Alkylation of enol silyl ethers by Pummerer-generated vinylthionium ions: a novel masked Michael reaction

Abstract

Reaction of the enol silyl ethers (3) and (4) with the allyl sulphoxides (2), (7), (10), and (14) under Pummerer conditions using trimethylsilyl trifluoromethanesulphonate (TMSOTf) and di-isopropylethylamine (Hunig's base) affords the E-masked Michael adducts (5), (6), (8), (9), (12), (13), and (15) in good yields. The vinylthionium ion intermediate is competitively intercepted by Hunig's base in the reaction to afford, for the allyl sulphoxide (7), the quaternary ammonium Inflate salt (18), which is the sole product in the absence of silyl ether. The amine interception occurs with other pyramidal amines (triethylamine, quinuclidine, and diethyltrimethylsilylamine). The adduct (18) can be alkylated in moderate yield by less reactive enolates, e.g. malonate, using palladium(0) as catalyst.