3- Pyrazolin-5-ones ring cleavage

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... 

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>. 

---