(Trialkylsilyl)vinylketenes: Synthesis and application as diene components in Diels-Alder cycloadditions

New strategies for the synthesis of (trialkylsilyl)vinylketenes ("TAS-vinylketenes") are described based on the photochemical Wolff rearrangement of alpha-silyl-alpha'-diazo-alpha,beta-unsaturated ketones and the 4 pi electrocyclic ring opening of cyclobutenones. These remarkably robust vinylketenes undergo highly regioselective [4 + 2] cycloadditions with reactive olefinic and acetylenic dienophiles to produce highly substituted cyclohexenones and phenols in which the ketene carbonyl dominates in controlling the regiochemical course of the reaction. The stereochemical course of these cycloadditions follows the Alder endo rule, as illustrated in the reaction of nitropropene with TBS-vinylketene 22.