Z-E Isomerization of fi-Methoxychalcones: Preferred Existence of E-Isomers in Naturally Occurring flmethoxychalcones

Methylation of dibenzoylmethanes with diazomethane always gave Z-β-methoxychalcones, which are the products of kinetically controlled methylation of chelated enol forms of /1-hydroxychalcones. Z-β-Methoxychalcones were thermodynamically unstable and readily isomerized into more stable E-isomers on contact with silica gel, on keeping in polar solvents, or on exposure to light. The structures of both isomers of the simplest fl-methoxychalcone were determined by X-ray crystal structure analysis. All p-methoxychalcones prepared were fully characterized by spectroscopic methods, revealing that the E- and Z-isomers are distinguishable in terms of ultraviolet and 13 C-nuclear magnetic resonance spectra, and nuclear Overhauser effect between the /Jmethoxyl group and H-8 proton. The spectral data reported for natural ft-methoxychalcones methylpongamol and praecansone A, are attributable to the E-isomers.