Stereo- and Enantioselective Syntheses of 1,2-Oxaborinan-3-enes and δ-Boryl-Substituted Homoallylic Alcohols.

Stereo- and enantioselective syntheses of 1,2-oxaborinan-3-enes and δ-boryl-substituted homoallylic alcohols are reported. We developed a practical approach to synthesize α-boryl-substituted allylboronate. This reagent was utilized to generate α,α-disubstituted allylboronates, and such reagents cannot be accessed via the Pd-catalyzed alkene isomerization approach. Chiral Brønsted-acid-catalyzed aldehyde addition with these reagents gave 1,2-oxaborinan-3-enes with excellent stereo- and enantioselectivities. Lewis-acid-catalyzed aldehyde addition also worked well, affording δ-boryl-substituted homoallylic alcohols with high stereoselectivities. The enantioselective variant of the reaction was achieved via a chiral Brønsted-acid-catalyzed aldehyde addition and Pd-catalyzed alkene isomerization approach.