Chemical Investigations Of Four Strains Of Insect-Associated Fungi

Natural products are valid sources of innovative drugs,presumably due to their diverse structural features and various biological activities.As prolific producers of active secondary metabolites,fungi play an important role in the discovery of drugs and lead compounds.Insect-associated fungi usually inhabit in the surface or inside the body of inscets and interact with the hosts in various aspects,gradually leading to formation of close and complicated relationships,such as parasitism,mutulism,commensalism,competition,and others.Currently,over 100 genera,800 species of insect-associated fungi have been recorded,and new species are being discovered continuously.During the long-term co-evolution,the insect-associated fungi gradually evolved unique metabolisms,and are more likely to produce new secondary metabolites.However,the insect-associated fungi have been largely underexplored due to the complicated environments they residing and the difficulty in obtaining fungal strains.With the availability of ever-increasing genome sequences,new strategies have been developed to generate new natural products or “unnatural” natural products based on genomic data,and the insect-associated fungi are attracting more attention.Based on the results from chemical investigations of a series of insect-associated fungi,and identification of the verticillin biosynthetic gene cluster,the mutant ?verG of the Opiocordyceps-associated fungus Clonostachys rogersoniana,and three other scale insect-associated fungi,were cultured under different conditions to prepare the organic solvent extracts.Guided by chemical fingerprints established by HPLC analyses,and by initial antimicrobial bioassay results,four fungal strains were further grown in larger scales under the optimal conditions to produce enough extracts.The EtOAc extracts were subsequently fractionated by silica gel and/or Sephadex LH-20 column chromatography,and reversed-phase high performance liquid chromatography(RP HPLC)to afford 32 compounds,23 of which were new structures.The structures of new compounds were elucidated primarily by mass spectrometry(MS),nuclear magnetic resonance(NMR),together with circular dichroism(CD)and chemical derivatization.The new compounds obtained in this work were also evaluated for their cytotoxicity against a panel of human tumor cell lines.Fractionation of the mutant ?verG led to the isolation of two new nonribosomal peptide-polyketide hybrids incorporating an unusual imidazolone N-oxide moiety;fractionation of the scale insect-associated fungus Hypocrea sp.resulted in the isolation of six new heterodimeric botryane ethers;fractionation of the scale insect-associated fungus Pestalotiopsis sp.led to the isolation of nine new hexahydrobenzopyran derivatives;fractionation of the scale insect-associated fungus Paraconiothyrium hawaiiense led to the isolation of three new tricyclic terpenoids,two ethers of tricyclic terpenoids and diketone cyclopentene,and a new compound structurally related to p-terphenyl.Chemical studies of four strains of insect-associated fungi indicated that this unique group of fungi deserve more systematic investigations.