Abstract The structure and absolute configuration of fraxinellonone isolated fromFagaropsis glabra was established as 3α-(3′-furanyl)-3α,7-dimethyl-4,5-dihydro-1,6-isobenzofurandione, by NMR and CD comparison with fraxinellone. Complete carbon assignments for fraxinellone and isofraxinellone were achieved by 13 C- 1 H COSY experiments.