2-Aminopurine (Pacific Chemical Laboratories, 3.0 g, 22.2 mmoles) and 1-(2-deoxy-2-fluoro-β-D-ribofuranosyl)uracil (0.5 g; 2.0 mmoles) which may be prepared according to J. F. Codington et al., (J. Org. Chem. 29:558, 1964) were suspended in 25 ml of 5 mM potassium phosphate buffer, pH 7.0, which contained 0.04% (w/v) potassium azide. Thymidine phosphorylase (41,700 I.U.) and purine nucleoside phosphorylase (83,300 I.U.) (T. A. Krenitsky et al., Biochemistry 20:3615, 1981 and U.S. Pat. No. 4,381,344) which had been absorbed onto 10.5 g of DEAE-cellulose (25 ml) were added and the suspension shaken at 37° C. After 24 hours 3.0 g of 2,6-diaminopurine was added and the temperature increased to 50° C. After another 24 hours the reaction was filtered. The filter cake was washed with water and the filtrates combined and the solvent removed under vacuum. The residue was suspended in water and filtered. The filter cake was extracted with water (25° C.) until no product remained in the filter cake. The filtrates were combined, the pH was adjusted to pH 9.4 with NH4OH, and the solution applied to a 2.5×20 cm column of anion exchange resin (Bio-Rad AG1X2-hydroxide form). After washing the column with water, the product was eluted with methanol/water (9/1). Product containing fractions were combined and the solvent removed under vacuum. The residue was dissolved in chloroform/methanol/water (75:25:4) and applied to a 5×25 cm column of silica gel 60. The product was eluted with chloroform/methanl/water (75:25:4). Product containing fractions were combined and the solvent removed under vacuum. The residue was dissolved in hot water and filtered through a 0.22 μm-pore membrane filter. Lyophilization of the filtrate yielded 0.50 g of title compound that analyzed as a 0.5 hydrate.
