An NMR study on the keto-enol tautomerism of 1,3-dicarbonyl drug precursors.

The effective implementation of drug precursor legislation has driven the innovation and design of new alternative substances. The application of 1,3-dicarbonyl precursors as alternative precursors for the synthesis of 1-phenyl-2-propanone (P2P) and 3,4-methylenedioxyphenyl-2-propanone (MDP2P) has created new challenges to legal control. Their 1,3-dicarbonyl structure allows the precursors to exist as an equilibrium mixture of the tautomeric diketo and keto-enolic forms during the nuclear magnetic resonance (NMR) analysis. In this study, the keto-enol tautomerism of four 1,3-dicarbonyl drug pre-precursors, α-phenylacetoacetamide (APAA), methyl α-phenylacetoacetate (MAPA), ethyl α-phenylacetoacetate (EAPA), and methyl 2-(benzo[d][1,3]dioxol-5-yl)-3-oxobutanoate (MAMDPA) were investigated through NMR. One-dimensional (1D) and 2D NMR were combined to assign signals for the diketo and keto-enolic tautomers. Results showed that the keto-enol tautomerism was solvent-dependent but was also influenced by the substituent present in the molecule. Further, the analysis results indicated that majority of substances existed mainly in the diketo form. The enol-keto equilibrium constant (K_eq) was stable in dimethyl sulfoxide-d₆ and chloroform-d, while unstable for some compounds in acetone-d₆ and deuterated methanol. The presence of impurities in the seized sample may disrupt the equilibrium between keto-enol tautomers in 1,3-dicarbonyl precursors. After the optimization of several key quantitative parameters, a quantitative NMR method for the quantification of 1,3-dicarbonyl drug precursors were also developed to facilitate their quantitative analysis. This is the first study to investigate the keto-enol tautomerism and quantification of 1,3-dicarbonyl drug precursors by NMR, providing a new approach for structure analysis and quantification of new precursor analogues.