the problem of constructing the characteristic structural elements of the corrin chromophore, the vinylogous amidine systems bridging the four peripheral rings. Early in 1967, the Harvard group accomplished the synthesis of the model A-D-component, with the f-side chain undifferentiated, bearing a methyl ester function like all other side chains. From then on, the two groups systematically exchanged samples of their respective halves of the corrinoid target structure. By 1970, they had collaboratively connected Harvard's undifferentiated A-D-component with ETH's B-C-component, producing dicyano-cobalt(III)-5,15-bisnor-heptamethyl-cobyrinate 1 (fig. 4). The ETH group identified this totally synthetic corrinoid intermediate by direct comparison with a sample produced from natural vitamin B12. In this advanced model study, reaction conditions for the demanding processes of the C/D-coupling and the A/B-cyclization via sulfide contraction method were established. Those for the C/D-coupling were successfully explored in both laboratories, the superior conditions were those found at Harvard, while the method for the A/B-ring closure via an intramolecular version of the sulfide contraction was developed at ETH. Later it was shown at Harvard that the A/B-ring closure could also be achieved by thio-iminoester/enamine condensation. By early 1971, the Harvard group had accomplished the synthesis of the final A-D-component, containing the f-side chain carboxyl function at ring D differentiated from all the carboxyl functions as a nitrile group (as shown in 2 in fig. 4; see also fig. 3). The A/D-part of the B12 structure incorporates the constitutionally and configurationally most intricate part of the vitamin molecule; its synthesis is regarded as the apotheosis of the Woodwardian art in natural product total synthesis. The alternative approach to cobyric acid synthesis As far back as 1966, the ETH group had started to explore, once again in a model system, an alternative strategy of corrin synthesis in which the corrin ring would be closed between rings A and D. The project was inspired by the conceivable existence of a thus far unknown bond reorganisation process. This if existing would make possible the construction of cobyric acid from one single starting material. Importantly, the hypothetical process, being interpreted as implying two sequential rearrangements, was recognized to be formally covered by the new reactivity classifications of sigmatropic rearrangements and electrocyclizations propounded by Woodward and Hoffmann in the context of their orbital symmetry rules! By May 1968, the ETH group had demonstrated in a model study that the envisaged process, a photochemical A/D-seco-corrinate→corrinate cycloisomerization, does in fact exist. This