PURPOSE: To obtain a new compd. useful as an inhibitor for reverse transcriptase of virus for the treatment of infectious diseases caused by retroviruses.

CONSTITUTION: This compd. is expressed by formula I (R is H or a blocking group; A is H, OH, a halogen, SH, NH2 or azide; B is a (deoxy)nucleoside base; (n) is 0 to 30). The compd. is obtd., for example, in the following processes. First, 5'-O-dimethoxytrimethyl-N4-benzoyldeoxycytidine is reacted with bis- pyrrolidinyl chlorophosphine and 4-chlorobenzylmercaptan to obtain deoxycytidine phosphorothioamidide expressed by formula II. The obtd. compd and a dried acetonitrile soln. of tetrazole are reacted with a deoxynucleoside on a silica expressed by formula III ((P) is a silica base body). The obtd. product is oxidized with sulfur, and successively subjected to acylation and detritylation of unreacted deoxynucleoside. Then the produced compd. expressed by formula IV is used to extend oligonucleotide.