After 10.0 g (49.2 mmol) of indole-3-pyruvic acid was added to and dissolved in 98 ml of aqueous saturated sodium carbonate solution, the resulting solution was adjusted to pH 12.4 using aqueous 25% sodium hydroxide solution. After 16.3 g (147.6 mmol) of sodium pyruvate was added for reaction at ambient temperature for 2 hours, a reaction solution containing 4-hydroxy-4-(3-indolylmethyl)-2-ketoglutaric acid was obtained. 13.7 g (197 mmol) of hydroxylamine hydrochloride salt was added to the reaction solution while the reaction solution was adjusted to a pH value around neutrality using aqueous 25% sodium hydroxide solution, and agitated at ambient temperature for 4 hours. Using conc. hydrochloric acid, the reaction solution was adjusted to an acidic pH value, to extract the organic matter in ethyl acetate. The organic layer was rinsed with aqueous saturated sodium chloride, dried over anhydrous magnesium sulfate and subsequently concentrated, to obtain the residue. The residue was recrystallized in aqueous 28% ammonia and ethanol, to obtain 5.51 g of 4-hydroxy-4-(3-indolylmethyl)-2-hydroxyiminoglutaric acid (16.2 mmol; a yield of 32% vs. indole-3-pyruvic acid) in crystal form.
