Corrin Syntheses. Part III

Since our work on corrin synthesis was intended to be a model study for the construction of the ligand system of vitamin B12 (cf. Part I of this series), the choice of an A/D component that would contain an angular Me group at the junction between rings A and D seemed crucial. Such a Me group was expected to direct (through its steric impact) a regioselective functionalization of the two carbonyl groups of a corresponding dilactam structure that appeared to be the obvious type of intermediate in the construction of an A/D component. The target A/D structure envisaged originally was the methyl ester derivative 1a (cf. Fig. 1). However, observations in an early phase of our work made it clear that we better aim at the nitrile derivative 1b, instead of the corresponding methyl ester. Nitrile 1b was in fact synthesized, but not used as A/D component, because, on the way to its synthesis, the opportunity emerged to prepare structure 2. As A/D component, 2 differs from 1b by the isomeric positioning of the angular Me group, it could be prepared regio‐ as well as stereoselectively and, therefore, much more efficiently than 1b.