Nicolaou & E. G. Sorensen, summarized by J. Mulzer & D. Riether, and G. W. Craig, besides many other publications where these epochal syntheses are discussed. The Harvard/ETH approach to the synthesis of cobyric acid: the path to the common corrinoid intermediate via A/B-corrin-ring closure In the A/B approach to cobyric acid, the Harvard A-D-component was coupled to the ETH B-C-component between rings D and C, and then closed to a corrin between rings A and B. Both these critical steps were accomplished by C,C-coupling via sulfide contraction, a new reaction type developed in the synthesis of the B-C-component at ETH. The A-D-component was synthesized at Harvard from a ring-A precursor (prepared from achiral starting materials), and a ring-D precursor prepared from (−)-camphor. A model A-D-component was used to explore the coupling conditions; this component differed from the A-D-component used in the final synthesis by having as the functional group at the ring-D f-side chain a methyl ester group (like all other side chains) instead of a nitrile group. The ETH approach to the synthesis of cobyric acid: the path to the common corrinoid intermediate via A/D-corrin-ring closure 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). ETH/Harvard: the jointly executed final steps from the common corrinoid intermediate to cobyric acid The final steps from the common corrinoid intermediate E-37/HE-44 to cobyric acid E-44/HE-51 were carried out by the two groups collaboratively and in parallel, the ETH group working with material produced by the A/D approach, and the Harvard group with that from the A/B approach. What the two groups in fact accomplished thus were the common final steps of two different syntheses. The tasks in this end phase of the project were the regioselective introduction of methyl groups at the two meso positions C-5 and C-15 of E-37/HE-44, followed by conversion of all its peripheral carboxyl functions into primary amide groups, excepting that in side chain