Rabenchromenone and Rabenzophenone, Phytotoxic Tetrasubstituted Chromenone and Hexasubstituted Benzophenone Constituents Produced by the Oak-Decline-Associated Fungus Fimetariella rabenhorstii

A new phytotoxic tetrasubstituted chromen-4-one ( 1 ) and a new hexasubstituted benzophenone ( 2 ), named rabenchromenone and rabenzophenone, respectively, were isolated from the culture filtrates of Fimetariella rabenhorstii , an oak-decline-associated fungus in Iran. Rabenchromenone and rabenzophenone, isolated together with known moniliphenone ( 3 ) and coniochaetone A ( 4 ), were characterized as methyl 3-chloro-1,8-dihydroxy-6-methyl-9-oxo-1,9-dihydrocyclopenta[ b ]chromene-1-carboxylate and methyl 4-chloro-2-(2,6-dihydroxy-4-methylbenzoyl)-3-hydroxybenzoate, respectively, by spectroscopic methods (primarily nuclear magnetic resonance and high-resolution electrospray ionization mass spectrometry). The R absolute configuration at C-1 of rabenchromenone was determined by quantum chemical calculations and electronic circular dichroism experiments. All metabolites ( 1 - 4 ) were tested by leaf puncture on tomato and oak plants. All compounds were active in this assay by causing in both plants a necrosis diameter in the range of 0.2-0.7 cm. Specifically, rabenzophenone ( 2 ) was found to be the most phytotoxic compound in both plants.