Hetero [4 + 2] cycloadditions of (trialkylsilyl)vinylketenes. Synthesis of alpha,beta-unsaturated delta-valerolactones and -lactams.

[formula: see text] Hetero Diels-Alder reactions of (trialkylsilyl)vinylketenes (TAS-vinylketenes) with carbonyl and imino dienophiles are described. TAS-vinylketenes participate as electron-rich dienes in [4 + 2] cycloadditions with diethyl ketomalonate to afford alpha,beta-unsaturated delta-valerolactones in good yield. Nonenolizable N-alkyl- and N-(trimethylsilyl)imines combine with TAS-vinylketenes to furnish alpha,beta-unsaturated delta-valerolactams. In contrast to most Diels-Alder reactions involving unactivated imines, these cycloadditions do not require promotion by Lewis acids and in general proceed with a high degree of stereoselectivity.