A solution of 3.4 g (16 mmol) of sodium periodate in 30 ml of water was added dropwise to 5.3 g (15 mmol) of the thioether from Example 10 in a mixture of 150 ml of ethanol and 40 ml of dimethylformamide at 0° C., and the mixture was then stirred at the same temperature for 2 hours and at room temperature overnight. The reaction solution was then poured into water, the pH was adjusted to 5 with 2N hydrochloric acid, the mixture was extracted with dichloromethane, and the organic extracts were washed with water, dried over magnesium sulfate and evaporated under reduced pressure. Crystallization of the oily residue from ethanol yielded 2.2 g of title compound of melting point 198°-200° C.
