Dimericbiscognienyne A: A Meroterpenoid Dimer from Biscogniauxia sp. with New Skeleton and Its Activity

Dimericbiscognienyne A (1), an unusual diisoprenyl-cyclohexene-type meroterpenoid dimer, was isolated from Biscogniauxia sp. together with three new monomeric diisoprenyl-cyclohexene-type meroterpenoids (2–4) and one new isoprenyl-benzoic acid-type meroterpenoid (5). All structures were determined by extensive NMR spectroscopic methods, quantum chemical calculations, chemical derivatization, and X-ray crystallography. The formation of 1 is related to a unique intermolecular redox coupling Diels–Alder adduct reaction. Their cytotoxicities and short-term memory enhancement activities against Alzheimer’s disease were assessed.