Double Diastereoselective Approach to Chiral syn- and anti-1,3-Diol Analogues through Consecutive Catalytic Asymmetric Borylations

Homoallylic boronate carboxylate esters derived from unsaturated aldehydes via an imination, β-borylation, imine hydrolysis, and Wittig trapping sequence, were subjected to a second boryl addition to give 1,3-diborylated carboxylate esters. Control of the absolute and relative stereochemistry of the two new 1,3-stereogenic centers was achieved through: (1) direct chiral catalyst controlled asymmetric borylation of the first stereocenter on the unsaturated imine with high e.e.; and (2) a double diastereoselectively controlled borylation of an unsaturated ester employing a chiral catalyst to largely overcome directing effects from the first chiral boryl center to give poor (mismatched) to good (matched) diastereocontrol. Subsequently, the two C–B functions were transformed into C–O systems to allow unambiguous stereochemical assignment of the two borylation reactions involving oxidation and acetal formation.