A solution of p-toluenesulfonic acid (7.79 g, 0.45 mol) in dry acetone (100 ml) is added dropwise to a solution of adenosine (10 g, 0.037 mol) in dry acetone (300 ml). 2,2-Dimethoxypropane (18.18 ml, d=0.847) is then added to the reaction mixture, and the mixture stirred for 48 hours (TLC methylene chloride/ethanol 9:1). The solution slowly becomes clear and is made basic with 3% ammonium hydroxide (800 ml). The solvent is removed at reduced pressure, keeping the temperature below 30° C. until formation of a solid that is collected by filtration. This procedure is repeated several times by concentrating the filtrate. A white solid is obtained (10.08 g, 0.0328 mol, 87.7% yield).
