Here we report the direct chemical synthesis of coenzyme B12 (AdoCbl) from Coβ‐5′‐deoxyadenosylcobyric acid (AdoCby) and the preparation of the latter from crystalline CN,H2O‐cobyric acid (CN,H2OCby). AdoCby is a suggested common key intermediate in the biosynthesis of AdoCbl and of other cobamides in microorganisms. AdoCby was thoroughly characterized by spectroscopic means, including homo‐nuclear and hetero‐nuclear NMR, as such data are not available in published work. AdoCbl was prepared from AdoCby in one‐step in over 85% yield, by covalent attachment in aqueous solution of the integral B12‐nucleotide moiety using 1‐ethyl‐3‐(3‐dimethylaminopropyl)‐carbodiimide (EDC·HCl) and N‐hydroxybenzotriazole (HOBt) as coupling reagents. By the same procedure crystalline vitamin B12 (CNCbl) was also prepared in 92% yield from CN,H2OCby. Coordination of the B12‐nucleotide base at the Coα‐face of AdoCby or of CN,H2OCby was indicated to assist in the efficient covalent coupling at the activated f‐side chain function to furnish the complete corrinoids AdoCbl and CNCbl.