Silicon in organic synthesis—17 : Cyclopentannulation by thermolysis of (1-trimethylsilylcyclopropyl)ethylenes—the 1-trimethylsilylcyclopropyl anion

Abstract

Several alternatives for preparing (1-trimethylsilylcyclopropyl)ethylenes have been examined. Although 1-lithio-1-(trimethylsilyl)ethylene does add satisfactorily to carbonyl compounds and the subsequent Simmons-Smith cyclopropanation provides the desired cyclopropyl carbinols, shorter routes involving the 1-trimethylsilylcyclopropyl anion were sought. The sequence based upon reductive lithiation of 1-(trimethylsilyl)-1-phenyl-thiocyclopropane proved less than satisfactory. In contrast, metalation of 1-bromo-1-(trimethylsilyl)cyclopropane leads efficiently to the desired carbinols which dehydrate without rearrangement. Pyrolysis of the resulting (1-trimethylsilylcyclopropyl)ethylenes affords silyl substituted cyclopentenes in high yield. Subsequent reaction of these products with varied electrophiles forms the basis of a new and versatile 5-ring annulation sequence. An intramolecular competitive study of the vinylcyclopropane rearrangement is documented as are other attempts to induce ring expansion of the Si-substituted 3-membered ring by ionic pathways.