The Chemoenzymatic Total Synthesis of Phellodonic Acid, a Biologically Active and Highly Functionalized Hirsutane Derivative Isolated from the Tasmanian Fungus Phellodon melaleucus

摘要

A total synthesis of the title natural product, 1, has been achieved using the cis-1,2-dihydrocatechol 7 as starting material. Compound 7 is readily obtained in large quantity and in an enantiomerically pure form through the whole-cell biotransformation of toluene using the genetically engineered microorganism E. coli JM109 (pDTG601) that overexpresses the enzyme toluene dioxygenase (TDO). Three key chemical steps were employed in the synthesis, the first of which was the microwave-promoted Diels-Alder cycloaddition reaction between thene 8 and cyclopent-1-en-2-one to give adduct 9. The second key step was the photochemically promoted oxa-di-π-methane rearrangement of the bicyclo[2.2.2]octenone derivative 15 of 9 to give the epimers 16 and 17, and the third key step was the reductive cleavage of the last pair of compounds so as to afford the linear triquinane 19. Elaboration of compound 19 to target 1 followed established procedures. Single-crystal X-ray analyses were carried out on compounds 11 and 19.