Isoxazoles. 10. Degradation and enolization kinetics of 4-aminoisoxazolyl-1,2-naphthoquinone in basic aqueous solution.

The kinetics of enolization and degradation of N-(5-methyl-4-isoxazolyl)-4-amino-1,2-naphthoquinone (1) was investigated in aqueous solutions over a pH range of 7.30 to 12.25, at 35 degrees C and at constant ionic strength (mu = 0.5) using reversed-phase HPLC. Pseudo-first-order kinetics was observed throughout the pH range studied. The rate of enolization (ke), the keto-enol equilibrium constant (Kt), and specific base catalysis rate constant (kCH) were determined. Good agreement between the theoretical pH-rate profile and the experimental data supports the proposed transformation process. The average recovery for 1 and its tautomerization product 2-hydroxy-N-(5-methyl-4-isoxazolyl)-1,4-naphthoquinone 4-imine (2) from mixtures of different composition was evaluated.