Enantioselective Syntheses of ()-6'-Boryl--1,2-oxaborinan-3-enes via a Dienylboronate Protoboration and Asymmetric Allylation Reaction Sequence.

The enantioselective synthesis of 6'-boryl-1,2-oxaborinan-3-enes is reported. A Cu-catalyzed highly stereoselective 1,4-protoboration of 1,1-bisboryl-1,3-butadiene is developed to generate ()-α,δ-bisboryl-crotylboronate. The chiral phosphoric-acid-catalyzed asymmetric allylboration of aldehydes with the boron reagent produces 6'-boryl-1,2-oxaborinan-3-enes with excellent Z-selectivities and enantioselectivities. The product contains a vinyl and alkyl boronate unit that can directly participate in a variety of subsequent transformations.