Step 2: Preparation of 2,5-dimethyl-2H-pyrazole-3-carbaldehyde (15)
A mixture of compound 14 (1,3-dimethyl-5-hydroxymethyl-1H-pyrazole, 3 g, 23.8 mmol) with pyridinium dichromate (13.4 g, 35.7 mmol) in dichloromethane (100 ml) was reacted for 16 hrs at 25°C under nitrogen protection with stirring. After completion of the reaction, the reaction mixture was filtered and the filtrate was collected. The filtrate was dried over anhydrous Na2SO4 and concentrated under reduced pressure to remove the solvent. The resulting crude product was purified by silica gel column chromatography to afford the target product 2,5-dimethyl-2H-pyrazole-3-carbaldehyde (15) as a 1 g brown solid in 34% yield.
Product characterization data:
1H NMR (200 MHz, CDCl3): δ 9.8 (s, 1H), 6.6 (s, 1H), 4.1 (s, 3H), 2.3 (s, 3H).
Mass spectrum (CI method, isobutane): 125 (M + 1, 100%).
IR spectrum (KBr, cm-1): νmax 1688.
