Green Synthesis of 9-Aryl Xanthene Diones via MCRs Using MgO@CSCA Catalyst: Characterization and Reusability

Abstract An innovative, eco-friendly, and effective approach was established for synthesizing derivatives of 3,3,6,6-tetramethyl-9-aryl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (2a-2j) through a one-pot, multicomponent reaction involving dimedone and diverse substituted benzaldehydes. The reactions were catalyzed by magnesium oxide nanoparticles (MgO-NPs) derived from coconut seed coat ash (CSCA), prepared using a modified procedure. Among several investigated synthetic approaches, including conventional heating, microwave irradiation, solvent-free grinding, and ionic liquid catalysis, the MgO@CSCA-mediated protocol in aqueous ethanol proved most efficient, affording high product yields (81.14–89.88%) within short reaction times (as low as 10 min). Structural elucidation of the synthesized compounds was achieved through FTIR, ¹H and ¹³C NMR spectroscopy, HRMS, and elemental analysis. The inclusion of electron-withdrawing and electron-donating groups on the aryl ring significantly affected the spectral characteristics and reaction rates. Comparative catalyst screening revealed that MgO@CSCA-NPs outperformed conventional acidic and heterogeneous systems in terms of yield, rate, and reusability, with successful recyclability over five consecutive cycles. These findings highlight the significant potential of biogenic MgO@CSCA nanocatalysts for sustainable organic synthesis and reinforce their utility in the rapid generation of pharmacologically promising xanthene scaffolds under eco-friendly conditions.