Formation of Five-Membered Cyclic Orthoesters from Tribromides with Participation of a Neighboring Carbonyl Group

Abstract

The Mg-mediated conjugate addition of bromoform to enones followed by alcoholysis of the resulting keto tribromides proceeds through an unusual course, affording cyclic orthoester (dihydrofuran) intermediates under neutral workup conditions. However, acid workup of the reaction mixture or treatment of the isolated dihydrofurans with acid provides the expected but synthetically useful gamma-keto esters. The intermediacy of dihydrofurans in the one-pot transformation of keto tribromides into gamma-keto esters provides evidence for the active participation of the neighboring carbonyl group in the alcoholysis of the tribromide moiety.