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Paper | Regular issue | Vol 83, No. 6, 2011, pp.1329-1353
Published online, 20th April, 2011
DOI: 10.3987/COM-11-12193
Asymmetric Cyclization Reactions of N-Benzoyl-α-dehydroarylalanine Alkyl Esters via Photoinduced Electron Transfer from Achiral and Chiral Amines

Yuji Sasaki, Haruo Watanabe, Tetsutaro Igarashi, and Tadamitsu Sakurai*

*Material and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan

Abstract

Irradiation of the title compounds [(Z)-1] having (S)-(+)-sec-butyl, (–)-menthyl, and related chiral auxiliaries in methanol and 1,2-dichloroethane containing 2-(diethylamino)ethanol afforded chiral auxiliary-substituted (4S,5S)-, (4R,5R)-, (4R,5S)-, and (4S,5R)-4,5-dihydrooxazole derivatives (2) along with (E)-1. It was found that the photoinduced electron transfer-initiated cyclization of (Z)-1 gives either of the two diastereomers for cis-2 and trans-2 in diastereomeric excess whose value varies from 8 to 81% depending on the property of solvent as well as on the steric bulkiness of chiral auxiliary. On the other hand, the asymmetric photocyclization of chiral auxiliary-unsubstituted (Z)-1 in 1,2-dichloroethane proceeded cleanly in the presence of the chiral amine, (S)-(–)-1-methylpyrrolidine-2-methanol, to give (4R,5R)-2 and (4R,5S)-2 in enantiomeric excess of 17–38% and 20–33%, respectively, depending on the steric bulkiness of the aryl substituent. A mechanistic consideration of these diastereoselective and enantioselective photocyclization reactions enabled us to propose major factors controlling the extent of asymmetric photoinduction.