SYNTHESIS AND CONFORMATIONAL ANALYSIS OF TRICYCLIC RING-C AROMATIC 20-NOR DITERPENOID RESIN ACID ANALOGS

A simple synthesis of the tricyclic unsaturated acid 8 and its conversion into lactone 9 is described. All four possible racemates of ring-C aromatic 20-nor diterpenoid resin acid analogs 1, 2, 3, and 4 have been synthesized by catalytic and chemical reduction of 8 and 9. Lithium-ammonia reduction of the benzylic lactone 9 proceeds with retention of configuration at C-12 to give trans acid 1, while catalytic hydrogenation of 9 proceeds with inversion at C-12 to give cis acid 3. Lithium-ammonia reduction of 8 yields trans acid 2 exclusively, whereas catalytic hydrogenation of 8 gives 75% cis acid 3 and 9% cis acid 4. Some chemical and conformational properties of 1, 2, 3, and 4 are reported. In contrast to the corresponding cis resin acid analogs where the conformation of ring A is “steroid,” ring A for the cis acids 3 and 4 is “nonsteroid.”. © 1969, American Chemical Society. All rights reserved.