Sc(OTf)/BF·OEt-Catalyzed Annulation of 3-Formylchromones with Functionalized Alkenes: Access to Diverse 2-Hydroxybenzophenones.

The Sc(OTf)/BF·OEt-catalyzed annulation of 3-formylchromones with functionalized alkenes for the direct construction of 2-hydroxybenzophenones is described. Sc(OTf)/BF·OEt acts as a synergistic catalyst, providing rapid synthetic access to diversely and highly functionalized 2-hydroxybenzophenones. This reaction has excellent regio- and chemoselectivities and is suitable for late-stage functionalization. The reaction proceeds via [3 + 3] and [4 + 2] cycloaddition processes, through carbonyl-ene, Diels-Alder, or aldol-type reactions. Furthermore, this protocol tolerates the various functional groups present in natural terpenes and steroids.