Electrofugal Fragmentation of Alkylcobalmin Derivatives Using Cob(I)alamin and Heptamethyl Cob(I)yrinate as Catalysts

Article abstract of DOI:10.1002/hlca.19820650406

The cob(I)alamin- (1(I) and the heptamethyl cob(I)yrinate- (2(I)) catalyzed transformation of an epoxide to the corresponding saturated hydrocarbon 3 -> 4 -> 5 is examined (see Schemes 1 and 3-5).Under the reaction conditions, the epoxyalkyl acetate 3 is opened by the catalysts with formation of appropriate (β-hydroxyalkyl)-corrinoid derivatives (13, 14, 17, 18, see Schemes 12 and 14).Triggered by a transfer of electrons to the Co-corrin-? system, the Co,C-bond of the intermediates is broken, generating the alkenyl acetate 4 (cf.Schemes 12 and 14) following an electrofugal fragmentation (cf.Schemes 2 and 12).The double bond of 4 is also attacked by the catalysts, leading to the corresponding alkylcorrinoids (15, 19, see Schemes 12 and 14) which is turn are reduced by electrons from metallic zinc, the electron source in the system, inducing a reductive cleavage of the Co,C-bond with production of the saturated monoacetate 5 (see Schemes 2, 5 and 12).In the cascade of steps involved, the transfer of electrons to the intermediate alkylcorrinoids (13-15, 17-19, see Schemes 12 and 14) is shown to be rate-limiting.Comparing the two catalytic species 1(I) and 2(I), it is shown that the ribonucleotide loop protects intermediate alkylcobalmins to some extent from an attack by electrons.The protection function of the ribonucleotide side-chain is shown to be present in alkylcobalamins existing in the base-on form (cf.Chap. 4 and see Scheme 14).