General procedure for the synthesis of methyl 3-amino-5-methylthiophene-2-carboxylate from 2-butenenitrile and methyl mercaptoacetate: a mixture of cis- and trans-2-butenenitrile (298.00 g, 4.44 mol) was mixed with pyridine (61.49 g, 0.78 mol) and the resulting mixture was cooled to 0 °C. Chlorine gas (314.92 g, 4.44 mol) was passed at a rate of 60-65 g/h while maintaining the internal temperature at 10-13 °C. After addition, the reaction mixture was slowly warmed to room temperature and stirred overnight. The resulting oil was dissolved in dichloromethane (1.5 L) to allow the solution to pass through a silica gel column for initial purification. The filtrate was concentrated under vacuum to give the complex reaction mixture (508.61 g) as a brown oily substance. This oily substance (493.80 g) was mixed with methyl mercaptoacetate (422.77 g, 3.98 mol), and the resulting solution was slowly (1.5 h) added dropwise to a methanol (2.2 L) slurry of potassium carbonate (1651 g, 11.95 mol) at 100 °C. The final slurry was allowed to warm to room temperature and stirred at ambient temperature for 5 hours. The solid salt was filtered out and washed with methanol (2L). Combine the organic solutions and evaporate the solvent. The residue was redissolved in ethyl acetate (3L) and subjected to secondary purification via silica gel column. The silica gel column was washed with additional ethyl acetate (2L). All organic phases were combined and the solvent was evaporated under vacuum to give a brown oil (627 g). The mixture was vacuum distilled at 6 mmHg under reduced pressure. The fraction collected at 138°C was crystallized in a receiving flask (303.07 g). The target product methyl 3-amino-5-methylthiophene-2-carboxylate (179.00 g, 26%) was obtained as white crystals by recrystallization from methanol. The structure of the product was confirmed by 1H NMR (CDCl3, 400 MHz) and 13C NMR (CDCl3, 100 MHz).
