Synthetic studies relevant to biosynthetic research on vitamin B12. Part 9. Synthesis of 20-methyl and 20-cyano isobacteriochlorins

Abstract

Isobacteriochlorins carrying a C-methyl group at C-20 of the macrocycle are important for research on the biosynthesis of vitamin B12. Several different approaches are studied which allow the introduction of a C-20 methyl group into model isobacteriochlorins, the most successful involving the stepwise reduction of a nitrile residue to a methyl group. Successful syntheses are described of two 20-methylisobacteriochlorins and two 20-cyanoisobacteriochlorins. All the routes used depend finally on the photochemical cyclisation of an 18π-electron open-chain precursor.