|Isopropylidine adenosine was converted to the p-toluene sulphonyl (tosyl) ester by reaction with tosyl chlorine solution, following the method of Clark et al. (1951) [J. Chem. Soc. 2952]. Because of its tendency to cyclization, the reagent was used directly it was ready. A reaction flask with separating funnels was set up in such a way that the whole system could be evacuated and filled with pure nitrogen two or three times, to eliminate all oxygen, and reagents could then be added when desired, in the closed system.
The flask contained 700.0 mg hydroxocobalamin in 20 ml of water, one funnel 200.0 mg sodium borohydride in 10 ml of water, and another the crude isopropylidine adenosine tosyl ester made from 500 mg isopropylidine adenosine dissolved in 10 ml of 50% aqueous methanol. On adding the borohydride to the vitamin, the color changed instantly from red to brown, then slowly to a greenish black. After 15 min the isopropylidine adenosine tosyl ester was added, and the colour slowly changed to a red-brown. After 45 min at room temperature air was admitted and the mixture was shaken to reoxidise any remaining reduced vitamin B12. The alkaline solution was neutralized with dilute hydrochloric acid and extracted with phenol carbon tetrachloride 3:1 in small portions till the aqueous layer was nearly colorless. The combined extracts were washed with water, mixed with about ten parts of carbon tetrachloride- 10:1 and shaken with small portions of water till all red color was removed.
The product was purified by chromatography on columns of DEAE (diethyl aminoethyl) cellulose (3 x 1) followed by CM (carboxymethyl) cellulose (6 x 1), developed with water. Nearly all the color washed quickly through DEAE cellulose. The effluent and washes were applied to the CM cellulose column, which was further developed with water. Elution was continued as long as this fraction continued to emerge, in a total of 850 ml. One half of this fraction (425 ml) was concentrated to a few ml under reduced pressure; it crystallized slowly after adding to slight turbidity. So cobamamide was obtained.