Sc(OTf)3/BF3·OEt2‑Catalyzed Annulation of 3‑Formylchromones with Functionalized Alkenes: Access to Diverse 2‑Hydroxybenzophenones

The Sc­(OTf)3/BF3·OEt2-catalyzed annulation of 3-formylchromones with functionalized alkenes for the direct construction of 2-hydroxybenzophenones is described. Sc­(OTf)3/BF3·OEt2 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.