Sclerotinin A (3,6,8-trihydroxy-3,4,5,7-tetramethyl-3,4-dihyroisocoumarin: 1), a metabolite of Sclerotinia sclerotiorum, was first totally synthesized. The pentenone ring of 2,3,4,6-tetramethyl-5,7-dimethoxyindanone (VIIa) which was obtained from 2,4-dimethylresorcinol dimethyl ether (VI) through a new synthetic process, was cleavaged oxidatively, and demethylated to I. Sclerotinin A was also yielded from indanone dibenzyl ether (VIIb) by the same method and reductive debenzylation.