Determination of the pK_aand pH-Solubility Behavior of an Ionizable Cyclic Carbamate, (S)-6-Chloro-4-(cyclopropylethynyl)-1,4-dihydro-4-(trifluoromethyl)-2H-3,1-benzoxazin-2-one (DMP 266)

Abstract

The solubility of a nonnucleoside reverse transcriptase inhibitor, (S)-6-chloro-4-(cyclopropylethynyl)-1,4-dihydro-4-(trifluoromethyl )- 2H-3,1-benzoxazin-2-one (DMP 266), was investigated as a function of pH. A dramatic increase in the aqueous solubility was observed at pH > or = 10, which was consistent with going from a neutral to a charged species. The ionization of the proton positioned on the carbamate functionality was confirmed spectrophotometrically (pKa = 10.1). The spectrophotometric result was in excellent agreement with that obtained from the solubility studies (pKa = 10.2). The ionization behavior of DMP 266 represents a unique case in which the pKa for a carbamate functional group is quite low. The anomalous pKa value may be attributed to stabilization of the negatively charged species through inductive effects, which originate from the surrounding substituents and delocalization of the negative charge via resonance effects.