The intermediate phosphorochloridate (III) has been obtained as follows: The reaction of 4-bromophenol (IV) with POCl3 and triethylamine in ethyl ether gives the corresponding phosphorodichloridate (V), which is finally condensed with L-alanine methyl ester (VI) by means of triethylamine in dichloromethane.
The dehydration of thymidine (I) by known methods (J Med Chem 1989, 32: 461) gives 2,3-anhydrothymidine (II), which is then esterifired with the phosphoryl chloride (III) by means of 1-methylimidazole in THF.