Synthesis of Alkenylboronic Esters: An Alternative Route to Trisubstituted Homoallylic Alcohols

New alkenylboronic esters were synthesised from halo-substituted alkenylboronic esters through cross-coupling reactions. Upon Johnson rearrangement, enantiomerically pure allylboronates bearing a stereogenic centre in the position a to the boron moiety were obtained in moderate yield (53 %; 29 % over six steps from the protected propargylic alcohols). The products of these reactions were separable by medium-pressure liquid chromatography, and could be used in highly stereoselective allylation reactions to synthesise enantiomerically enriched homoallylic alcohols containing a 1,1,2-trisubstituted (Z)-configured double bond.