A solution of 2,3-dibromomaleic acid (1 g, 3.65 mmol) in AcOH (25 mL) was heated to reflux and left to stir for 30 min prior to addition of S2 (1.3 g, 3.15 mmol) in AcOH (5 mL). The solution turned from colourless to brown. The reaction mixture was heated to reflux for an additional 4 h before concentration in vacuo to give a brown oil. The crude product was purified by flash column chromatography, eluting 25-60% EtOAc [1% AcOH] in cyclohexane on a 120 g silica column over 20 CV. Fractions containing product were combined and concentrated in vacuo to afford S3 as a pale yellow solid (985 mg, 2.5 mmol, 79% yield).