Diplobifuranylones A and B, 5′-monosubstituted tetrahydro-2H-bifuranyl-5-ones produced by Diplodia corticola, a fungus pathogen of cork oak

Two new 5'-monosubstituted tetrahydro-2H-bifuranyl-5-ones, named diplobifuranylones A and B ( 1 and 2), were isolated from the culture filtrates of Diplodia corticola, the causal agent of a canker of cork oak ( Quercus suber). The same fungus also produced eight known metabolites, namely, the diplopyrone, (3S, 4R)-trans- and (3R, 4R)-cis-4-hydroxymellein, sapinofuranone B and its ( S,S)-enantiomer, and sphaeropsidins A-C. Diplobifuranylones A and B ( 1 and 2) were characterized, using spectroscopic and chemical methods, as two diastereomeric 5'-(1-hydroxyethyl)-3,4,2',5'-tetrahydro-2H-[ 2,2'] bifuranyl-5-ones. While the relative stereochemistry of the two metabolites ( 1 and 2) was deduced by NOESY and ROESY experiments, the absolute stereochemistry of the chiral carbon of the hydroxyethyl side chain at C-5', determined by application of Mosher's method, proved to be S and R in 1 and 2, respectively. Assayed on a nonhost plant, diplobifuranylones A and B did not show phytotoxic activity. In an Artemia salina larvae lethality bioassay neither 1 nor 2 was toxic at the highest concentration tested ( 300 mu g/mL).