To 215 g (about 400 mmol) of the crude crystal of A-SO3Na, 300 mL of acetonitrile and 200 mL of sulfolane as a solvent were added, and further phosphoryl trichloride (245.3 g, 1.6 mole) was added and then reacted at 70° C. to obtain a reaction mixture. Further, 5 mL of N,N-dimethyl acetoamide was added thereto and the resulting yellow suspension was stirred at 71 to 73° C. for 40 min and then cooled to 3° C. To the suspension, 1 L of cool water was added at a rate such that the temperature of the reaction system was not over 10° C. The resulting precipitate was collected, washed with cool water and re-crystallized with 350 mL of toluene to obtain 153 g of the aimed white crystalline A-SO2Cl having a melting point of from 130.5 to 131.5° C. (yield: 87% on the basis of A). The IR spectrum is shown in FIG. 4 and the NMR spectrum is shown in FIGS. 5 and 6.
