A mixture of 3.63 g (16 mmol, 1 eq) of (3-amino-2-hydroxyphenyl)-((R)-3-hydroxypyrrolidin-1-yl)methanone and 3.40 g (20 mmol, 1.2 eq) of 3,4-diethoxy-3-cyclobutene-1,2-dione in 70 ml of ethanol was heated at 60° C. for 16 hours. The reaction medium was filtered and the filtrate was chromatographed on silica gel (column puriFlash IR-50SI-300G, Spot II with solid deposition) eluted with dichloromethane/methanol (gradient). 3.67 g of 3-ethoxy-4-[2-hydroxy-3-((R)-3-hydroxypyrrolidine-1-carbonyl)phenylamino]cyclobut-3-ene-1,2-dione were obtained in the form of a beige solid. Yield=66%.
