Palladium(II)-catalyzed S(N)2 ' reactions of alpha-allenic acetates. Stereoconvergent synthesis of (Z,E)-2-bromo-1,3-dienes

The reaction of acetylated a-allenic alcohols with LiBr in the presence of 1.5 mol % of Pd(OAc)(2) provides easy access to substituted (Z,E)-2-bromo-1 ,3-dienes in good yields with excellent diastereoselectivity. Both secondar y and tertiary acetates as well as terminal and nonterminal allenes were st udied to investigate the scope and the limitations of the reaction. A mecha nism is proposed to clarify how a diastereomeric mixture of the starting co mpound is transformed into a single diastereomer of the product.