A study of the claisen rearrangement of 7-cinnamyloxy benzo-γ-pyrones

The Claisen rearrangement of 7-O-cinnamyl noreugenin (2) yields 4′,5′ - dihydro - 5 - hydroxy - 2,4′ - dimethyl - 5′ - phenyl - furo (2′,3′: 7,8) chromone (6) as established by its NMR spectrum, whereas that of 7-cinnamyloxyflavone (10a) and 5 - hydroxy - 7 - cinnamyloxy - 2 - methylisoflavone (10b) afford the corresponding furo derivatives (12a and 12b respectively). All these rearrangements are symmetry-allowed but are accompanied by further reactions of thermal cyclisation via 3-membered cyclic intermediate (5) and/or dehydrogenation.