PURPOSE: To prepare the titled compound useful as a synthetic intermediate of an X-ray contrast medium, in high yield and purity, by reacting an allofuranose derivative with triphenyl phosphine and iodine in the presence of imidazole.

CONSTITUTION: An allofuranose derivative of formula I (R1 and R2 are lower alkyl) is made to react with triphenyl phosphine and iodine in a solvent such as toluene, benzene, etc., in the presence of imidazole, at 110W120°C for 2W3hr to obtain the objective compound of formula II in high safety. The preferably molar ratios of imidazole : triphenyl phosphine : iodine : the compound of formula I are 1.5:1.2:1.3:1.0W3:2:3:1. The process is industrially advantageous in view of safety, reaction time, purity, yield, etc.