Stereoselective, borane-catalysed synthesis of syn-β-hydroxyketones from α,β-unsaturated ketones.

The reductive aldol reaction is a powerful tool for the regiocontrolled coupling of α,β-unsaturated compounds with aldehydes. The use of stoichiometric organoborane reductants has previously been reported. Here these reagents have been rendered catalytic through B-O transborylation (B-O/B-H metathesis). This one-pot dialkylborane-catalysed method allows for the synthesis of β-hydroxycarbonyl compounds in good yields with high diastereo- and enantioselectivity. This protocol was applied across a broad substrate scope including those containing reducible functional groups and intramolecular coupled examples.