Synthesis of the Oxepinochromone Natural Products Ptaeroxylin (Desoxykarenin), Ptaeroxylinol, and Eranthin

An improved synthesis of the oxepinochromone ptaeroxylin is reported, together with the syntheses of the related natural products ptaeroxylinol and eranthin. Ptaeroxylin and ptaeroxylinol were obtained from the chromenone noreugenin by selective reaction of the 7-hydroxyl group, allylation of the 5-hydroxyl, followed by Claisen rearrangement under microwave conditions with concomitant deprotection of the 7-hydroxyl. Alkylation of the 7-hydroxyl with the appropriate allyl bromide provides a precursor for ring-closing metathesis to deliver the oxepinochromone ring system. Eranthin was obtained by a similar strategy involving Claisen rearrangement to transfer an allyl group from the C-7 hydroxyl of noreugenin to C-8 regioselectively.