Treatment of ribonucleosides with thionyl chloride/pyridine/acetonitrile (0° C to ambient temperature) resulted in essentially quantitative formation of 5'-chloro-5'-deoxy-2', 3'-O- sulfinylnucleoside derivatives. These diastereomeric sulfite esters underwent deprotection readily with aqueous methanolic ammonia. This gave 5'-chloro-5'-deoxynucleosides without use of the suspected carcinogen, hexamethylphosphoramide (HMPA). Nucleophilic ...