General procedure for the synthesis of 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine from N-[3-(methylamino)propyl]-acetamide: N-[3-(methylamino)propyl]-acetamide obtained from the previous step was dissolved in 200 mL of polyphosphoric acid ester (PPE) and placed in a microwave reactor. The reaction temperature was controlled to be 80 °C and the reaction time was 2 hours. After completion of the reaction, the reaction mixture was purified to afford the target product 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine 240 g in 94.42% yield.