Synthesis of 7-acetyloxy-3,7-dimethy1-7,8-dihydro-6H-isochromene-6,8-dione and its analogues



7-Alkanoyloxy-3,7-dimefhyl-7,8-dihydro-6H-isochromene-6,8-diones 12-15 were synthesized in 69-16% yields from the reaction of 2,4-dihydroxy-3-methyl-6-(2-oxopropyl)benzaldehyde 11 with p-toluenesulfonic acid in various carboxylic acids such as acetic acid, propionic acid, butyric acid and heptanoic acid followed by oxidation with lead tetraacetate. On the other hand, (±)-daldinin A 5 (oleate) was not obtained using oleic acid as a medium. In the cases of heptanoic acid and oleic acid, esters 16 and 17 were produced in 23 and 9% yields, respectively. 6,8-Dihydroxy-3,7-dimethyl-2-benzopyrylium p-toluenesulfonate 31 is considered as the intermediate for the production of 12-15. Overall yields of isochromenes 12-15 were 26-6% starting from 2-methylresorcinol for seven steps.