PURPOSE: To obtain the titled compound in high yield and simply, by reacting a specific fluorinated saccharide with a silylated base in the presence of Lewis acid catalyst.

CONSTITUTION: A fluorinated saccharide (e.g. O2', O3', O5'-tribenzyl-1-fluorinated-α- D-ribofuranose, etc.) shown by formula I [R1 and R2 are OH-protective group; l, m and n are 0W3 and l+m+n=2W3) is reacted with a compound [e.g. bis(trimethylsilyl)uracil, etc.] shown by formula B-[Si(CH3)3]p [B is pyrimidine or purine residue which may be replaced; p is 1W3) in the presence of Lewis acid catalyst (preferably silicon tetrafluoride, trimethylsilyl triflate or boron trifluoride) usually in an inert solvent such as diethyl ether, dichloromethane, etc., preferably at -78W100°C for 30minW72hr to give the aimed compound shown by formula II (R3 and R4 are H or OH-protecting group).