Morusalones A−D (1−4), a new class of Diels−Alder adducts featuring unprecedented 6/7/6/6/6/6 hexacyclic core skeletons with a unique bridged cycloheptenone ring, were isolated from Morus alba cell cultures. The biosyntheses for 1−4 were proposed through an unusual Diels−Alder cycloaddition with quinostilbenes as dienophiles and prenyl 2-phenylbenzofuran as a diene to yield the typical methylhexene unit and a rare intramolecular nucleophilic addition to form the cycloheptenone ring. Compounds 1−4 exhibited protein tyrosine phosphatase 1B inhibitory activity.