A total synthesis of mycophenolic acid, some analogues and some biogenetic intermediates

Mycophenolic acid (28) has been obtained by a convergent synthesis starting from methyl 6-bromo-4-methylhex-4-enoate (13) and 5,7-dihydroxy-4-methylphthalide (24). For the total synthesis of 5,7-dihydroxy-4-methylphthalide, 1-carbethoxy-2,3-dimethylcyclohexa-4,6-dione (14) was aromatized and transformed into 4,6-dimethoxy-2,3-dimethylbenzamide. The photolysis of the corresponding N-chloroamide and subsequent hydrolysis gave 5,7-dimethoxy-4-methylphthalide which was hydrolysed to 5,7-dihydroxy-4-methylphthalide (24). The bromoester (13) was obtained starting from geraniol. Condensation between 13 and 24 with silver oxide in dioxane afforded the methyl ester of nor-O-methyl mycophenolic acid. Selective methylation and hydrolysis furnished mycophenolic acid.