[Br:1][CH:2]1[CH2:7][S:6][C:5]2[S:8][C:9]([S:11]([NH2:14])(=[O:13])=[O:12])=[CH:10][C:4]=2[C:3]1=[O:15].[BH4-].[Na+].Cl>CO.O>[Br:1][CH:2]1[CH2:7][S:6][C:5]2[S:8][C:9]([S:11]([NH2:14])(=[O:13])=[O:12])=[CH:10][C:4]=2[CH:3]1[OH:15]A suspension of 5,6-dihydro-5-bromo-4H-thieno[2,3-b]thiopyran-4-one-2-sulfonamide (2.58 g, 0.0079 m) in methanol (100 ml) was cooled to -10° C. and vigorously stirred while a solution of sodium borohydride (0.45 g, 0.012 m) in water (10 ml) was added over 25 minutes while maintaining the temperature at -10° to -5° C. The mixture was stirred at ambient temperature for 2 hours, then acidified with 6N hydrochloric acid and concentrated in vacuo below 40° C. The residue was distributed between ethyl acetate (100 ml) and water (100 ml), the aqueous layer was separated and again extracted with ethyl acetate (2×75 ml). The combined extracts were washed with water, dried over sodium sulfate and concentrated in vacuo to give 2.42 g (93%) of crude product. Partial purification was effected by chromatography on silica gel, eluting with 5% methanol-chloroform, to yield 1.71 g of product which was used in the subsequent step without further purification.
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