The Reversible Proxibarbal-Valofan Isomerisation. Part II. Kinetic studies in a biphasic octanol/water system

The tautomerisation between proxibarbal (I) and the two diasterloisomers of valofan (IIX and IIY) was investigated at pH 7.4 in a biphasic octanol/H2O system. The rate constants of isomerisation and the equilibrium constants of (partition coefficients) were calculated by compartmental analysis. The rate constants of isomerisation were comparable with those determined in monophasic aqueous solutions, whereas at pH 7.4 and 37° the duration necessary for a global equilibrium to be reached was 4–6 times longer in the biphasic system. These reduced rates and the higher lipophilicity of IIX and IIY as compared to I may be of pharmacokinetic and pharmacodynamic significance. They may also have relevance for a number of drugs known or suspected to form lactonic or lactamic metabolites.