Synthesis and special characterization through X-ray analysis of 1,8-dioxooctahydroxanthenes

Abstract

Pyran moiety containing heterocyclic compounds have the capability for binding to either side to cyclohex-2-enone rings and to form xanthene derivatives (1,8-dioxooctahydroxanthenes also known as xanthenodiones). These xanthene derivatives display a wide range of biological activities and find applications in laser technologies and photodynamic therapy. This paper describes the preparation, X-ray structural analysis, and theoretical investigation of a series of 1,8-dioxooctahydroxanthenes. The compounds were synthesized via Knoevenagel condensation between different aldehydes and β-diketones. The reactions were performed free of solvents and the 1,8-dioxooctahydroxanthenes were obtained in good yields (70−92%). All the compounds were fully characterized by NMR and IR spectroscopy as well as mass spectrometry. Among the synthesized compounds, seven had their crystal structures elucidated for the first time. In all the new crystal structures, the three fused rings did form an almost completely planar xanthenodione core, except for the side rings that adopt half-chair conformation with both carbons at the flaps oriented toward to the aromatic substituent, or with one of the two carbons pointing opposite to the substituent. Another conformational difference among the new compounds investigated by X-ray diffraction resides in the rotation around the bond axis connecting the xanthenodione core to its aromatic substituent. It was found that different bent levels resulted from weak intermolecular contact patterns. In addition, theoretical calculations for single molecule and dimmers have provided insights into the balance between intramolecular and intermolecular forces driving both conformational features.