Long-distance control in stereoselective reduction of 3-[3-(4?-bromo[1,1?-biphenyl]-4-yl)-3-keto-1-phenylpropyl]-4-hydroxy-2H-1-benzopyran-2-one: Relative configuration of prevailing diastereomer and absolute configuration of its enantiomers

Abstract

Depending on the reducing agent and reaction conditions, diastereoselective reduction of 3‐[3‐(4′‐bromo[1,1′‐biphenyl]‐4‐yl)‐3‐keto‐1‐phenylpropyl]‐4‐hydroxy‐2H‐1‐benzopyran‐2‐one (2) proceeds with different stereoselectivity; a surprisingly high, approximately 90% d.e. of 4A is achieved with NaBH4 in MeOH at low temperature. Resulting diastereomeric racemates 4A and 4B are separated and their respective syn and anti configurations are assigned on the bases of mechanic considerations, supported by the 1H‐NMR spectra and conformational analysis based on MM2 calculations. The syn diastereomer 7A, 4‐OMe derivative of 4A, was partially resolved by acylation at C(3)‐OH with S‐(−)‐camphanic acid to camphanyl ester 12 of (−)‐7A, leaving (+)‐enantiomer 7A. The assignment of absolute 1R,3S‐configuration to (−)‐7A is based on comparison of its CD spectrum with those of the model compounds S‐14 and R‐15, which represent partial chromophores 4‐hydroxy‐2H‐1‐benzopyran‐2‐one (4‐hydroxycoumarin) A, and 4′‐bromo‐1,1′‐biphenyl B; their exciton coupling in (−)‐7A is suggested. Chirality 9:512–522, 1997. © 1997 Wiley‐Liss, Inc.