Enantioselective Addition of Boronates to Chromene Acetals Catalyzed by a Chiral Brønsted Acid/Lewis Acid System

Abstract

Chiral α,β-dihydroxy carboxylic acids catalyze the enantioselective addition of alkenyl and aryl boronates to chromene acetals. The optimal carboxylic acid is the easily available tartaric acid amide shown in the scheme. Spectroscopic and kinetic mechanistic studies demonstrate that an exchange process generates a reactive dioxoborolane intermediate leading to enantioselective addition to the pyrylium ion formed from the chromene acetal.