Diastereoselective Syntheses Of (E)-Alpha-Trialkylsilyl Alpha,Beta-Unsaturated Esters, Alpha-Silane-Substituted Conjugated Silyl Ketene Acetals, And Alpha,Gamma-Substituted Allylsilanes

The vicinal functionalization of propiolate esters via a catalytic carbocupration/silicon group migration sequence has been further investigated to include the syntheses of a wide variety of beta-alkyl- and beta-aryl-substituted (E)-alpha-silyl alpha,beta-unsaturated esters. The ester substrates were transformed into their beta,gamma-unsaturated isomers by means of an LDA-mediated gamma-deprotonation, extended silyl ketene acetal formation, and final alpha-protonation sequence. The silyl ketene acetal intermediates were also isolated, and their stereo chemistries were established by NOE. The isolation of the intermediate extended silyl ketene acetals afforded further understanding of the lithium-extended dienolate structure and furnished additional support for Snieckus's proposed cyclic eight-membered transition state in the deprotonation of (Z)-alpha,beta-unsaturated carbonyls with metallodialkylamides.