Rearrangement of 4-ylidenebutenolides to cyclopentene-1,3-diones: synthesis of calythrone and related compounds

Treatment of 4-ylidenebutenolides with NaOMe in MeOH results in rearrangement to the corresponding cyclopentene-1,3-diones in high yield; the general method is applied in a synthesis of calythrone (1) from Calythrix tetragona, and the related cyclopentene-1,3-diones (6), (8), and (13).