Synthesis of calythrone and related cyclopentene-1,3-diones via rearrangement of 4-ylidenebutenolides

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Abstract
Treatment of 4-ylidenebutenolides with sodium methoxide in methanol 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 (18)] from Calythrix tetragona, and the related cyclopentene-1,3-diones [(15) from (14); (25) from (24); (27) from (26); (33) from (32); and (37) from (36)]. Attempts to rearrange the butenolide (21) to natural lucidone (2b), from Lindera lucida were unsuccessful, and instead led to the corresponding 4-oxo-unsaturated ester (23). The relevance of the facile rearrangement of 4-ylidenebutenolides to cyclopentene-1,3-diones, to a possible bio-synthetic route is considered.