Synthetic and Structural Studies on Novel 4,3′-Bicoumarins
A series of directly linked 4-3′ bicoumarins have been synthesized by both Knoevenagel and Perkin reactions. This single-step transformation was accomplished by the reaction of coumarin-4-acetates with substituted salicylaldehydes in presence of piperidine using ethanol as solvent and by the reaction of coumarin-4-acetic acids with substituted salicylaldehydes in the presence of sodium hydride in acetic anhydride. Greater yields have been obtained in the sodium hydride and acetic anhydride condition. An intermediate chalcone with an ortho-hydroxyl group, which is a precursor for lactone formation, has been isolated and its structure has been confirmed by x-ray analysis. The nonplanar S-cis arrangement of two C4-C3’ double bonds has been confirmed by nuclear Overhauser spectroscopy (NOE) and x-ray studies. The UV-fluorescence studies support the formation of a conjugated bicoumarin system.