CONFIGURATIONAL AND CONFORMATIONAL-ANALYSIS OF SILYL KETENE THIOACETALS BY NMR-SPECTROSCOPY AND COMPUTATIONAL STUDIES

The configuration and conformation of silyl ketene thioacetals derived from 2-pyridyl thioesters were studied by combined NMR and computatio nal analysis. The results revealed that the (E)-silyl ketene thioaceta l is always the thermodynamically favoured diastereoisomer; the equili brium ratio is reversed /E/Z>2:98/ for steric reasons when two O-silyl substituents are present on the C=C double bond, The preferred confor mational minima for the E and Z isomers are proposed on the basis of M onte Carlo conformational searches performed on selected model compoun ds; the computational results are in excellent agreement with the NMR data.