Reduction‐Labile Organo‐cob(III)alamins via Cob(II)alamin: Efficient Synthesis and Solution and Crystal Structures of [(Methoxycarbonyl)methyl]cob(III)alamin

An efficient synthesis of Coβ‐[(methoxycarbonyl)methyl]cob(III)alamin ( 6 ) is reported as an example of a new method for the preparation of some easily reducible organo‐cob(III)alamins via the alkylation of cob(II)alamin. The procedure represents a considerable improvement compared to earlier methods that were based on an alkylation of cob(I)alamin. Thus, aquacob(III)alamin chloride ( 5 ⁺?Cl) was reduced to cob(II)alamin ( 4 ), either by controlled potential electrolytic reduction or with an excess of sodium formate as reducing agent. The solution of 4 was then treated with an excess of methyl bromoacetate while being reductively poised potentiostatically or kept reduced by the formate, to give crystalline 6 in a yield of up to 91%. The structure of 6 in aqueous solution was mainly established by the completely assigned ¹H‐ and ¹³CNMR spectra (Table 1). The NOE data (Table 2) were best rationalized by the presence of a single main conformation of the (methoxycarbonyl)methyl ligand. Single crystals of 6 were obtained by crystallization from an aqueous solution, and the crystal structure was determined by X‐ray analysis at cryotemperatures. The NMR and crystallographic data of 6 indicated similar structures in aqueous solution and in the crystal with the (methoxycarbonyl)methyl ligand preferring a ‘southern' orientation in each case.