A C2-Symmetric Biphenyl-Bridged Bis-Aurone: Synthesis, Z/E Photoisomerization, and Conformational Insights from NMR Spectroscopy and DFT Analysis

Aurones are naturally occurring flavonoids characterized by a benzofuran-3(2H)-one scaffold bearing an exocyclic benzylidene double bond. Herein, we report the synthesis and NMR-based structural characterization of a highly oxygenated biphenyl-bridged bis-aurone and its mono-condensation derivative. UV irradiation at 254 nm in deuterated acetone generated the corresponding E-containing photoisomers, whose formation was monitored by 1H NMR spectroscopy. For the bis-aurone, weak additional resonances indicated a minor species possibly corresponding to the doubly isomerized (E,E′) form. NOESY correlations provide constraints on the spatial arrangement of the bis-aurone in solution. Both isolated E-containing derivatives exhibit a pronounced downfield shift of one biphenyl aromatic proton. Conformational analysis is used to examine possible geometrical and magnetic-anisotropy contributions to this observation. The study focuses on synthesis, NMR observations of Z/E photoisomerization, and conformational interpretation; intrinsic photochemical efficiencies were not determined.