Phaeosphspirone (1/1′), a pair of unique polyketide enantiomers with an unusual 6/5/5/6 tetracyclic ring from the desert plant endophytic fungus Phaeosphaeriaceae sp. (February 2022) - Xu, Zhen-Lu
Abstract: Phaeosphspirone, an undescribed polyketide with a unique 6/5/5/6-fused tetracyclic system, and two known analogues, herbarin and O -methylherbarin, were purified from the endophytic fungus Phaeosphaeriaceae sp. isolated from the desert plant Bassia dasyphylla . The connectivity and relative configuration of phaeosphspirone was elucidated by comprehensive HR-ESI-MS and NMR analysis together with a computer-assisted structure elucidation (CASE) method. A pair of enantiomers existing in phaeosphspirone were separated by HPLC chromatography after reacting with chiral reagents, from which the absolute configuration of phaeosphspirone was simultaneously determined based on Mosher's rule. This tandem strategy provides a useful approach for the separation and stereochemical determination of enantiomers possessing secondary hydroxyl groups. The structural feature of phaeosphspirone, herbarin and O -methylherbarin together with gene cluster analysis suggested their polyketide biosynthetic origin. Herbarin and O -methylherbarin exhibited moderate cytotoxicity against three cancer cell lines. Graphical abstract: A pair of novel polyketide enantiomers were isolated and their ab solute configurations were determined by a unique tandem strategy. Image 1 Highlights: Phaeosphspirone (1 ) possessing a unique 6/5/5/6-fused tetracyclic system, and herbarin (2 ) and O -methylherbarin (3 ), were isolated. A pair of enantiomers (1/1′ ) were separated by HPLC chromatography after reacting with chiral reagents. The biosynthetic pathway of 1 –3 was suggested according to their structural features and gene cluster analysis. Compounds 2 and 3 exhibited moderate cytotoxicity against three cancer cells.