ENANTIOSELECTIVE CYCLIZATION REACTIONS OF (Z)-N-BENZOYL-α-DEHYDRO(1-NAPHTH YL)ALANINE N'-ARYLAMIDES INITIATED BY PHOTOINDUCED ELECTRON TRANSFER FROM CHIRAL PROLINOL

摘要

An investigation was undertaken to explore substituent effects on the photoinduced electron transfer-initiated enantioselective cyclization of the title a-dehydronaphthylalanine N-(2-hydroxyphenyl)amide [(Z)-1a], N';-(3-hydroxyphenyl)amide [(Z)-1b], N'-(4-hydroxyphenyl)amide [(Z)-1c], and N'-phenylamide [(Z)-1d] in 1,2-dichloroethane containing chiral N-methylprolinol or prolinol. ~1H NMR spectral and HPLC chiral analyses of the target photoproducts, cis- and trans-4,5-dihydrooxazole isomers, confirmed that enantiomeric excess (ee) for these two isomers (estimated in the presence of the former prolinol) displayed a clear tendency to decrease in order of (Z)-1d > (Z)-1c > (Z)-1b > (Z)-1a. Hydrogen bonding and charge transfer interactions in radical ion pair and biradical intermediates were suggested to be key factors controlling ee, the value of which varied from 0 to 58%.