Letter Cite This: Org. Lett. 2017, 19, 5936-5939
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Catalytic Asymmetric Roskamp Reaction of Silyl Diazoalkane: Synthesis of Enantioenriched α‑Silyl Ketone Jae Yeon Kim,† Byung Chul Kang,† and Do Hyun Ryu* Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, Korea S Supporting Information *
ABSTRACT: A catalytic enantioselective Roskamp reaction of silyl diazoalkane to synthesize a highly optically active α-silyl ketone from aldehydes is described. In the presence of a chiral oxazaborolidinium ion catalyst, the reaction provides α-chiral silyl ketones with good yields (up to 97%) and high enantioselectivities (up to >99% ee). In addition, a one-pot procedure using an asymmetric Roskamp/reduction strategy gives highly optically active syn-β-hydroxysilane in good yields (up to 94%) with high enantioselectivities (up to 99% ee) and syn stereoselectivities (>20:1).
E
The Roskamp reaction is a homologation of an aldehyde with a diazo compound in the presence of a Lewis acid; catalytic asymmetric methods have recently been reported by both the Feng laboratory7a and our group.7b−d Strategically, we envisioned that the catalytic asymmetric Roskamp reaction with a silyl diazoalkane8 could provide a transition-metal-free coupling9 approach to prepare α-chiral silyl ketones (Scheme 1, eq 2). While trimethylsilyldiazomethane was used in various reactions in place of the unstable, toxic, and potentially explosive diazomethane,10 the reactions with silyl diazoalkanes have not been well studied.11 To the best of our knowledge, the use of silyl diazoalkanes for the asymmetric reaction is without precedent.12 Herein, we describe the successful development of the first catalytic enantioselective Roskamp reaction of silyl diazoalkane to afford a highly optically active α-silyl ketone. Initially, an asymmetric Roskamp reaction between dimethylphenylsilyldiazoethane and benzaldehyde was examined in the presence of 20 mol % chiral oxazaborolidinium ion (COBI)13 1a activated by triflic acid (Table 1, entry 1). When the reaction was carried out at −78 °C in toluene, the desired optically active α-silyl ketone 2 was obtained as the major product via a selective 1,2-hydride shift (path a). A minor epoxide product 3 was also isolated in 20% yield (path b).14 First, we screened the catalyst structure and found that the catalyst system giving the best result was a 3,5-dimethylphenyl Ar1 substituent and a 2isopropoxyphenyl Ar2 substituent, activated by triflic acid (Table 1, entries 1−6). It is notable that chirality inversion15 took place when the Ar2 of the catalyst was changed to an orthoalkoxy-substituted phenyl group. Introduction of one more phenyl group to the silyl diazoethane led to an improved yield of 79%, excellent enantioselectivity of 98% ee, and 2/3 ratio of >20:1 (Table 1, entry 7); however, during subsequent purification on a silica gel column protodesilylation of α-silyl
nantioenriched chiral silanes are useful in organic synthesis and medicinal chemistry as versatile intermediates1 and stereodetermining groups for C−C bonds2 and C−O bonds,3 and the development of synthetic methods to prepare them has been an important and challenging goal. Among the available catalytic asymmetric strategies, two general methods are currently reliable and well developed. Asymmetric conjugate addition of silyl reagents to α,β-unsaturated carbonyl compounds efficiently produces β-chiral silyl carbonyl compounds.4 A reliable method to produce α-chiral silyl carbonyl compounds is the asymmetric insertion of a metal carbenoid derived from an α-diazocarbonyl compound into a Si−H bond.5 Generally, α-aryl substituted diazoesters are good substrates to provide α-chiral silyl esters in the presence of a transition metal catalyst.5 Compared to enantioselective syntheses of α-chiral silyl esters, those of α-chiral silyl ketones have rarely been reported,6 and to the best of our knowledge, no catalytic asymmetric example of this synthetic method has been reported to date. In 1987, the Enders group reported the first enantioselective synthesis of chiral α-silyl ketone including the use of SAMP/RAMP hydrazine as a chiral auxiliary (Scheme 1, eq 1).6a Scheme 1. Asymmetric Methods To Prepare Chiral α-Silyl Ketone Compounds
Received: September 19, 2017 Published: October 20, 2017 © 2017 American Chemical Society
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DOI: 10.1021/acs.orglett.7b02928 Org. Lett. 2017, 19, 5936−5939
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Organic Letters Table 1. Optimization of the Asymmetric Roskamp Reaction Using Silyl Diazoethane and Benzaldehydea
Table 2. Asymmetric Roskamp/Reduction Reaction of Methyldiphenylsilyldiazoalkane with Aromatic Aldehydesa
entry
entry
R
cat.
2/3b
yield (%)c
ee (%)d
1 2 3 4 5 6 7
(CH3)2PhSi (CH3)2PhSi (CH3)2PhSi (CH3)2PhSi (CH3)2PhSi (CH3)2PhSi CH3Ph2Si
1a 1b 1c 1d 1e 1f 1f
2:1 6:1 4:1 5:1 4:1 9:1 >20:1
41 40 62 68 63 73 79
−34 −50 68 81 92 92 98
1 2d,e 3 4 5 6 7 8 9 10 11 12 13 14g,h
4a 4a 4b 4c 4d 4e 4f 4g 4h 4i 4j 4k 4l 4m
yield (%)b
Ar
4
Ph Ph 4-MePh 4-MeOPh 4-BrPh 4-CF3Ph 4-NO2Ph 2-FPh 3-MePh 3-BrPh 2-thienyl 2-furyl 2-naphthyl Ph
84 80 90 94 93 74 81 70 89 84 88 94 84 56
(79)
f
ee (%)c 97 98 99 99 95 88 90 97 97 95 94 96 94 90
(93)f (97)f (78)f (63)f
a
The reaction of methyldiphenylsilyldiazoethane (0.28 mmol) with aromatic aldehyde (0.23 mmol) was performed in the presence of 1f (20 mol %) in 1.0 mL of toluene at −78 °C for 30 min. bIsolated yield of 4. cThe ee of 4 was determined by chiral HPLC. dWith 1.0 mmol scale. e10 mol % of 1f was used. fIsolated yields of α-silyl ketone are given in parentheses. gMethyldiphenylsilyldiazopropane was used instead of methyldiphenylsilyldiazoethane. h20 mol % of 1e was used.
a
The reaction of silyl diazoethane (0.28 mmol) with benzaldehyde (0.23 mmol) was performed in the presence of 1 (20 mol %) in 1.0 mL of toluene at −78 °C for 30 min. bDetermined by 1H NMR analysis of the crude reaction mixture. cIsolated yield of 2. dThe ee of 2 was determined by chiral HPLC.
ketone 2 was observed to cause considerable loss of yield (∼10%). Next, we investigated diastereoselective reduction conditions for the synthesis of syn-β-hydroxysilane 4a.16 Due to difficulty in isolating 2 without loss of yield (Table 2, entries 1−6), a one-pot direct reduction of the ketone of 2 was applied to prepare 4a. As the reducing reagent, diisobutylaluminum hydride (DIBAL-H) appeared to be superior to lithium borohydride or sodium borohydride in terms of syn stereoselectivity. When DIBAL-H was added at −78 °C to 2 prepared in situ by the optimized conditions given in Table 1, syn-βhydroxysilane 4a was isolated in 84% yield and 97% ee with an excellent syn/anti ratio (>20:1) (Table 2, entry 1). With the optimized one-pot reaction conditions known, we evaluated this methodology with a range of substituted aromatic aldehydes (Table 2). Regardless of the electronic properties of the substituents on the aromatic aldehyde, highly optically active syn-β-hydroxysilanes 4 were obtained (Table 2, entries 1−13). Notably, o-fluorobenzaldehyde gave the desired product 4 in lower yield because of a competing epoxide formation (Table 2, entry 8) via an undesired Darzens reaction (Table 1, path b).14 Methyldiphenylsilyldiazopropane reacted with benzaldehyde to provide the corresponding syn-β-hydroxysilane 4m in moderate yield and high enantioselectivity (Table 2, entry 14). Analysis of the 1H NMR spectrum of 4a indicated a large vicinal coupling constant (J = 9.6 Hz) between the α and β protons, which confirmed the syn relationship of the two chiral centers.17c The absolute (1S,2S) configuration of the βhydroxysilanes was confirmed by a modified Mosher ester analysis.17 Encouraged by the promising results exhibited in Table 2, we applied this methodology to react a range of aliphatic aldehydes. Unlike the aromatic α-silyl ketone 2, the aliphatic α-silyl ketone 5 was stable during purification on a silica gel
column. As summarized in Table 3, secondary aldehydes reacted well with silyl diazoethane, giving good yields and high Table 3. Asymmetric Roskamp Reaction of Silyl Diazoethanes with Aliphatic Aldehydesa
entry
5
cat.
R1
R2
yield (%)b
ee (%)c
1 2 3 4 5 6 7
5a 5b 5c 5d 5e 5f 5g
1f 1f 1f 1g 1g 1g 1g
(CH3)2PhSi CH3Ph2Si CH3Ph2Si (CH3)2PhSi CH3Ph2Si (CH3)2PhSi CH3Ph2Si
iPr iPr Cyd Et Et n-Hex n-Hex
60 89 80 73 85 73 77
92 96 85 95 83 93 85
a
The reaction of silyl diazoethane (0.28 mmol) with aliphatic aldehyde (0.23 mmol) was performed in the presence of 1 (20 mol %) in 5.0 mL of toluene at −78 °C for 1 h. bIsolated yield of 5. cThe ee of 5 was determined by chiral HPLC. dCy = cyclohexyl.
enantioselectivities (Table 3, entries 1−3). Introduction of one more phenyl group to silyl diazoethane provided the desired product with improved yield and enantioselectivity (Table 3, entries 1 and 2). However, the best chiral oxazaborolidinium ion catalyst for aromatic and secondary aldehydes, 1f, was not the optimal catalyst for primary aldehydes; catalyst 1g, having a phenyl group on the boron, was found to be more suitable for generating higher yields and enantioselectivities (Table 3, entries 4−7). The reaction of propionaldehyde and long-chain 5937
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Organic Letters
of 8. As a result, aromatic (S)-α-silyl ketone 2b was formed through intermediate 9. The stereochemistry of (1S,2S)-βhydroxysilane 4a is explained by the Felkin−Anh model 10. In summary, we have developed the first catalytic asymmetric Roskamp reaction of silyl diazoalkane with both aromatic and aliphatic aldehydes. This mild and chemoselective silylation method gives access to a variety of highly optically active α-silyl ketones. Moreover, a one-pot procedure using an asymmetric Roskamp/reduction strategy gives highly optically active syn-βhydroxysilane in good yields with high enantioselectivities and syn stereoselectivities (>20:1). We believe that the resulting αsilyl ketone and syn-β-hydroxysilane could be valuable precursors for the highly selective formation of carbon−carbon and carbon−heteroatom bonds. Additional applications of this methodology and extension of the substrate scope are underway.
heptaldehyde with silyl diazoethane proceeded well, with good yields and high enantioselectivities (Table 3, entries 4−7). Notably, the reaction with methyldiphenylsilyldiazoethane produced the α-silyl ketone 5 with improved yield but reduced enantioselectivity (Table 3, entries 4−7). Contrastingly, reactions of tertiary aliphatic aldehyde substrates were unfruitful. The absolute (R) configuration of the α-silyl ketone was confirmed by a modified Mosher ester analysis16 and comparison of the optical rotation data to literature values [5d: [α]D25 +197.0 (C6H6, c = 2.0; 95% ee); lit.18 [α]D −201.4 (C6H6, c = 1.25; ≥96% ee)]. The observed stereochemistry for the asymmetric Roskamp reaction of the silyl diazoalkane using the chiral oxazaborolidinium ion catalysts 1f or 1g can be explained by the transition-state model shown in Figure 1. The mode of
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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.7b02928. Experimental procedures and full analytical data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Do Hyun Ryu: 0000-0001-7615-4661 Author Contributions †
J.Y.K. and B.C.K. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIP) (Nos. 2016R1A2B3007119, 2016R1A4A1011451).
Figure 1. Transition-state model for the asymmetric Roskamp reaction of silyl diazoalkane with aldehyde catalyzed by 1f or 1g and the reduction mechanism with DIBAL-H.
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coordination of aldehyde to 1f or 1g is the same as previously postulated for the asymmetric formal C−H insertion reaction with aryldiazoalkanes, diazoesters, and α-aryl diazo Weinreb amides.7b−d In the pretransition-state assemblies 6 and 8, shown in Figure 1, the aldehyde group is placed above the 3,5dimethylphenyl group, and those groups can effectively block the re face from attack by the silyl diazoalkane. In the case of aliphatic aldehydes, the silyl diazoalkane approaches the aldehyde for nucleophilic addition with the methyldiphenylsilyl group situated away from the aldehyde group due to steric interaction between the silicon group of the silyl diazoalkane and the boron aryl substituent of the catalyst.7d Nucleophilic addition of the silyl diazoalkane from the si face of the aldehyde leads to the intermediate 7. Chemoselective hydride shift with loss of nitrogen gas provides the aliphatic (R)-α-silyl ketone 5 as the major enantiomer. Interestingly, nucleophilic addition of the silyl diazoalkane from the si face of the aromatic aldehydes unexpectedly yields (S)-α-silyl ketone 2b. Presumably, when the phenyl substituent of silyl diazoalkane is placed near the aryl ring of the aldehyde due to π−π interaction,19 the diazo group of silyl diazoalkane is situated away from the aldehyde group as shown in the model
REFERENCES
(1) (a) Chan, T. H.; Wang, D. Chem. Rev. 1992, 92, 995. (b) Langkopf, E.; Schinzer, D. Chem. Rev. 1995, 95, 1375. (c) Fleming, I.; Barbero, A.; Walter, D. Chem. Rev. 1997, 97, 2063. (d) Bolm, C.; Kasyan, A.; Drauz, K.; Günther, K.; Raabe, G. Angew. Chem., Int. Ed. 2000, 39, 2288. (e) Showell, G. A.; Mills, J. S. Drug Discovery Today 2003, 8, 551. (f) Franz, A. K.; Wilson, S. O. J. Med. Chem. 2013, 56, 388. (g) Wei, L.; Zhou, Y.; Song, Z.-M.; Tao, H.-Y.; Lin, Z.; Wang, C.J. Chem. - Eur. J. 2017, 23, 4995. (2) (a) Enders, D.; Prokopenko, O. F.; Raabe, G.; Runsink, J. Synthesis 1996, 1996, 1095. (b) Jain, N. F.; Takenaka, N.; Panek, J. S. J. Am. Chem. Soc. 1996, 118, 12475. (c) Panek, J. S.; Jain, N. F. J. Org. Chem. 2001, 66, 2747. (d) Hu, T.; Takenaka, N.; Panek, J. S. J. Am. Chem. Soc. 2002, 124, 12806. (e) Lipomi, D. J.; Langille, N. F.; Panek, J. S. Org. Lett. 2004, 6, 3533. (f) Huang, H.; Panek, J. S. Org. Lett. 2004, 6, 4383. (g) Tinsley, J. M.; Roush, W. R. J. Am. Chem. Soc. 2005, 127, 10818. (h) Kesavan, S.; Panek, J. S.; Porco, J. A., Jr. Org. Lett. 2007, 9, 5203. (3) (a) Fleming, I.; Henning, R.; Parker, D. C.; Plaut, H. E.; Sanderson, P. E. J. J. Chem. Soc., Perkin Trans. 1 1995, 317. (b) Barrett, A. G. M.; Head, J.; Smith, M. L.; Stock, N. S.; White, A. J. P.; Williams, D. J. J. Org. Chem. 1999, 64, 6005. (c) Mader, M. M.; Norrby, P.-O. J.
5938
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Organic Letters Am. Chem. Soc. 2001, 123, 1970. (d) Tenenbaum, J. M.; Woerpel, K. A. Org. Lett. 2003, 5, 4325. (e) Restorp, P.; Fischer, A.; Somfai, P. J. Am. Chem. Soc. 2006, 128, 12646. (f) Gilles, P.; Py, S. Org. Lett. 2012, 14, 1042. (g) Gribble, M. W., Jr.; Pirnot, M. T.; Bandar, J. S.; Liu, R. Y.; Buchwald, S. L. J. Am. Chem. Soc. 2017, 139, 2192. (4) For selected examples of asymmetric conjugate addition of silyl metal reagents, see: (a) Walter, C.; Oestreich, M. Angew. Chem., Int. Ed. 2008, 47, 3818. (b) Lee, K.-S.; Hoveyda, A. H. J. Am. Chem. Soc. 2010, 132, 2898. (c) Pace, V.; Rae, J. P.; Procter, D. J. Org. Lett. 2014, 16, 476. (d) Wu, H.; Garcia, J. M.; Haeffner, F.; Radomkit, S.; Zhugralin, A. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2015, 137, 10585. (5) For selected examples of Si-H bond insertion with diazoester compounds, see: (a) Doyle, M. P.; Forbes, D. C. Chem. Rev. 1998, 98, 911. (b) Zhang, Y.-Z.; Zhu, S.-F.; Wang, L.-X.; Zhou, Q.-L. Angew. Chem., Int. Ed. 2008, 47, 8496. (c) Yasutomi, Y.; Suematsu, H.; Katsuki, T. J. Am. Chem. Soc. 2010, 132, 4510. (d) Sambasivan, R.; Ball, Z. T. J. Am. Chem. Soc. 2010, 132, 9289. (e) Chen, D.; Zhu, D.-X.; Xu, M.-H. J. Am. Chem. Soc. 2016, 138, 1498. (6) (a) Enders, D.; Lohray, B. B. Angew. Chem., Int. Ed. Engl. 1987, 26, 351. (b) Enders, D.; Lohray, B. B. Angew. Chem., Int. Ed. Engl. 1988, 27, 581. (c) Lohray, B. B.; Zimbiniski, R. Tetrahedron Lett. 1990, 31, 7273. (d) Enders, D.; Ward, D.; Adam, J.; Raabe, G. Angew. Chem., Int. Ed. Engl. 1996, 35, 981. (e) Enders, D.; Otten, T. Synlett 1999, 1999, 747. (7) (a) Li, W.; Wang, J.; Hu, X.; Shen, K.; Wang, W.; Chu, Y.; Lin, L.; Liu, X.; Feng, X. J. Am. Chem. Soc. 2010, 132, 8532. (b) Gao, L.; Kang, B. C.; Hwang, G.-S.; Ryu, D. H. Angew. Chem., Int. Ed. 2012, 51, 8322. (c) Shin, S. H.; Baek, E. H.; Hwang, G.-S.; Ryu, D. H. Org. Lett. 2015, 17, 4746. (d) Kang, B. C.; Nam, D. G.; Hwang, G.-S.; Ryu, D. H. Org. Lett. 2015, 17, 4810. (8) For preparation of silyl diazoalkane compounds, see: (a) Aoyama, T.; Shioiri, T. Tetrahedron Lett. 1986, 27, 2005. (b) Shioiri, T.; Aoyama, T.; Mori, S. Org. Synth. 1990, 68, 1. (9) Sun, C.-L.; Shi, Z.-J. Chem. Rev. 2014, 114, 9219. (10) (a) Aoyama, T.; Shioiri, T. Chem. Pharm. Bull. 1981, 29, 3249. (b) Zhang, S.; Zhang, X.-M.; Bordwell, F. G. J. Am. Chem. Soc. 1995, 117, 602. (c) Audubert, C.; Marin, O. J. G.; Lebel, H. Angew. Chem., Int. Ed. 2017, 56, 6294. (11) For use of silyl diazoalkane as a reagent, see: (a) Brook, A. G.; Jones, P. F. Can. J. Chem. 1971, 49, 1841. (b) Bassindale, A. R.; Brook, A. G. Can. J. Chem. 1974, 52, 3474. (c) Sekiguchi, A.; Ando, W. J. Chem. Soc., Chem. Commun. 1979, 575. (d) Aoyama, T.; Shioiri, T. Chem. Pharm. Bull. 1989, 37, 2261. (12) For selected examples of a catalytic asymmetric reaction of silyl diazoester, see: (a) Müller, P.; Lacrampe, F. Helv. Chim. Acta 2004, 87, 2848. (b) Bolm, C.; Saladin, S.; Claßen, A.; Kasyan, A.; Veri, E.; Raabe, G. Synlett 2005, 461. (c) Ghanem, A.; Lacrampe, F.; Schurig, V. Helv. Chim. Acta 2005, 88, 216. (d) Inoue, S.; Nagatani, K.; Tezuka, H.; Hoshino, Y.; Nakada, M. Synlett 2017, 28, 1065. (13) Selected recent examples for asymmetric reactions with the COBI catalyst, see: (a) Lee, S. I.; Hwang, G.-S.; Ryu, D. H. J. Am. Chem. Soc. 2013, 135, 7126. (b) Gao, L.; Kang, B. C.; Ryu, D. H. J. Am. Chem. Soc. 2013, 135, 14556. (c) Shim, S. Y.; Kim, J. Y.; Nam, M.; Hwang, G.-S.; Ryu, D. H. Org. Lett. 2016, 18, 160. (d) Kang, K.-T.; Kim, S. T.; Hwang, G.-S.; Ryu, D. H. Angew. Chem., Int. Ed. 2017, 56, 3977. (e) Shim, S. Y.; Cho, S. M.; Venkateswarlu, A.; Ryu, D. H. Angew. Chem., Int. Ed. 2017, 56, 8663. (14) For selected examples for the Darzens reaction of aldehyde, see: (a) Davies, H. M. L.; DeMeese, J. Tetrahedron Lett. 2001, 42, 6803. (b) Illa, O.; Namutebi, M.; Saha, C.; Ostovar, M.; Chen, C. C.; Haddow, M. F.; Nocquet-Thibault, S.; Lusi, M.; McGarrigle, E. M.; Aggarwal, V. K. J. Am. Chem. Soc. 2013, 135, 11951. (c) Kuang, Y.; Lu, Y.; Tang, Y.; Liu, X.; Lin, L.; Feng, X. Org. Lett. 2014, 16, 4244. (15) (a) Bartók, M. Chem. Rev. 2010, 110, 1663. (b) Inagaki, T.; Ito, A.; Ito, J.-I.; Nishiyama, H. Angew. Chem., Int. Ed. 2010, 49, 9384. (c) Awano, T.; Ohmura, T.; Suginome, M. J. Am. Chem. Soc. 2011, 133, 20738. (d) Zhang, L.; Zhang, J.; Ma, J.; Cheng, D.-J.; Tan, B. J. Am. Chem. Soc. 2017, 139, 1714.
(16) (a) Enders, D.; Nakai, S. Helv. Chim. Acta 1990, 73, 1833. (b) Roush, W. R.; Pinchuk, A. N.; Micalizio, G. C. Tetrahedron Lett. 2000, 41, 9413. (c) Tinsley, J. M.; Mertz, E.; Chong, P. Y.; Rarig, R.-A. F.; Roush, W. R. Org. Lett. 2005, 7, 4245. (d) Canales, E.; Gonzalez, A. Z.; Soderquist, J. A. Angew. Chem., Int. Ed. 2007, 46, 397. (e) Lira, R.; Roush, W. R. Org. Lett. 2007, 9, 4315. (f) Binanzer, M.; Fang, G. Y.; Aggarwal, V. K. Angew. Chem., Int. Ed. 2010, 49, 4264. (17) (a) Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113, 4092. (b) Hoye, T. R.; Jeffrey, C. S.; Shao, F. Nat. Protoc. 2007, 2, 2451. (c) Chen, M.; Roush, W. R. Org. Lett. 2011, 13, 1992. (18) Enders, D.; Lohray, B. B.; Burkamp, F.; Bhushan, V.; Hett, R. Liebigs Ann. 1996, 1996, 189. (19) For selected reviews on π−π intereaction, see: (a) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. J. Chem. Soc., Perkin Trans. 2 2001, 651. (b) Waters, M. L. Curr. Opin. Chem. Biol. 2002, 6, 736. (c) Salonen, L. M.; Ellermann, M.; Diederich, F. Angew. Chem., Int. Ed. 2011, 50, 4808.
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