Further secondary metabolites produced by Diplodia corticola, a fungal pathogen involved in cork oak decline

Abstract

A new nor-pimarane diterpene and a new monosubstituted bifuranylone, named sphaeropsidin G (1) and diplobifuranylone D (2), were isolated from the organic extract of Diplodia corticola an oak pathogen together with the already known 4-hydroxyscytalone (3) and diorcinol (4). Sphaeropsidin G and diplobifuranylone D were characterized by using spectroscopic methods as 1,1,7-trimethyl-7-vinyl-2,3,4,4a,4b,5,6,7,10,10a-decahydro-1H-phenanthren-9-one and 5′-(1-hydroxyethyl)-2′,5′-dihydro-2H-[2,2′]bifuranyl-5-one, respectively. The relative configurations of sphaeropsidin G and diplobifuranylone D were assigned by comparison of their spectroscopic properties with those of sphaeropsidin A and diplobifuranylones A–C and confirmed by NOESY spectra. Each compound was tested for phytotoxic, antifungal, antioomycetes and zootoxic activity. Compound 4 was active in all bioassays exhibiting remarkable phytotoxicity on Quercus afares, Quercus suber, Quercus ilex and Celtis australis leaves at 1 mg/mL causing necrotic lesions. It completely inhibited the mycelial growth of Athelia rolfsii, Lasiodiplodia mediterranea and Phytophthora cinnamomi. In the Artemia salina assay, 1, 3 and 4 proved to be active, while 2 was found to be inactive in all bioassays.