Hydrochloric Acid → 1-(3,4-Dichlorophenyl)-5-pyridin-3-yl-1H-pyrazole-3-carboxylic acid methyl ester hydrochloride

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Reaction Conditions

Reaction Steps

Acetyl pyridine (1.81 mL, 16.5 mmol) and methyl oxalate (3.12 g, 26.4 mmol) were dissolved in MeOH (30 mL, anhydrous). Sodium methoxide (6.9 mL, 25% in MeOH) was added over 10 min. Reaction solidified and was complete after 15 minutes. The solid mass was dissolved when acidified with HCl (10%, aqueous). The pH was then adjusted with NH4OH (conc) until precipitation ceased. The resulting solid was taken up in EtOAc. The aqueous layer was removed and extracted twice with EtOAc. The combined EtOAc layers were washed with water and brine, dried over MgSO4, filtered, and concentrated in vacuo. Collected 1 (2.90 g, 85%) as an off-white solid. 1-(3,4-Dichlorophenyl)-5-pyridin-3-yl-1H-pyrazole-3-carboxylic acid methyl ester hydrochloride (2). To a solution of 1 (2.90 g, 14.0 mmol) in EtOH (60 mL, anhydrous) was added 3,4-dichlorophenyl hydrazine hydrochloride (3.29 g, 15.4 mmol). The solution was heated to reflux for 30 min and then cooled to 0° C. The precipitate was collected by filtration and washed with H2O and MeOH to yield 2 (3.79 g, 70%) as an off-white powder. 1-(3,4-Dichlorophenyl)-5-pyridin-3-yl-1H-pyrazole-3-carboxylic acid methyl ester hydrochloride (3). A suspension of 2 (2.0 g, 5.74 mmol) in THF (40 mL) and H2O (11 mL) was treated with NaOH pellets (581 mg, 14.5 mmol) and heated to reflux for 1 h. The THF was removed in vacuo and the pH of the remaining aqueous portion was adjusted to 1.5 with HCl (10%, aqueous). The resulting solid was dissolved in EtOAc. The aqueous layer was removed and extracted with EtOAc-MeOH (4:1). The combined organic layers were dried (brine and MgSO4) and concentrated in vacuo to yield 3 (1.61 g, 84%) as a white solid. 1-(3,4-Dichlorophenyl)-5-pyridin-3-yl-1H-pyrazole-3-carbonyl azide (4). A solution of 3 (100 mg, 0.27 mmol) and t-butyl alcohol (28.4 μL, 0.30 mmol) in DMF (5 mL, anhydrous) was cooled to 0° C. Diphenylphosphoryl azide (64 μL, 0.30 mmol) was added to the solution. Triethylamine (103 μL, 0.60 mmol) was then added over 10 min. The solution was stirred 1 h at 0° C. and allowed to warm to room temperature and stir 16 h. The reaction was quenched with H2O and extracted with EtOAc. The combined organic layers were washed with H2O, dried (brine and MgSO4), filtered, and concentrated in vacuo. The resulting oil was purified by flash chromatography by eluting with hexane-EtOAc (1:1). Collected 4 (86 mg, 90%) as a yellow crystalline solid. [1-(3,4-Dichlorophenyl)-5-pyridin-3-yl-1H-pyrazol-3-yl]carbomic acid tert-butyl (5). A solution of 4 (98 mg, 0.24 mmol) and t-butyl alcohol (3 mL) were heated to reflux for 4 h. The solution was cooled and concentrated. The resulting oil was purified by flash chromatography. Collected 4 (74 mg, 76%) as a clear oil. 1-(3,4-Dichlorophenyl)-5-pyridin-3-yl-1H-pyrazol-3-yl amine (6). The BOC-protected compound (5, 74 mg, 0.18 mmol) was dissolved in MeOH (10 mL, anhydrous) and HCl (g) was bubbled through for 10 min. The solution was stirred 3 h at room temperature. Concentrated in vacuo. The remaining oil was dissolved in H2O and neutralized with NaHCO3 (sat. aqueous). The aqueous solution was extracted with CHCl3; the combined organic layers were dried (brine and MgSO4), filtered, and concentrated. The resulting oil was purified by flash chromatography (chloroform-MeOH—NH4OH 95:5:0.5) to yield 6 (39 mg, 70%) as a yellow crystalline solid.

Provenance & Evidence