Rare benzannulated spiroketal dimers from an endophytic <i>Chaetomium</i> sp.

Three undescribed dimeric benzannulated spiroketals (1 − 3) were isolated from the fermented broth of Chaetomium sp. UJN-EF006, an endophytic fungal strain derived from Vaccinium bracteatum. Structures of these distinctive fungal products were elucidated by comprehensive spectroscopic analyses, and their absolute configurations were determined via electronic circular dichroism analyses. Of note, compounds 2 and 3 were unstable under normal storage condition, and three transformation products (3a − 3c) derived from 3 were further separated and structurally characterised. The inhibitory activities of available compounds against α-glucosidase, along with human MDA-MB-231 and A549 carcinoma cells, were also evaluated, with 3c exhibiting significant cytotoxicity.