After 1.0 g (4.92 mmol) of indole-3-pyruvic acid was added to and dissolved in 10 ml of aqueous saturated sodium carbonate solution, the resulting solution was adjusted to pH 12.55 using aqueous 25% sodium hydroxide solution. After 1.3 g (14.8 mmol) of pyruvic acid was added, the resulting solution was adjusted to pH 12.6 using aqueous 25% sodium hydroxide solution, for reaction at ambient temperature for 2 hours, to obtain a reaction solution containing 4-hydroxy-4-(3-indolylmethyl)-2-ketoglutaric acid. 1.37 g (19.7 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 stirred 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 0.52 g of 4-hydroxy-4-(3-indolylmethyl)-2-hydroxyiminoglutaric acid (1.5 mmol; a yield, 31% vs. indole-3-pyruvic acid) in crystal form.
