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The absolute configurations of regiolone and isosclerone, two enantiomeric bioactive naphthalenone pentaketides produced by fungi and plants, have been unambiguously assigned by ab initio computational prediction of their theoretical optical rotatory powers and electronic circular dichroism spectra. Isosclerone is produced by the plant pathogen Botrytis cinerea, whereas regiolone is produced by B. fabae. The phytotoxic activities of the two compounds were tested for comparison on faba bean (host of both pathogens) and vine plants (host of only B. cinerea); the (R) configuration at C-4 was found to be a fundamental structural feature for bioactivity

A strain of Botrytis fabae isolated from faba bean (Vicia faba L.) plants displaying clear chocolate spot disease symptoms produced phytotoxic metabolites in vitro. The phytotoxins isolated from the culture filtrate organic extract were characterized by spectroscopic and optical methods. A new naphthalenone pentaketide, named botrytone, was isolated and characterized as (4R)-3,4-dihydro-4,5,8-trihydroxy-1(2H)-naphthalenone together with other well-known closely related naphthalenones such as regiolone and cis- and trans-3,4-dihydro-2,4,8-trihydroxynaphthalen-1(2H)-ones. When tested on leaves of the host plant, with the cis- and trans-3,4-dihydro-2,4,8-trihydroxynaphthalen-1(2H)-ones assayed in mixture, regiolone demonstrated the highest level of phytotoxicity together with cis- and trans-3,4-dihydro-2,4,8-trihydroxynaphthalen-1(2H)-ones. Botrytone showed moderate phytotoxic activity at 1 mg/mL and was still phytotoxic at 0.5 mg/mL.

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