[4 + 1] annulation reactions of (trialkylsilyl)ketenes : synthesis of substituted indanones and cyclopentenones

Abstract

(Trialkylsilyl)vinylketenes ("(TAS)vinylketenes") and (trialkylsilyl)arylketenes ("(TAS)- arylketenes") function as versatile four-carbon building blocks for the synthesis of carbocyclic and heterocyclic compounds. A new [4 + 1] annulation strategy for the synthesis of substituted 2-indanones, based on the reaction of TAS-arylketenes with trimethylsilyl diazomethane, has been developed. In addition, a new class of carbenoid reagents for our previously reported [4 + 1] cyclopentenone annulation has been identified. Studies have shown that the reaction of a- benzotriazolyl organolithium compounds (prepared via metallation of readily available N- substituted benzotriazole derivatives) with (TAS)vinylketenes generates dienolate intermediates which cyclize to form cyclopentenones. Most cases of the annulation proceed with a high level of diastereoselectivity, and deliver highly substituted and functionalized 2-silylcyclopentenones in good yield. Furthermore, the vinylsilane moiety incorporated in the [4 + 1] annulation products provides a useful handle for further synthetic transformations. Preliminary studies focused on elaboration of the cyclopentenone products have laid the groundwork for future applications of this [4 + 1] annulation methodology.