Control of Stereochemistry: A General Synthesis of cis- or trans-β,γ-Disubstituted-γ-butyrolactones Following Z-Crotylboration

A general and practical procedure for the highly diastereoselective preparation of either the cis- or trans-β,γ-disubstituted-γ-butyrolactones by appropriate choice of Lewis or Bronsted acid catalysts during crotylboration or lactonization is reported. The cis-stereochemistry of the Z-crotylboration product can be inverted with strong acids during lactonization. A carbocation mechanism and catalytic cycle has been proposed.