2-Substituted-1H-perimidines: Synthesis, crystal structure and DFT calculations

• 1H-perimidines exhibit 1,3-annular tautomerism resulting in the imine or enamine forms.
• The imine forms of 2-substituted-1H-perimidines can exhibit cis or trans orientation.
• Synthesis of four 2-substituted perimidine derivatives are reported.
• The compounds are characterized by MS, IR, NMR spectroscopy.
• Single-crystal X-ray diffraction, powder X-ray diffraction and DFT calculations are applied.

Perimidines or 1H-perimidines represent an important class of heterocyclic compounds built up of a dihydropyrimidine ring peri-fused to a naphthalene moiety. They exhibit 1,3-annular tautomerism due to two possible hydrogen positions at one or the other endocyclic nitrogen atoms or both resulting in the imine or enamine forms, respectively. The imine forms of 2-substituted-1H-perimidines can exhibit cis or trans orientation of 2-substituent in respect of the double CN bond.Here we report synthesis of four 2-substituted perimidine derivatives: 2-hydroxymethyl-perimidine 2, ethyl-1H-perimidine-2-carboxylate 3, perimidine-2-carboxylic acid 4 and 2-cyanomethyl-perimidine 5.The compounds are characterized by MS, IR spectroscopy, single-crystal X-ray diffraction (SCXRD) (2), and powder X-ray diffraction (PXRD) (5), as well as DFT calculations (3, 4 and 5) and by the NMR spectroscopy in the solution (2–5).Single-crystal X-ray diffraction proved that the compound 2 exists in the solid state as imine tautomer.The molecular structure that compound 5 adopt in its crystalline state could not be determined reliably. The results from the PXRD structural analysis cannot univocally indicate which of the forms is more likely to be present in the solid state.On the contrary, DFT calculation shows that the enamine form of 5 is more stable than imine form.