Dimericbiscognienyne A: A Meroterpenoid Dimer from <i>Biscogniauxia</i> sp. with New Skeleton and Its Activity

Dimericbiscognienyne A (<b>1</b>), an unusual diisoprenyl-cyclohexene-type meroterpenoid dimer, was isolated from <i>Biscogniauxia</i> sp. together with three new monomeric diisoprenyl-cyclohexene-type meroterpenoids (<b>2</b>–<b>4</b>) and one new isoprenyl-benzoic acid-type meroterpenoid (<b>5</b>). All structures were determined by extensive NMR spectroscopic methods, quantum chemical calculations, chemical derivatization, and X-ray crystallography. The formation of <b>1</b> 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.