Selective conjugate addition of nitromethane to enoates derived from D-mannitol and L-tartaric acid

摘要

The conjugate addition of nitromethane to enoates prepared from D-(+)-mannitol, substituted at the alpha-position by a methyl or a benzyl group, was investigated. While excellent syn-selectivity (d.e. >90%) was obtained from a-benzyl enoates (used as a mixture of epimers, E/Z=1.8:1), for alpha-methyl enoates the selectivity depended on the stereochemistry of the double bond in the acceptor (d.e. >90% for the (Z)-enoate and 50% for the (E)-enoate). In all cases, a mixture of epimers was formed at the newly generated stereocenter at the a-position. The epimeric syn-adducts were transformed into the corresponding pure alpha,beta,gamma-trisubstituted gamma-butyrolactones by cyclization in acid medium followed by epimerization of the stereocenter at the a-position in DBU/CH2Cl2. When enoates derived from L-tartaric acid were used as acceptors, syn-selective conjugate additions were also observed (d.e. >90% for the (Z)-isomer and 50% for the (E)-isomer). The configuration at the newly generated stereogenic centers were assigned based on X-ray analyses, H-1-H-1 coupling constants and NOE experiments in NMR spectroscopy. (C) 2002 Elsevier Science Ltd. All rights reserved. [References: 16]