2-(Pyridin-2-ylmethylene)indan-1-ones: The first study of E/Z isomerization of cyclic chalcones with electron-deficient hetaryl moiety

The products of condensation of pyridine-2-carbaldehyde with 1-indanone (4) 5,6-dimethoxy-1-indanone (5), and 7-acetamido-1-indanone (6) were synthesized and studied as derivatives of cyclic chalcones with electron-deficient heterocycle (pyridine) as distinct from the earlier studied analogues with electron-rich heterocycles (azoles). All products are formed as the E-isomers. The structure of compound 4 was proved by X-ray analysis. The UV-induced E/Z isomerization of compounds 4–6, as well as the transformation occurring upon protonation, were studied experimentally by NMR and IR spectroscopy and theoretically by DFT calculations. The rate of isomerization for compound 4 is much higher than those of 5 and 6. All possible isomers relative to the exocyclic C[dbnd]C bond and rotamers about the C–C bond with the pyridine ring were calculated. General scheme of transformation of compounds 4–6 upon UV irradiation and protonation is proposed.