Chromones Featuring a [6,6]-Spiroketal Moiety Produced by Coculture of the Endophytic Fungi <i>Chaetomium virescens</i> and <i>Xylaria Grammica</i>

Under the guidance of HPLC-DAD analysis, ten new chromones featuring a rare [6,6]-spiroketal moiety, namely chaetovirexylariones A–J (<b>1</b>–<b>10</b>), together with two known congeners (<b>11</b>–<b>12</b>), were isolated from coculture of the endophytic fungi <i>Chaetomium virescens</i> and <i>Xylaria grammica</i>, from the rhizome of the medicinal plant <i>Smilax glabra</i> Roxb. Their structures were elucidated via a combination of NMR and HRESIMS data, and the absolute configurations of <b>1</b>–<b>10</b> were determined by the chemical conversion and single-crystal X-ray diffraction (Cu Kα) experiments, as well as the comparison of the experimental and calculated electronic circular dichroism (ECD) data. Compound <b>6</b> is the first report as a racemate of this type of natural product. Compound <b>10</b> represents the first example of a [6,6]-spiroketal chromone bearing a 5-amino-3-methyl-2-pentenoic acid fragment. Compound <b>8</b> demonstrated a reduction in PTX resistance of SW620/AD300 by a factor of 45, and had the potential to be an effective P-gp inhibitor and an antitumor chemotherapy sensitizer.