1,3-DIPOLAR CYCLOADDITIONS TO BAYLIS-HILLMAN ADDUCTS - RATIONALE FOR THE OBSERVED DIASTEREOSELECTIVITY

Diazomethane and benzonitrile oxide cycloadditions were performed on Baylis-Hillman adducts derived from methyl acrylate and aldehydes (beta'-alkoxy-alpha,beta-unsaturated esters). The reactions proceed in good chemical yields, and their stereochemical outcome can be explained by the ''inside alkoxy effect'' theory. In the case of diazomethane cycloadditions, however, electrostatic factors play a reduced role in comparison to the corresponding nitrile oxide reactions, while steric effects are of major importance in governing the stereoselectivity. This different behavior of the two 1,3-dipoles finds a rationale in the analysis of the atomic charges as calculated at the RHF/3-21G level of theory for the transition structure of these reactions.