Boerelasins A-D, Four Unprecedented Cytochalasins from the Endophytic Fungus Boeremia Exigua .

Four undescribed cytochalasins ( 1-4 ) were isolated from the endophytic fungus Boeremia exigua . Structurally, boerelasin A ( 1 ) represents the first example of a cytochalasin with a rare 5/5 bicyclic carbon core. Boerelasin B ( 2 ) possesses an unprecedented 5/6/5/6/8 pentacyclic ring system. Boerelasin C ( 3 ), a derivative from the common biosynthetic intermediate to 1 , is a macrocyclic ring-opening cytochalasin, and boerelasin D ( 4 ) contains an uncommon six-carbon alkyl acid side chain. The structures were elucidated based on spectroscopic methods, electronic circular dichroism, spin-spin coupling constants, and calculated nuclear magnetic resonance with DP4+ analysis. These compounds exhibited significant cytotoxicity against the tumor cells.