Enantiomeric Dibenzo-α-Pyrone Derivatives from Alternaria alternata ZHJG5 and Their Potential as Agrochemicals

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Three pairs of enantiomeric dibenzo-α-pyrone derivatives ( 1 – 3 ) including two pairs of new racemates (±)-alternaone A ( 1 ) and (±)-alternaone B ( 2 ) and one new enantiomer (−)-alternatiol ( 3 ), together with five known compounds ( 4 – 8 ) were isolated from the fungus Alternaria alternata ZHJG5. Their structures were confirmed by spectroscopic data and single-crystal X-ray diffraction analysis. All enantiomers were separated via chiral high-performance liquid chromatography, with their configurations determined by electronic circular dichroism calculation. Biogenetically, a key epoxy-rearrangement step was proposed for the formation of skeletons in 1 – 3; (+) 1, (−)- 1, and 5 presented moderate antibacterial inhibition on phytopathogenic bacteria Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicola . In the antifungal test, compounds 7 and 8 showed a moderate protective effect against Botrytis cinerea in vivo.