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

Samaneh Bashiri,Marcin Górecki,Gennaro Pescitelli,Antonio Evidente,Marco Masi,Jafar Abdollahzadeh,Daniela Alioto,Roberta Di Lecce

doi:10.1021/acs.jnatprod.9b01017

Abstract

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.

Highlights

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 oakdecline-associated fungus in in Kurdistan (Iran)
Lundq. (Ascomycota, Sordariales, Lasiosphaeriaceae), recently isolated as endophytic fungus associated with Aquilaria sinensis in mainland China,[12] was detected for the first time from oak trees (Quercus brantii), showing decline and wood necrosis symptoms from the Zagros forest, in the west of Iran (Figure 1)
Article chromenone and a new hexasubstituted benzophenone, named rabenchromenone and rabenzophenone, respectively (1 and 2; Figure 2) and two already known compounds, which were identified as moniliphenone and coniochaetone A (3 and 4; Figure 2)

Summary

■ RESULTS AND DISCUSSION

The EtOAc extract of F. rabenhorstii culture filtrates, showing a strong phytotoxicity, was fractionated by thin-layer chromatography (TLC) and column chromatography (CC), as detailed in the Experimental Section. The 1H and COSY NMR spectra showed the presence of two doublets (J = 8.3 Hz) at δ 7.56 (H-6) and 7.43 (H-5) and a broad singlet at δ 6.23 as a result of the overlapping of two aromatic meta-coupled protons (H-3′ and H-5′) typical of 1,2,4,6- and 1,2,3,4-tetrasubstituted benzene rings, respectively This second ring, from the signals observed in the 13C NMR spectrum, appeared symmetrically substituted. The structure assigned to rabenzophenone (2) was supported from the other couplings observed in the HMBC spectrum and its HRESIMS data The latter showed a pseudomolecular ion [M + H]+ and typical isotopic peaks of 37Cl at m/z 339.0458 and 337.0469.

These are closely related to the already known moniliphenone

Other Authors

■ ACKNOWLEDGMENTS

■ REFERENCES