6-Farnesyl-5,7-dihydroxy-4-methylphthalide oxidation mechanism in mycophenolic acid biosynthesis

Abstract

The farnesyl-chain oxidation-mechanism in mycophenolic acid biosynthesis has been studied by incorporation experiments and isotopic-trap experiments with advanced precursors. Two different approaches were used for the synthesis of the precursors : a semisynthetic route, starting from mycophenolic acid, and a total-synthesis route. Our results show that 6-farnesyl-5,7-dihydroxy-4-methylphthalide (3) is converted into mycophenolic acid (1) by at least two pathways : a direct oxidation of the central double bond, and a two-stage removal of the terminal and central groups. The incorporation percentages indicate that the two pathways are equally important. As regards the oxidation mechanism, our data suggest the sequence: epoxidation of the double bond, rearrangement of the epoxide to ketone, hydroxylation to α-hydroxyketone, and finally Woodward reaction with C–C bond cleavage and formation of the acid. The last three biosynthetic steps leading to mycophenolic acid (1) in P. brevicompactum(prenylation of the aromatic nucleus, oxidation of the farnesyl chain, and O-methylation) are characterized by a very low enzyme specificity.