To a solution of 3-{2-[(S)-1-(2-fluorophenyl)ethylamino]-6,6-dimethyl-4,4-dioxo-5,6-dihydro-4H-4lambda*6*-[1,4,3]oxathiazin-5-yl}benzoic acid methyl ester (1.7 mg) in dioxane (15 ml) were added water (5 ml) and sulfuric acid (0.3 ml), and the mixture was stirred at 100° C. for 16 h. The mixture was alkalized with sodium carbonate solution, diluted with water and washed with ethyl acetate. The aqueous phase was acidified, extracted with ethyl acetate, dried over magnesium sulfate and concentrated. This gave the product (551 mg) with a molecular weight of 420.5 g/mol (C20H21FN2O5S).
