan, B.; Liu, X.-Y. J. Am. Chem. Soc. 2016, 138, 9357−9360. (c) Wang, F.; Wang, D.; Wan, X.; Wu, L.; Chen, P.; Liu, G. J. Am. Chem. Soc. 2016, 138, 15547−15550. (d) Cheng, Y.-F.; Dong, X.-Y.; Gu, Q.-S.; Yu, Z.-L.; Liu, X.-Y. Angew. Chem., Int. Ed. 2017, 56, 8883−8886. (e) Wu, L.; Wang, F.; Wan, X.; Wang, D.; Chen, P.; Liu, G. J. Am. Chem. Soc. 2017, 139, 2904− 2907. (10) For selected catalytic asymmetric examples with α-CF3-enolates, see: (a) Yin, L.; Brewitz, L.; Kumagai, N.; Shibasaki, M. J. Am. Chem. Soc. 2014, 136, 17958−17961. (b) Brewitz, L.; Arteaga, F. A.; Yin, L.; Alagiri,
■
NOTE ADDED AFTER ASAP PUBLICATION Publications were added to reference 17 on February 16, 2018.
1073
DOI: 10.1021/acs.orglett.7b04039 Org. Lett. 2018, 20, 1070−1073
