169. A formal total synthesis of (+)-methyl trachyloban-18-oate and (+)-methyl 16-oxo-17-norkauran-18-oate: Regio- and diastereoselective preparation of methyl (13S)-13-hydroxyisoatisiren-18-oate from (-)-abietic acid

A novel preparation of methyl (13S)-13-hydroxyisoatisiren-18-oate (4), a key-intermediate in a synthesis of (+)-methyl trachyloban-18-oate ((+)-1), from (-)-abietic acid, is described. Since (-)-1 has been previously converted into (-)-methyl 16-oxo-17-norkauran-18-oate ((-)-16), our preparation of 4 constitutes also a formal total synthesis, from (-)-abietic acid, of (+)-16. Key steps in this approach were the allene photoaddition to podocarp-8(14)-en-13-one (5) and the conversion of the endo-toluene-4-sulfonate 11 into the exo-benzoate 12b.