Stereoselective Synthesis of Trisubstituted Vinyl Bromides by Addition of Alkynes to Oxocarbenium Ions.

We have developed an efficient method for the synthesis of (E)-trisubstituted vinyl bromides via a Friedel-Crafts-type addition of alkynes to oxocarbenium ions formed in situ from acetals. The success of this reaction relies on identification of MgBr2·OEt2 as both a Lewis acid promoter and bromide source. This reaction employs simple, inexpensive starting materials and proceeds under mild conditions to allow the preparation of a range of vinyl bromide products in high yields and E:Z selectivities. Furthermore, the vinyl bromide products also contain an allylic ether functional group. Both the vinyl bromide and allylic ether are effective handles for the elaboration of these useful synthetic intermediates.