Radical Cyclization vs. Elimination in the Vitamin-B₁₂-Catalyzed Reduction of Unsaturated Bromo Acetals and Bromo Orthoesters

Abstract

Reduction of the 2-bromoacetaldehyde acetals of cinnamyl alcohol or propargyl alcohol with Zn/NH₄ in presence of catalytic amounts of B₁₂ in DMF affords oxolanes by free radical 5-exo-cyclization and the corresponding alcohols by reductive elimination. 2-(Bromoethyl) dimethyl orthopropiolate mainly decomposes by reductive elimination and by hydrogenolysis of the bromide.