Hydroxy-3-pyrazolin-5-ones, 63,

Acyl-, 4-alkoxycarbonyl- and 4-phenylazo-pyrazolin-5-ones present the possibility of a fourth tautomer with an exocyclic double bond and a chelated structure. The molecular structure of (138) has been determined by X-ray crystallography (Table 5). It was shown that the hydroxy group participates in an intramolecular hydrogen bond with the carbonyl oxygen atom of the ethoxycarbonyl group at position 4 (8OCSCII21). On the other hand, the fourth isomer is the most stable in 4-phenylazopyrazolones (139), a chelated phenyl-hydrazone structure. 

The problem of tautomerism is simpler in the case of 1-substituted pyrazolin-3-ones since only two forms, the OH (140a) and the NH (140b), are possible. The OH form is the more stable and is the only one present in the crystal (Section 4.04.1.3.1). In protic solvents, like water or methanol, the equilibrium position is much more evenly balanced between the OH and NH forms. Finally, 4-hydroxypyrazoles (141) exist as such. A CNDO/2 calculation justifies the result that 4-hydroxy tautomers are relatively more stable than... 

Irradiation of 4-hydroxy- and 4-alkoxy-3-pyrazolin-5-one derivatives (163 R = OH, OR) leads to ring cleavage with the formation of /3-diamides (165) 69TL271). The methylene blue sensitized rearrangement of the same pyrazolinone (R = H) to the oxindole (166) also... 

Pyrazolines substituted at position 4 or 5 with hydroxy or amino groups readily eliminate a molecule of water or amine yielding pyrazoles. The 4-substituted derivatives are relatively more stable than the 5-substituted ones, because for the last group the lone pair at N-1 assists the elimination (407) -> (408) -> (409). The sulfonyl group at position 1 is also easily eliminated and this property is taken advantage of in Dorn s elegant synthesis of 3-aminopyrazole (Section 4.04.3.3.1). 

Pyrazolin-2-one, 5-hydroxy-3-methyl-1-phenyl- C NMR, 5, 194 (77JCS(P2)1024) 2-Pyrazolin-5-one, 3-methyl-1 -phenyl- C NMR, 5, 194 (77JCS(P2)1024)... 

The addition of diazomethane to 17j -hydroxy-5a-androst-l-en-3-one (7) gives the A -pyrazoline (8) in which the C=N bond is conjugated with the 3-keto group. °°... 

Pyrazolin-5-one, 3-alkyl-(l,2,4-thiadiazol-5-yl)-reactions, 6, 483 2-Pyrazolin-5-one, 3-amino-tautomerism, 5, 215 2-Pyrazolin-5-one, 4,4-diazido-rearrangement, 5, 720 2-Pyrazolin-5-one, 3-hydroxy-tautomerism, 5, 215 2-Pyrazolin-5-one, 3-methyl-l -phenyl-reactions, 5, 252... 

Pyrazottne-5-ones Treatment of 5-hydroxy-3,5-diphenyl-A2-pyrazoline-4-one (l)withmethanolicpotassiumhydroxidegives4-hydroxy-3,4-diphcnyl-A -pyrazoline-5-one (2) in 76-80% yield. A pinacol rearrangement is probably involved. Compound (2)... 

C10H9NO 2-hydroxy-4-methylquinoline 607-66-9 24.83 1.1516 2 19050 C10H10N2O 3-methyl-1-phenyl-2-pyrazoline-5-one 89-25-8 25.00 1.1646 2... 

The tautomeric composition in solution of 4-(arylmethyl)isoxazol-5-one derivatives has been determined on the basis of H NMR and infrared (IR) data. The CH form was predominant in chloroform solution, while the NH and OH forms are more common in polar solvents and in the solid state <1996T1443>. 5-Hydroxy- and 5-amino-2-isoxazo-lines show different tautomeric forms in solution. The presence of cyclic hemiacetal or hemiaminal moieties in such molecules allows the easy cleavage of the C-5-0 bond to form linear structures. Subsequent intramolecular addition of nucleophiles to the C=N bond gives rise to cyclic structures. Compounds 20 exist, in the crystalline state, as the isoxazoline form A. In solution, a ring-ring tautomeric equilibrium was observed between the isoxazoline form A and the pyrazoline form C. The tautomeric ratio depended on steric factors and on the solvent used. The tautomeric equilibrium was established after several days (Scheme 2) <2000CHE722>. 

---