α-Hydroxyalkyl Heterocycles via Chiral Allylic Boronates: Pd-Catalyzed Borylation Leading to a Formal Enantioselective Isomerization of Allylic Ether and Amine

An efficient catalytic enantioselective preparation of synthetically useful pyranyl and piperidinyl allylic boronates was achieved via a palladium-catalyzed borylation/isomerization reaction on the corresponding alkenyl triflates. The influence of the base and solvent was found to be crucial on the regio- and enantioselectivity of this reaction. The overall borylation process constitutes a successful example of formal asymmetric isomerization of allylic ether/amine. The resulting allylic boronate reagents add to various aldehydes in a one-pot process to give synthetically useful alpha-hydroxyalkyl derivatives in high stereoselectivity.