Synthetic studies towards complex diterpenoids-171: synthesis and oxidative cleavage of (+)-19,20-cycloabieta-19-oxo-8,11,13-triene

Oxidative cleavage of the enolates of (+)-19,20-cycloabieta-19-oxo-8,11, 13-triene (1a) and the related tetracyclic ketone (1b) with molecular oxygen leads to (+)-abieta-8,11,13-triene-19,20-dioic acid (20a) and the dicarboxylic acid (20b) respectively. The synthesis of the tetracyclic ketone (1a) has been realized by two different methods through Ni(acac)₂ catalyzed intramolecular carbon-hydrogen insertion of the α-diazomethyl ketone (13), prepared from (+)-20-nor-4-epidehydroabietic acid (7) via lactone 6 and the stereo-specific rearrangement of the cyclobutanone (16), obtained from the easily accessible unsaturated acid (4) via the unsaturated cyclobutenone (15). Lithium-ammonia reduction of 4 gave (+)-20-nor-dehydroabietic acid (11) exclusively, whereas catalytic hydrogenation of 4 over Pd-C (10%) resulted in the A/B-ring cis-acid (9) as the major product, which was obtained exclusively from the lactone 6 by hydrogenolysis under the same condition.