The Eschenmoser Cyclization Strategy

75 blsnorcobyrinic acid abdeg pentamethyl ester c dlmethylamlde f nitrile 

Friedrich, W. Gross, G. Bemhauer, K. Zeller, P. Helv. Chim. Acta 1960, 43, 704. 

In March 1976, M.A. Wuonola and R.B. Woodward accomplished the conversion of cobyric acid (4) to vitamin B12 (1). The total synthesis of vitamin B12 can thus be claimed, see reference Id, footnote 11, p. 1420. The formal total synthesis of 1 had been accomplished in 1973. 

We are grateful to Prof. A. Eschenmoser for helpful comments on this chapter. 

Intermediate 7, a viable precursor of intermediate 6, possesses a y, 5-unsaturated ester, the structural prerequisite, or retron, for the ortho ester Claisen transform.5 In the synthetic direction, the convergent union of intermediates 9 and 10 could give mixed-ketene acetal 8 the intermediacy of 8 should be brief, for it should readily 

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