text: In the A/D approach to the synthesis of cobyric acid, the four ring precursors (ring-C precursor only formally so) derive from the two enantiomers of one common chiral starting material. All three vinylogous amidine bridges that connect the four peripheral rings were constructed by the sulfide contraction method, with the B-C-component  already prepared for the A/B-approach  serving as an intermediate. The photochemical A/D-secocorrin→corrin cycloisomerization, by which the corrin ring was closed between rings A and D, is a novel process, targeted and found to exist in a model study (cf. fig. 2).