0.15 g (0.31 mmol) of 5'-tritylamino-5'-deoxy-thymidine [prepared as described in step (a) above] was dried by azeotropic distillation with pyridine and then dissolved in 2 ml of methylene chloride. 0.23 ml (1.2 mmol) of N,N-diisopropyl-N-ethylamine was then added to the resulting solution, after which 0.08 ml (0.36 mmol) of 2-cyanoethyl N,N-diisopropylchlorophosphoramidite was added over a period of 2 minutes in a stream of nitrogen. The resulting mixture was then stirred at room temperature for 60 minutes. At the end of this time, it was confirmed that the starting compound had disappeared by thin layer chromatography, and the reaction mixture was diluted with 30 ml of ethyl acetate. The resulting organic solution was washed with a saturated aqueous solution of sodium hydrogencarbonate and with a saturated aqueous solution of sodium chloride, in that order. The organic layer was through a 1PS (trade mark) filter paper (Whatman), and the filtrate was freed from the solvent by distillation under reduced pressure. The resulting residue was purified by column chromatography through 15 g of silica gel (70-230 mesh), using ethyl acetate as the eluent, to give 0.19 g (yield 89%) of the title compound as a foamy substance.
