A Formal Total Synthesis of (+)‐Methyl Trachyloban‐18‐oate and (+)‐Methyl 16‐Oxo‐17‐norkauran‐18‐oate: Regio‐ and Diastereoselective Preparation of Methyl (13 S )‐13‐Hydroxyisoatisiren‐18‐oate from (−)‐Abietic Acid

A novel preparation of methyl (13 S )‐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 .