General procedure 6: 3,5-dimethyl-2-pyrrolecarboxaldehyde (1 eq.) was suspended or dissolved in carbon tetrachloride (CCl4) under argon protection, followed by the addition of N-bromosuccinimide (NBS, 1.05 eq.) and benzoyl peroxide (0.025 eq.). The reaction mixture was heated to reflux and the reaction progress was monitored by liquid chromatography-mass spectrometry (LC/MS). Upon completion of the reaction, the crude product was cooled to room temperature and further cooled in an ice bath. If a precipitate formed, it was filtered and the product was purified by fast column chromatography. If no precipitate was formed, the solvent was removed under reduced pressure and the crude product was similarly purified by fast column chromatography. Preparation 23: The synthesis of 4-bromo-3,5-dimethyl-1H-pyrrole-2-carboxaldehyde was carried out according to General Method 6, specifically using the reaction of 3,5-dimethyl-1H-pyrrole-2-carboxaldehyde (8 g, 65 mmol), NBS (12.16 g, 68.3 mmol) and benzoyl peroxide (394 mg, 1.63 mmol) in carbon tetrachloride (300 mL). Purification gave 11.9 g of 4-bromo-3,5-dimethyl-1H-pyrrole-2-carbaldehyde as a dark solid in 92% yield.1H NMR (DMSO-d6) δ 12.06 (broad peak, 1H), 9.50 (single peak, 1H), 2.22 (single peak, 3H), 2.19 (single peak, 3H).
