5-(3-Amino-2-hydroxy-5-methyl-phenyl)-thiophene-2-carboxylic acid hydrobromide 24d (138 mg, 0.42 mmol) was dissolved in 1.5 mL of 1 N hydrochloric acid upon cooling by an ice-water bath followed by dropwise addition of 1.5 mL of aqueous sodium nitrite (85 mg, 1.22 mmol). After the mixture was reacted for 20 minutes, 2-indan-5-yl-5-methyl-2,4-dihydro-pyrazol-3-one 1i (81 mg, 0.38 mmol), sodium bicarbonate (527 mg, 6.27 mmol) and 2 mL of ethanol were added successively. The reaction mixture was reacted overnight at room temperature. The reaction was monitored by TLC until the disappearance of the starting materials. The mixture was filtered and the filter cake was washed with 20 mL of water, and then dissolved in 10 mL of water. Upon cooling by an ice-water bath, the mixture was adjusted to pH 3-4 with concentrated hydrochloric acid, filtered and dried to obtain the title compound 5-{2-hydroxy-3-[N′-(1-indan-5-yl-3-methyl-5-oxo-1,5-dihydro-pyrazol-4-ylidene)-hydrazino]-5-methyl-phenyl}-thiophene-2-carboxylic acid 24 (30 mg, yield 16.7%) as a red solid.
