619-12-5

• A pH-sensitive fluorescent probe 4,4′-(hydrazine-1,2-diylidene bis(methanylylidene)) bis(3-hydroxybenzoic acid (HDBB) by one-step condensation reaction with hydrazine.
• 2-Oxo-2H-1-benzopyran-3,7-dicarboxylic acid by condensation reaction with diethyl malonate.
• Schiff base ligands for the preparation of stable and functional Schiff base metal complexes.

100-97-0

99-06-9

619-12-5

GENERAL METHOD: Hexamethylenetetramine (0.3 mmol) and cuprous oxide (0.15 mmol) were sequentially added to a solution of trifluoroacetic acid (5 mL) of the substrate (1a-1q, 0.15 mmol). The reaction mixture was heated to reflux for 5 h and subsequently cooled to room temperature. 3N hydrochloric acid (5 mL) was slowly added to the reaction system and stirring was continued for 1 hour. Upon completion of the reaction, the mixture was concentrated under reduced pressure to remove the solvent. The crude product was purified by silica gel column chromatography (200-300 mesh) to afford the target compound 4-formyl-3-hydroxybenzoic acid.