Concentrated hydrochloric acid (0.34 ml) is added dropwise to a solution of 372 mg of (3R,5S)-1-allyloxycarbonyl-3-t-butyldimethylsilyloxy-5-[1-hydroxy-1-(imidazo[5,1-b]thiazol-3-yl)methyl]pyrrolidine (stereoisomer B), described in Synthesis Example 4-a), in 8.5 ml of dry acetonitrile under ice cooling (internal temperature 3° C.). The mixture is stirred in this state for 20 min and diluted with 50 ml of ethyl acetate, and the diluted solution is adjusted to pH 9.2 by addition of a 5% aqueous sodium hydrogencarbonate solution, followed by shaking. The organic layer is separated. The aqueous layer is extracted twice with 50 ml of ethyl acetate while adjusting the pH value to 9.2 by addition of a 5% aqueous sodium hydrogencarbonate solution. The organic layer is combined with the above organic layer, and the combined organic layers are dried over magnesium sulfate, followed by filtration and the removal of the solvent by evaporation under reduced pressure to give 313 mg of crude (3R,5S)-1-allyloxycarbonyl-3-hydroxy-5-[1-hydroxy-1-(imidazo[5,1-b]thiazol-3-yl)methyl]pyrrolidine (stereoisomer B) as a slight yellow viscous material.
