Isolation, structure determination, and total synthesis of the dibenzofurans α- and β-pyrufuran, new phytoalexins from the wood of Pyrus communis L.

Abstract

The antifungal compounds α- and β-pyrufuran have been isolated from the wood of perry pear trees [Pyrus communis L. (Rosaceae), cv. Hendre Huffcap) infected with Chondrostereum purpureum(Pers. ex Fr.) Pouzar, the causative fungus of silver leaf disease. Spectroscopic and chemical evidence show that the compounds are 1,2,3,4-substituted trimethoxydibenzofuranols. Three of the four possible positional isomers, compounds (11), (12), and (13), have been synthesized by cyclisation of the corresponding trimethoxydiphenyl ethers using palladium(II) acetate, followed by oxidation of the organo-lithium derivative using lithium t-butyl peroxide. The latter stage proved unsuccessful in the attempted synthesis of 2,3,4-trimethoxydibenzofuran-1-ol (10). Comparison of spectroscopic and chromatographic properties of the natural products and synthesized compounds show α-pyrufuran and β-pyrufuran to be 1,3,4-trimethoxydibenzofuran-2-ol (11) and 1,2,4-trimethoxydibenzofuran-3-ol (12), respectively.