Improved solvent-free synthesis and structure elucidation of (E)- and (Z)-4-(arylmethylene)-3-isochromanones

A series of new 4-(arylmethylene)-3-isochromanones 1-17 has been prepared by base-catalyzed Knoevenagel condensations. Stereochemical and conformational analyses were performed by ¹H and ¹³C NMR methods, and X-ray crystallography was used to complement the configurational assignment for a representative product. The reaction generally yields the (E) isomer, but the isomeric composition of the products was influenced by the aromatic aldehyde and usually governed by the stability of the products. Several aldehydes exclusively afforded the (Z) isomer, due to intramolecular steric/electronic interactions reflected in the increased stability of the product. The condensation performed with ortho-hydroxyarenecarbaldehydes furnished the unexpected 3-arylcoumarins or 3-arylbenzocoumarins.