One pair of new cyclopentaisochromenone enantiomer from <i>Alternaria</i> sp. TNXY-P-1 and their cytotoxic activity

<p>One pair of new cyclopentaisochromenone derivatives, (+)-(<i>S</i>)-6-hydroxy-1,8-dimethoxy-3a-methyl-3,3a-dihydrocyclopenta[c]isochromene-2,5-dione (<b>1a</b>) and (-)-(<i>R</i>)-6-hydroxy-1,8-dimethoxy-3a-methyl-3,3a-dihydrocyclopenta[c]isochromene-2,5-dione (<b>1b</b>), together with seven known analog <b>2‒8</b>, were isolated from a rice solid culture of the endophytic fungus <i>Alternaria</i> sp. TNXY-P-1, obtained from fresh leaf of <i>Arisaema heterophyllum</i>. Their structures were elucidated on the basis of detailed 1D, 2D NMR, and HRESIMS analysis. Among them, compounds <b>1a</b> and <b>1b</b> were enantiomer<u>s</u> separated from <b>1</b> by chiral HPLC. The absolute configurations of <b>1a</b> and <b>1b</b> were assigned by quantum chemical calculations of the electronic circular dichroic spectra. All isolated compounds were evaluated for cytotoxic activities. Interestingly, enantiomers (+)-<b>1a</b> and (−)-<b>1b</b> showed distinct selective antitumor activities against HL-60 cell lines with IC₅₀ values of >200, 75.3 <i>μ</i>M, respectively.