Asymmetric Synthesis of Chiral 1,4-Enynes through Organocatalytic Alkenylation of Propargyl Alcohols with Trialkenylboroxines

A highly enantioselective synthesis of 1,4-enynes is described that proceeds through an organocatalytic reaction between propargyl alcohols and trialkenylboroxines. Our strategy relies on acid-mediated generation of the carbocationic intermediate from propargyl alcohols followed by enantioselective alkenylation with trialkenylboroxines. A range of chiral 1,4-enynes were obtained in moderate to good yields with high levels of enantioselectivity. Use of a highly acidic chiral N-triflyl phosphoramide catalyst, which has two distant Lewis basic oxygen atoms, was found to be crucial for both high reactivity and selectivity in the present reaction.