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S1, Preparation of methyl 5-methylnicotinate: Depending on the molar amount of 5-methylnicotinate, 16 equivalents of methanol were slowly added dropwise over a period of 20 minutes, followed by 3 equivalents of thionyl chloride. The reaction mixture was warmed up to 82°C and held at this temperature for 6 hours. Continuous stirring was required during the dropwise addition and during the holding phase. Upon completion of the reaction, the temperature was lowered to 55 °C and the excess methanol and thionyl chloride were removed by distillation under reduced pressure. The reaction mixture was cooled to room temperature and ice water was added to obtain solution A. The pH of solution A was adjusted with ammonia under ice bath conditions to 8. Subsequently, ethyl acetate was added for extraction. The separated ethyl acetate phase was dried by adding anhydrous sodium sulfate until the moisture content was reduced to 0.3% by weight. The dried solution was filtered and the ethyl acetate was removed by rotary evaporator to finally obtain methyl 5-methylnicotinate.