Photochemical diastereoselective synthesis and spectral characterization of (E)-3-(2-benzoylstyryl)-4H-chromen-4-ones

Abstract

An efficient, concise, and green photochemical vinylogous aldol reaction for the diastereoselective synthesis of novel trans-3-(2-benzoylstyryl)-4H-chromen-4-ones starting from readily available methylbenzophenone and 3-formylchromone in the absence of any bases, metals, or other catalysts is described. The products could be obtained under UV LED (365 nm) irradiation at room temperature in 9 h with perfect diastereoselectivity. To discuss the special patterns of alkene protons, to identify its trans-conformation and verify the two trans-alkene protons (β-H and γ-H), NMR spectra analysis (1D-NMR and 2D-NMR) was conducted in details by the chemical shifts, couplings and correlations. Single crystal X-ray analysis confirmed the trans-conformation and all the compounds were unequivocally confirmed by NMR, HRMS and XRD analysis. XRD analysis of 3a further aids to disclose the reason why the alkene moiety presented a better planarity than that of benzophenone moiety of the title compounds.