STEREOCHEMICAL CONTROL IN THE ESTER ENOLATE CLAISEN REARRANGEMENT .1. STEREOSELECTIVITY IN SILYL KETENE ACETAL FORMATION

Methods for the stereoselective deprotonation and silylation of esters were systematically investigated. A kinetically controlled enolization in combination with a kinetic resolution process accounts for the selective formation of (E)- and (Z)-silyl ketene acetals in THF and THF/dipolar solvent systems with bases such as LDA, LHMDS, and KHMDS. A thermodynamic equilibration mechanism seems to be of minor significance with ester enolates. Improved reaction conditions were exemplified in a highly stereoselective Claisen rearrangement in THF/45% DMPU.