(S)-2-(2-Hydroxy-ethyl)-piperazine-1,4-dicarboxylic acid 4-benzyl ester 1-tert-butyl ester (cf. WO 96/31501; S. Fustero et al., Org. Lett. 9 (2007), 5283; 2.862 g, 7.85 mmol) was dissolved in 78.5 ml of DCM and pyridinium dichromate (14.772 g, 39.27 mmol) was added. The mixture was stirred at room temperature overnight, then diluted into brine (330 ml) and extracted three times with EA (70 ml). The combined organic phases were washed with water (300 ml), filtered over diatomaceous earth, dried over sodium sulfate, filtered and evaporated to dryness under reduced pressure. The product was dissolved in EA and extracted three times with a 1 N solution of sodium hydroxide. The combined aqueous phases were acidified to pH 1 with 6 N hydrochloric acid, extracted with EA and the combined extracts were washed with water and brine, dried over sodium sulfate, filtered and evaporated to dryness. 2.226 g of the title compound were obtained as a thick colorless oil.
