But-2-ynoic acid (0.19 g, 2.36 mmol) followed by pyridine (0.6 ml), were added sequentially portion wise to a stirred solution of 4-(4-amino-benzenesulfonyl)-piperazine-1-carboxylic acid tert-butyl ester (0.2 g, 0.59 mmol) in THF/DMA (3:2, 5 ml) at room temperature. To this mixture was added EDC (0.6 g, 3.1 mmol) in one portion and the mixture was stirred at room temperature under a nitrogen atmosphere for 1 hour. After this time the mixture was diluted with ethyl acetate (50 ml) and washed with water (100 ml), saturated NaHCO3 (100 ml) and HCl (100 ml, 2M). The organic layer was separated, dried (MgSO4), filtered and concentrated under vacuum and the resulting residue was purified by flash column chromatography (elution, 20% ethyl acetate, 80% DCM) to give the title compound (0.05 g, 19% yield) as a white solid. Tr=4.12 min, m/z (ES+) (M+Na)+ 430.
