Abstract: Disclosed is a 5-O-pyrimidyl-2,3-dideoxy-1-thio-D-furanoside derivative represented by the following formula (I) or an L-form isomer thereof: ##STR1## wherein X represents a hydrogen atom, a fluorine atom or an azido group; R.sup.1 represents a C.sub.1-4 alkyl group; R.sup.2 represents a hydrogen atom, a methyl group or a fluorine atom or a trifluoromethyl group; R.sup.3 represents a C.sub.1-10 alkyl group or a C.sub.6-15 aryl group which is unsubstituted or substituted with at least one substituent selected from the group consisting of a chlorine atom, a bromine atom, a methyl group, an ethyl group and a nitro group. When this derivative is reacted with a sulfonium ion-generating reagent to thereby effect an intramolecular N-glycosylation of the derivative, a .beta.-2',3'-dideoxynucleoside derivative which is useful as a precursor of an antiviral agent, such as AZT, DDC or FLT, can be produced with an extremely high stereoselectivity for the .beta.-anomer and in high yield while very little or none of the .