Asymmetric Synthesis of 1,3-Butadienyl-2-carbinols by the Homoallenylboration of Aldehydes with a Chiral Phosphoric Acid Catalyst

Abstract

Asymmetric C(sp)-C(sp(2)) bond formation to give enantiomerically enriched 1,3-butadienyl-2-carbinols occurred through a homoallenylboration reaction between a 2,3-dienylboronic ester and aldehydes under the catalysis of a chiral phosphoric acid (CPA). A diverse range of enantiomerically enriched butadiene-substituted secondary alcohols with aryl, heterocyclic, and aliphatic substituents were synthesized in very high yield with high enantioselectivity. Preliminary density functional theory (DFT) calculations suggest that the reaction proceeds via a cyclic six-membered chairlike transition state with essential hydrogen-bond activation in the allene reagent. The catalytic reaction was amenable to the gram-scale synthesis of a chiral alkyl butadienyl adduct, which was converted into an interesting optically pure compound bearing a benzo-fused spirocyclic cyclopentenone framework.

Keywords: allylic alcohols; asymmetric dienylation; boron; enantioselectivity; organocatalysis.