To a mixture of 2-mercaptonicotinic acid (100 mg, 0.65 mmol) in DMF (5 mL) was added EEDQ (207 mg, 0.84 mmol). The resulting mixture was stirred for 10 minutes, followed by the addition of a premixed solution of 4-azido-phenyl-ammonium chloride (219 mg, 1.29 mmol) and DIPEA (224 uL, 1.29 mmol) in DMF (2 mL). The mixture was allowed to stir for 15 h and was poured into ethyl acetate. The resulting solution was washed with 0.1 N hydrochloric acid, water and brine. The organic phase was dried over magnesium sulfate, was filtered and concentration in vacuo. The resulting residue was taken up in ethyl acetate (10 mL) and solids were precipitated through the addition of hexanes. The solids were collected by filtration (92 mg) and were used directly without purification or characterization. The solids were dissolved in DMF (5 mL), ethyl bromoacetate (75 μL, 0.68 mmol) was added, followed by the addition of cesium carbonate (332 mg, 1.02 mmol). The reaction mixture was stirred for 15 minutes, poured into water and the resulting precipitate was collected via vacuum filtration on a fritted glass funnel. The solids were washed off the filter with ethyl acetate and the mixture was dried with magnesium sulfate. Filtration and concentration in vacuo gave a residue that was dissolved in ethyl acetate (10 mL). The solution was treated with hexanes and cooled resulting in precipitation of [5-(4-azido-phenylcarbamoyl)-pyridin-2-ylsulfanyl]-acetic acid ethyl ester, 46 mg (20% for two steps), isolated by filtration: 1H NMR (300 MHz, DMSO-d6) δ 10.38 (bs, 1H), 8.89 (m, 1H), 8.14 (m, 1H), 7.77 (m, 2H), 7.49 (m, 1H),7.12 (m, 2H), 4.10 (m, 4H), 1.16 (m, 3H); MS (EI) m/z 356.11 (M−H)−.
