[4+2] cycloaddition of ortho-quinone methides promoted by ionic liquids: an efficient and mild protocol for the synthesis of tetrahydropyranobenzopyrans

The stereoselective synthesis of trans-annelated pyrano[3,2-c]benzopyrans has been achieved by intramolecular [4+2] cycloaddition of o-benzoquinone methides that are generated in situ from o-hydroxybenzaldehydes and unsaturated alcohols using an air- and moisture-stable ionic liquid, i.e., 1-butyl-3-methylimidazolium tetrafluoroborate [bmim]BF4 under mild and neutral conditions