Zwitterionic Salts from the Reaction of Symmetrical and Unsymmetrical (thio)Barbituric Acids with 2-Pyridinecarbaldehyde under Solvent-free Condition

A simple and efficient synthesis for the preparation of unusual charge-separated pyridinium (thio)barbiturate zwitterion derivatives was achieved via a one-pot reaction of (thio)barbituric acid derivatives and 2-pyridinecarbaldehyde under solvent-free condition and also in methanol under refluxing. The structure of the compounds was confirmed by (HNMR)-H-1, (CNMR)-C-13, FT-IR, mass and X-ray analysis. The mechanism of the formation is discussed. Instead, no related pyridinium zwitterion was afforded from the reaction between dimedone and 2-pyridinecarbaldehyde under the same conditions and its xanthene derivative was obtained.