(<i>Z</i>)-α-Boryl-crotylboron reagents <i>via Z</i>-selective alkene isomerization and application to stereoselective syntheses of (<i>E</i>)-δ-boryl-<i>syn</i>-homoallylic alcohols

Abstract

Stereoselective synthesis of (<i>Z</i>)-α-boryl-crotylboronate is developed. Ni-catalyzed <i>Z</i>-selective alkene isomerization of α-boryl substituted homoallylboronate provided the targeted (<i>Z</i>)-crotylboronate with high selectivity. Stereoselective addition of the novel crotylboron reagent to aldehydes gave (<i>E</i>)-δ-boryl-substituted <i>syn</i>-homoallylic alcohols with excellent diastereoselectivities. The vinyl boronate unit in the products can be directly used for a subsequent C-C bond-forming transformation as illustrated in the synthesis of the C_1-7 fragment of the natural products nannocystin A and nannocystin Ax.