Reaction scheme:
N-acetylaminoisonitrosoacetanilide.
1. A suspension of N-acetylphenylhydrazine (10 g), hydroxylamine hydrochloride (15 g), sodium sulfate (62 g) in water (200 ml) containing 1M aqueous hydrochloric acid (67 ml) was heated to 100 *C.
2. To homogeneous solution, a solution of chloral hydrate (13,2 g) was rapidly dropped into the reaction mixture and allowed to react at 100 *C for 10 minutes.
3. After cooling to room temperature, the solvent was evaporated to half volume.
4. On standing in the refrigerator, a pale yellow solid precipitated.
5. After filtration in vacuo, washing with water and drying. N-acetylaminoisonitrosoacetanilide 11.6 g was obtained.
Indazole-3-carboxylic acid.
1. N-acetylaminoisonitrosoacetanilide 10 g was added portionwise to 96% sulphuric acid (50 ml) at 55 *C
2. The reaction mixture was warmed to 85 *C and reacted at this temperature for 15 minutes.
3. After cooling, the solution was poured into ice (140 g) and the resulting suspension refluxed for 2.5 hours.
4. The precipitate was filtered, washed with water and crystallized from glacial acetic acid. 5.7 g was obtained.
N-acetylaminoisonitrosoacetanilide.
1. A suspension of N-acetylphenylhydrazine (10 g), hydroxylamine hydrochloride (15 g), sodium sulfate (62 g) in water (200 ml) containing 1M aqueous hydrochloric acid (67 ml) was heated to 100 *C.
2. To homogeneous solution, a solution of chloral hydrate (13,2 g) was rapidly dropped into the reaction mixture and allowed to react at 100 *C for 10 minutes.
3. After cooling to room temperature, the solvent was evaporated to half volume.
4. On standing in the refrigerator, a pale yellow solid precipitated.
5. After filtration in vacuo, washing with water and drying. N-acetylaminoisonitrosoacetanilide 11.6 g was obtained.
Indazole-3-carboxylic acid.
1. N-acetylaminoisonitrosoacetanilide 10 g was added portionwise to 96% sulphuric acid (50 ml) at 55 *C
2. The reaction mixture was warmed to 85 *C and reacted at this temperature for 15 minutes.
3. After cooling, the solution was poured into ice (140 g) and the resulting suspension refluxed for 2.5 hours.
4. The precipitate was filtered, washed with water and crystallized from glacial acetic acid. 5.7 g was obtained.
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