Hydrochloric Acid → 4-(1,3-diethyl-2,3,6,7-tetrahydro-2,6-dioxo-1H-purin-8-yl)benzenesulfonic acid

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A mixture of 5,6-diamino-1,3-diethyl-2,4(1H,3H)-pyrimidinedione (41.2 g, 0.208 moles), 4-sulfobenzoic acid monopotassium salt (50.0 g, 0.208 moles), and water (250 ml) is prepared, and the pH adjusted to 6.0. Ethyl-3-(3-dimethylaminopropyl)carbodiimide (39.9 g, 0.208 moles) is added in one portion, and the pH of the resulting solution maintained at 5.5±0.5 with the dropwise addition of 4 N hydrochloric acid. When the pH of the solution ceases to rise, the mixture is concentrated on a rotary evaporator, and the residue taken up in 400 ml 10% potassium hydroxide and stirred for 18 hours. At the end of this time, the solution is boiled under reflux for one hour, cooled, filtered, and made acidic with the portionwise addition of 12 N hydrochloric acid. The resulting suspension is cooled and filtered; the solids are triturated with ethanol, filtered, and dried to yield 51.2 grams of 4-(1,3-diethyl-2,3,6,7-tetrahydro-2,6-dioxo-1H-purin-8-yl)-benzenesulfonic acid; mp greater than 360° C.

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