Stereoselectivity, regioselectivity and mechanism in the intramolecular 'citran' bicyclisation of chromenes

Stereochemical evidence suggests that acid catalysed citran formation proceeds via electrocyclisation of a dienone, and that pyrolysis of 'farnesylidenemalonic acid' proceeds with retention of configuration at the newly formed 8-olefin bond, indicating the operation of forward and reverse Diels-Alder processes respectively; 1,3-dihydroxynaphthalene reacts with citral in pyridine to yield chromene and citran products in a regioselective fashion.