8-(4-Hydroxy-bicyclo[2.2.2]oct-1-yl)-1,3-dipropyl-3,7-dihydro-purine-2,6-dione (Example 13a) (180 mg, 0.5 mmol) was dissolved in a mixture of 1H-tetrazole (350 mg, 5 mmol) in 10 ml of dry acetonitrile and 10 ml of dry methylene chloride. At room temperature dibenzyl diethylphosphoramidite (476 mg, 1.5 mmol) was added under nitrogen and the mixture was stirred for 2 h. Then 70% tert-butyl hydroperoxide solution (1 ml) was added and the mixture was stirred for 1 h. At 0° C. 15 ml of 10% NaHSO3 was added and the mixture was stirred for another 15 minutes. The mixture was then extracted three times with CH2Cl2, and the organic layer was washed water. Evaporation of solvent yielded Phosphoric acid dibenzyl ester 4-(2,6-dioxo-1,3-dipropyl-2,3,6,7-tetrahydro-1H-purin-8-yl)-bicyclo[2.2.2]oct-1-yl ester as a yellow oil, which was purified by column chromatography. Fractions contained Phosphoric acid dibenzyl ester 4-(2,6-dioxo-1,3-dipropyl-2,3,6,7-tetrahydro-1H-purin-8-yl)-bicyclo [2.2.2]oct-1-yl ester was combined and concentrated in vacuum to get 280 mg product. MS (M+1) 621.
