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

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 γ‐keto esters. The intermediacy of dihydrofurans in the one‐pot transformation of keto tribromides into γ‐keto esters provides evidence for the active participation of the neighboring carbonyl group in the alcoholysis of the tribromide moiety. Alcoholysis of γ‐keto tribromides to γ‐keto esters takes place with the intermediacy of isolable dialkoxydihydrofurans, confirmingthe participation of the carbonyl group in the transformation.