[Synthesis]
Methylhydrazine (24 cm3, 19.5 g, 0.423 mol) was slowly added dropwise to 225 cm3 of aqueous solution containing 17.0 g NaOH (0.423 mol) under ice bath cooling conditions. Subsequently, 4-(dimethylamino)-1,1-dimethoxy-3-buten-2-one (73.3 g, 0.423 mol) was added slowly dropwise at room temperature and the reaction mixture was stirred overnight. After completion of the reaction, it was cooled to room temperature, 36% HCl (28.2 g, 0.278 mol) was added and stirring was continued for 1 hour. The reaction mixture was extracted with dichloromethane (4 x 100 cm3), the organic phases were combined and dried over anhydrous sodium sulfate. The solvent was removed by evaporation under reduced pressure and the residue was dissolved in 420 cm3 1M HCl aqueous solution and stirred at room temperature for 3 hours. Subsequently, the pH was adjusted to 10 with 10% K?CO? aqueous solution and extracted with dichloromethane (3 x 200 cm3). The organic layer was dried over anhydrous sodium sulfate and the solvent was evaporated under reduced pressure. The residue was purified by vacuum distillation to give 1-methyl-1H-pyrazole-3-carbaldehyde (10a). Yield 32.8 g (70% yield); colorless liquid; boiling point: 70-72 °C (3 mbar); mass spectrum (ESI): m/z = 111 ([M + H]?) ; 1H NMR (500 MHz, CDCl?): δ = 9.84 (s, 1H), 7.36 (d, J = 1.4 Hz, 1H), 6.68 (d, J = 1.4 Hz, 1H), 3.91 (s, 3H) ppm; 13C NMR (126 MHz, CDCl?): δ = 185.8, 150.9, 131.8, 105.8, 39.2 ppm. |