2,2'-Dithiobis(5-nitropyridine)

Experimental procedure: 2,2'-Dithiobis(5-nitropyridine) (2.48 g, 8.0 mmol) is dissolved in 25 mL of methanol:DMF solution (1:1 v/v) and 3-mercaptopropionic acid (0.35 mL, 4.0 mmol) in methanol is added with vigorous stirring. Stirring is continued for 12 h, after which the solvent is removed by rotary evaporation to leave a thick yellow oil, which is redisssolved in 40 mL of 0.5 mol/L aqueous NaOH. The solution is then extracted with EtOAc (2 × 40 mL). The aqueous solution is then immersed into an ice bath, and acidified to pH 3.5 by the addition of ice-cold concentrated HCl. The organic layer is then extracted with EtOAc (3 × 50 mL). The organic layers are combined, washed with brine, dried over anhydrous Na₂SO₄, and evaporated to dryness, which is further purified by flash chromatography on neutral, activated alumina gel with ethanol/acetic acid (20:1 v/v) to yield 0.70 g (2.7 mmol, 67%) of 1 as a light yellow solid.