Synthetic studies relevant to biosynthetic research on vitamin B₁₂. Part 11. Modification of the east and west building blocks and study of different assembly methods for synthesis of isobacteriochlorins

The eastern and western building blocks required for the photochemical route to isobacteriochlorins have been synthesized by a C-C bond-forming step which leaves a cyano group on the bridge carbon, e.g. 5. Two methods have been developed to remove this cyano residue which use retro-Mannich reactions to eliminate either an aminomethyl group, resulting from reduction of the nitrile, or the corresponding sulfonamido group. Two effective ways to carry out the final steps of the isobacteriochlorin synthesis have also been developed.