rings A and D. The paths of the two syntheses met in a common corrinoid intermediate. The final steps from this intermediate to cobyric acid were carried out in the two laboratories again collaboratively, each group working with material prepared via their own approach, respectively. Synopsis of the Harvard/ETH collaboration The beginnings Woodward and Eschenmoser embarked on the project of a chemical synthesis of vitamin B12 independently from each other. The ETH group started with a model study on how to synthesize a corrin ligand system in December 1959. In August 1961, the Harvard group began attacking the buildup of the B12 structure directly by aiming at the most complex part of the B12 molecule, the "western half" that contains the direct junction between rings A und D (the A-D-component). Already in October 1960, the ETH group had commenced the synthesis of a ring-B precursor of vitamin B12. At the beginning, progress at Harvard was rapid, until an unexpected stereochemical course of a central ring formation step interrupted the project. Woodward's recognition of the stereochemical enigma that came to light by the irritating behavior of one of his carefully planned synthetic steps became, according to his own writings, part of the developments that led to the orbital symmetry rules. After 1965, the Harvard group continued work towards an A-D-component along a modified plan, using (−)-camphor as the source of ring D. Joining forces: the A/B approach to cobyric acid synthesis By 1964, the ETH group had accomplished the first corrin model synthesis, and also the preparation of a ring-B precursor as part of a construction of the B12 molecule itself. Since independent progress of the two groups towards their long-term objective was so clearly complementary, Woodward and Eschenmoser decided in 1965 to join forces and to pursue from then on the project of a B12 synthesis collaboratively, planning to utilize the ligand construction (ring coupling of components) strategy of the ETH model system. By 1966, the ETH group had succeeded in synthesizing the B-C-component ("eastern half") by coupling their ring-B precursor to the ring-C precursor. The latter had also been prepared at Harvard from (−)-camphor by a strategy conceived and used earlier by A. Pelter and J. W. Cornforth in 1961. At ETH, the synthesis of the B-C-component involved the implementation of the C,C-condensation reaction via sulfide contraction. This newly developed method turned out to provide a general solution to