General procedure for the synthesis of 1-methylpyrazole-5-carboxaldehyde using 1-methyl-1H-pyrazole-5-carboxaldehyde as starting material: a solution of N,N-dimethylformamide (DMF, 5 mL) of N-bromosuccinimide (NBS, 1.62 g, 9.09 mmol) was slowly added dropwise to a solution of 1-methyl-1H-pyrazole-5-carboxaldehyde (1.0 g, 9.09 mmol) solution in DMF (8 mL). The reaction mixture was stirred at room temperature overnight. Subsequently, 1N sodium hydroxide solution (9.1 mL) was added and stirring was continued for 10 min. The reaction solution was diluted with water (100 mL) and extracted with ethyl acetate (EtOAc, 30 mL x 3). The organic phases were combined, dried with anhydrous magnesium sulfate (MgSO4), filtered and concentrated to give an orange solid product (1.57 g, 92% yield). Liquid phase mass spectrometry (LRMS) detection showed m/z (M+H) of 187.9 and the measured value was 188.0.
