2-Cyano-1-t-butylethyl or 2-cyano-1-(1,1-diethyl-3-butenyl)ethyl 3'-O-phosphorimidazolidite of 5'-O-protected nucleoside was prepared in situ by the use of 2-cyano-1-t-butylethyl or 2-cyano-1-(1,1-diethyl-3-butenyl)ethyl phosphorobisimidazolidite as a new phosphitylating reagent. The phosphorimidazolidites were found to be key intermediates for preparing 5'-O-protected' nucleoside 3'-O-monoalkylphosphoramidites in situ useful in the solid-phase oligonucleotide synthesis, and for synthesizing conveniently 3'-OH free dinucleoside phosphates and phosphorothioates in solution.