Highly Stereoselective Syntheses of α,α-Disubstituted ( E )- and ( Z )-Crotylboronates.

We report herein stereoselective syntheses of α,α-disubstituted ( E )- and ( Z )-crotylboronates. Starting from α-boryl ( E )- or ( Z )-crotylboronate, base-mediated alkylation occurred exclusively at the position α to the boryl groups to give targeted boronates while retaining the geometries of the alkenes in the starting crotylboronates. Under proper conditions, the resulting α,α-disubstituted crotylboronates underwent aldehyde addition to give allylated products with high stereoselectivities.