Hydrazine hydrate (1:1) → 1H-Indazole

Participants

Reaction Conditions

Reaction Steps

A solution of 4-bromo-2-fluoro-N-methoxy-N-methyl-benzamide (CAS [801303-33-3]) (24.9 g, 95.0 mmol) in THF (100 mL) at −78° C. was treated with a 3 N solution of ethyl magnesium bromide in diethyl ether (33.25 mL, 115 mmol) and allowed to warm to 0 C and stir for a 6 h period, after which time the reaction mixture was quenched with a half saturated aqueous solution of NH4Cl (200 mL) and EtOAc (100 mL). The mixture was separated and the organic component, washed with brine (2×50 mL), dried over MgSO4, filtered, and evaporated to an oil which was purified by silica gel flash chromatography with 20% EtOAc/hexanes as eluant to afford 1-(4-bromo-2-fluorophenyl)-propanone as a pale yellow solid (16.25 g, 74%). The propanone (21.15 g, 91.6 mmol) was then dissolved in ethanol (100 mL), treated with hydrazine hydrate (4.9 mL, 100.7 mmol), and heated at reflux for 4 h. The reaction mixture was then evaporated and dissolved in ethylene glycol (100 mL) and heated at 160-165° C. for 8 h after which time the cooled reaction mixture was poured onto water (500 mL) with stirring. The aqueous mixture was neutralized, with rapid stirring, using a small amount of an aqueous saturated solution of NaHCO3 to afford a pale yellow precipitate. The solids were filtered, washed with water, and air dried to afford cyclized indazole product (16.1 g, 78%).

Provenance & Evidence