SYN OR ANTI SELECTIVE MICHAEL ADDITION OF ALLYL PHENYL SULFONE AND PHENYL PRENYL SULFONE TO ENOATES DERIVED FROM D-MANNITOL

Prenyl and allyl sulphones (3a, b) were deprotonated with BuLi in THF and the resulting carbanions were allowed to react with enoates prepar ed from D-(+)-mannitol, A major diastereomer (control at the two newly created stereogenic centers) was obtained from enoates Ef b-c (Ethyl and t-Butyl ester, respectively) in d.e. up to 90 %. Syn-sterecselecti vities were observed when the reactions were stopped at r.t. while at -78 degrees C anti-stereoselectivities predominated. Enoate E-1a (meth yl ester) led to anti-addition regardless the temperature at which the reaction was quenched. However, a syn-stereoselectivity was found whe n the reaction was run in the presence of HMPA. The stereochemical ass ignments for both newly generated stereogenic centers were based on th e transformation of the adducts into the corresponding beta, gamma-dis ubstituted gamma-lactones, followed by measurements of H-1, H-1 coupli ng constants and nOe experiments. (C) 1998 Published by Elsevier Scien ce Ltd: All rights reserved.