1,4-ADDITION TO ALPHA,BETA-UNSATURATED CARBONYL-COMPOUNDS BEARING A GAMMA-STEREOCENTER - A MOLECULAR MECHANICS MODEL FOR STERIC INTERACTIONS IN THE TRANSITION-STATE

A molecular mechanics model for the transition state of Michael additions to α,β-unsaturated carbonyl compounds was built. MM2 parameters were developed on the basis of the transition structure calculated ab initio by Weinstein and coworkers for the fluoride anion reaction with acrylic acid. Modeling the addition to γ-substituted crotonates provided a way of evaluating the steric interactions occurring between the γ-substituents and the incoming nucleophile and/or the double bond substituents. These calculations can be useful for discriminating between the various models which have been proposed to rationalize the stereoselectivity of Michael additions.