Two New Fungal Metabolites from an Epiphytic Fungus Paraphaeosphaeria Species

Abstract

As illustrated by a number of biologically active fungal metabolites such as penicillins, cyclosporins, and lovastatin, fungi are clearly an important source of useful secondary metabolites. To date, most of the useful compounds obtained from fungi have been encountered by application of random screening methods using fungi from soil environments. However, with several thousand fungal metabolites now known from these sources, screening such fungi for new bioactive compounds often leads to re-isolation of known metabolites. Therefore, as an alternative strategy to isolate new fungal metabolites, employing ecological and/or taxonomic considerations when selecting target organisms for chemical investigation has been stressed by several authors. For example, endophytic fungi, which reside in the tissues of living plants, are one of chemically unexplored fungal niches, and thus considered as potential sources of novel natural products. Similarly, fungi inhabiting plant surface (epiphytic fungi) represent another promising fungal niche because not only they are virtually unexplored from a chemical standpoint, but fungal interference competition on the phylloplane microflora has been observed. During the course of initiated studies of endophytic and epiphytic fungi as potential sources of new bioactive secondary metabolites, we investigated an isolate of Paraphaeosphaeria sp. collected from the needles of Pinus densiflora in Kwanak Mt. Chemical studies of the EtOAc extract of the filtered culture broth obtained from this fungus have led to the isolation and structure determination of two new fungal metabolites named 2,3-didehydropalitantin (1) and culpin-1β-galactopyranoside (2). Details of these studies are presented here. 2,3-Didehydropalitantin (1) has the molecular formula C14H20O4, as deduced from C NMR and HRESIMS data. This formula indicated five degrees of unsaturation. The H NMR and DEPT data revealed the presence of one methyl group, two methylene units (one oxygenated), six olefinic protons, and four sp methine units (two oxygenated). Comparison of the DEPT results and molecular formula indicated the presence of three exchangeable protons. These data, together with the signals corresponding to a carbonyl group and three double bonds observed in the C NMR data, indicated that compound 1 must be a monocyclic compound with three free hydroxy groups. COSY (Table 1) experiments defined the partial proton spin systems in 1 corresponding to C7-C11, C8-C13-C14, and C1-C2. The C NMR signal position assignments were confirmed by HMQC data (Table 1), and connectivities among the subunits in 1 were deduced from HMBC correlations (Table 1). For example, correlations of H-13 and H-14 with the carbonyl carbon C-12, along with correlations of H-10 and