Hydrochloric Acid → 2-(3-Ethyl-indan-5-yl)-5-methyl-2,4-dihydro-pyrazol-3-one

Di-tert-butyl 1-(3-ethyl-2,3-dihydro-1H-inden-5-yl)hydrazine-1,2-dicarboxylate 41c (3.43 g, 9.1 mmol) was dissolved in 50 mL of a solvent mixture of ethanol/water (V:V=3:2) followed by addition of 3-oxo-butanoic acid ethyl ester (1.18 g, 9.1 mmol) and 4.55 mL of hydrochloric acid (6 N). Upon completion of the addition, the reaction mixture was heated to reflux and reacted for 1 hour. The reaction was monitored by TLC until the disappearance of the starting materials. The mixture was concentrated under reduced pressure to remove ethanol and then extracted with dichloromethane (15 mL×3). The combined organic extracts were dried over anhydrous sodium sulfate, filtered to remove the drying agent and concentrated under reduced pressure. The residue was purified by silica gel column chromatography to obtain the title compound 2-(3-ethyl-indan-5-yl)-5-methyl-2,4-dihydro-pyrazol-3-one 41d (0.712 g, 39.8%) as a yellow oil.