Isomerization of 4′-methoxy-flavanone in alkaline medium.: Determination of the enolate formation constant

The varied biological activity of flavanones mainly depends on their physical and chemical properties. In order to obtain information on the acid-base behavior of flavanones of chemical and biological interest the isomerization of 4'-methoxyflavanone was studied and the formation constant of its enolate was determined by UV spectrometry. The kinetic experiments were performed in aqueous medium at 25 +/- 0.1degreesC, keeping the ionic strength (0.07, KCl) and pH (10.8) constant. The obtained results were interpreted by a four-step mechanism, which considered the existence of flavanone enolate as a key intermediate. Some stereochemical and mechanistic aspects of the reaction were explained by means of reactivity indexes and thermodynamic magnitudes calculated by ab initio methods in ethanol and water, which support the proposed mechanism. It is concluded that the kinetic methodology here proposed to study the enolization properties of flavanones seems to be quite convenient to overcome the marked intrinsic instability of these substances in alkaline media. (C) 2003 Elsevier B.V. All rights reserved.