L-Pyroglutamic Acid → tert-Butyl 5-oxo-L-prolinate

129 g of L-pyroglutamic acid were suspended in 2 l of tert-butyl acetate. 30 ml of a 70% strength solution of perchloric acid in water were added thereto. The mixture was then stirred at room temperature for 12 h. It was cooled to 0° C. and the pH of the solution was adjusted to 7 by cautious addition of a 2N NaOH solution. The phases were separated and the organic phase was washed twice with 300 ml of a saturated sodium chloride solution each time. The organic phase was then dried with sodium sulfate. The solvents were removed in vacuo, and the residue was stirred with heptane. The solid obtained in this way was filtered off with suction. The mother liquor was concentrated in vacuo, and the solid obtained in this way was also filtered off with suction.