Abstract On reaction of thionyl chloride with cytidine and adenosine in refluxing acetonitrile, the 5'-chloro-2', 3'-sulphinyl derivatives I and VII are formed in a quantitative yield. On heating in dimethyl-formamide, compound I affords 5'-chloro-5'-deoxycyclocytidine (II) which is hydrolyzed in alkali to the arabinosyl derivative III; reduction of III with tributyltin hydride gives the 5'-deoxyarabinosyl derivative IV. The sulphinyl derivative I is hydrolyzed to 5'- ...