Multinuclear NMR Measurements and DFT Calculations for Capecitabine Tautomeric Form Assignment in a Solution

The molecular structure of capecitabine (a widely applied prodrug of 5-fluorouracil) was studied by multinuclear NMR measurements and DFT quantum mechanical calculations. One or two tautomeric forms in a solution were detected depending on the solvent used. In the organic solvents, a mixture of two forms of capecitabine was observed: carbamate and imine tautomers. In the aqueous solution, only the carbamate form was found. The methylation of capecitabine yields mainly two products in different proportions: N³-methylcapecitabine and N⁷-methylcapecitabine. The protonation of capecitabine in organic solvents with perchloric acid occurs at the N3 nitrogen atom. DFT calculations strongly support the results coming from the analysis of the NMR spectra.