Hydrophobic Vitamin B12. XI. Preparation, Characterization, and Enantioselective Alkylation of Hydrophobic Vitamin B12 Bearing a Binaphthyl Moiety

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Abstract
Hydrophobic vitamin B12 derivatives bearing a chiral binaphthyl moiety, hexamethyl 71-decarboxy-71-[(R)-2′-methoxy-1,1′-binaphthyl-2-carboxymethyl]cobyrinate perchlorate [B12–BINAP(R)] and hexamethyl 71-decarboxy-71-[(S)-2′-methoxy-1,1′-binaphtyl-2-carboxymethyl]cobyrinate perchlorate [B12–BINAP(S)], were prepared from cyanocobalamin. These complexes were characterized by means of electronic and circular dichroism spectroscopy as well as by cyclic voltammetry in comparison with those data for a hydrophobic vitamin B12 without a binaphthyl moiety. The enantioselective alkylation of hydrophobic vitamin B12 derivatives at the β-axial site was examined in methanol with various 3-bromo-2-methylpropionic esters by means of 1H NMR spectroscopy. All the hydrophobic vitamin B12 derivatives used here, the one bearing methoxycarbonyl groups as peripheral substituents without a binaphthyl moiety, B12–BINAP(R), and B12-BINAP(S), were found to bind (S)-2-methylpropionates more favorably than the corresponding R-enantiomers; the highest S-selectivity was observed with the latter two derivatives, 65% e.e. The cause of such S-enantioselectivity was discussed with attention to stereochemical configurations of the peripheral substituents placed in the corrin ring.

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