Synthesis of Furo[3,2-c]benzopyrans via an Intramolecular [4 + 2] Cycloaddition Reaction of o-Quinonemethides

An intramolecular [4 + 2] cycloaddition reaction of o-quinonemethides generated from salicylaldehydes and α-prenylated alcohols is described. In the presence of a catalytic amount of benzenesulfonic acid (BSA), the reaction proceeded smoothly in EtOH to afford furo[3,2-c]benzopyrans through a three-bond forming process in moderate to excellent yields with high diastereoselectivity. This reaction provides a simple and straightforward protocol to efficiently construct furo[3,2-c]benzopyran skeletons. A possible mechanism involving hemiacetal formation/hetero-Diels-Alder reaction is proposed to rationalize the observed results.