Adenosyl-176-norcobinamide – A likely biosynthetic precursor to natural 176-norvitamin B12 derivatives

The “complete” corrinoid 176-norpseudovitamin B12 was recently isolated as the cyano-Co(III)-form of the corrinoid cofactor of tetrachlorethene reductive dehalogenase of the anaerobe Sulfurospirillum (formerly Dehalospirillum) multivorans. 176-Norpseudovitamin B12 represents the first example of (the cyano-Co(III)-form of) a naturally occurring “complete” B12 cofactor lacking a characteristic peripheral methyl group of the cobamide ligand. Its discovery has generated interest in 176-nor-B12 derivatives, “complete” corrinoids lacking the methyl group attached to carbon 176. Here, we report the preparation of Coβ-5′-adenosyl-176-norcobinamide by in situ alkylation of Co(I)-176-norcobinamide, obtained from electrochemical reduction of Coα,Coβ-dicyano-176-norcobinamide. Since Coβ-5′-adenosylcobinamide is a biosynthetic intermediate of the complete cobamides, Coβ-5′-adenosyl-176-norcobinamide is a “rational” biosynthetic precursor for natural 176-nor-B12 derivatives. The spectroscopic data for adenosyl-176-norcobinamide establish the suggested structure of the title compound and give further evidence for the extensive flexibility and conformational dynamics of the organometallic 5′-deoxy-5′-adenosyl ligand.