Patent ID: 8859755

Claim:
A method for preparing a ribonucleoside phosphorothioate represented by the following formula (I) or a salt thereof: wherein, R 1 represents hydrogen atom or a protective group of hydroxyl group, Bs represents a nucleobase which may have a protective group, n represents 0 or an integer of 1 or larger, and n of X independently represents a divalent group represented by the following formula (II-Sp) or (II-Rp): wherein, R 2 represents hydrogen atom or cyanoethoxymethyl group, which comprises the step of condensing a compound represented by the following formula (III): wherein, R 3 and R 4 independently represent a protective group of hydroxyl group, and one of R 3 and R 4 may represent a solid phase carrier bound via a linker as required, with an oxazaphospholidine ribonucleoside represented by the following formula (IVa) or (IVb): wherein, CEM represents cyanoethoxymethyl group, R 5 represents a protective group of hydroxyl group, and R 6 represents an aryl group which may have a substituent, and then sulfurizing the resulting product.