Kumbicins A–D: Bis-Indolyl Benzenoids and Benzoquinones from an Australian Soil Fungus, Aspergillus kumbius

Abstract

A soil survey conducted in southern Queensland, Australia, identified a novel isolate belonging to the genus Aspergillus subgenus Circumdati section Circumdati, Aspergillus kumbius FRR6049. Cultivation and fractionation of secondary metabolites from A. kumbius revealed a unique chemotype comprising three new bis-indolyl benzenoids, kumbicins A–C, and a new bis-indolyl benzoquinone, kumbicin D, along with the previously reported compounds asterriquinol D dimethyl ether, petromurins C and D, aspochracin, its N-demethyl analogue JBIR-15, and neohydroxyaspergillic acid. The structures of kumbicins A–D were assigned by detailed spectroscopic analysis. Kumbicin C was found to inhibit the growth of mouse myeloma cells (IC50 0.74 μg mL–1) and the Gram-positive bacterium Bacillus subtilis (MIC 1.6 μg mL–1).

Aspergillus hancockii sp. nov., a biosynthetically talented fungus endemic to southeastern Australian soils

“…hancockii adds further recognition of the interplay of novel chemical diversity and taxonomic uniqueness within subgenus Circumdati , as observed with our recent discovery of kumbicins as metabolites of A . kumbius [21]. …”

Bis-Indolyl Benzenoids, Hydroxypyrrolidine Derivatives and Other Constituents from Cultures of the Marine Sponge-Associated Fungus Aspergillus candidus KUFA0062

“…The structures of palmitic acid, clionasterol [ 22 ], ergosterol 5,8-endoperoxides [ 14 ], chrysophanic acid ( 1a ) [ 13 ], emodin ( 1b ) [ 14 ], asterriquinol D dimethyl ether ( 2a ) [ 15 ], petromurin C ( 2b ) [ 16 ], kumbicin B ( 2c ) [ 15 ], kumbicin A ( 2d ) [ 15 ], 2″-oxoasterriquinol D methyl ether ( 3 ) [ 17 ], kumbicin D ( 4 ) [ 15 ], preussin ( 5a ) [ 18 , 19 , 20 ], (3 S , 6 S )-3,6-dibenzylpiperazine-2,5-dione ( 6 ) [ 21 ] and 4-(acetylamino) benzoic acid ( 7 ) [ 22 ] were elucidated by analysis of their 1D and 2D NMR spectra as well as HRMS data, and also by comparison of their spectral data to those reported in the literature ( Supplementary Materials, Figures S2–S17, S20–S25, S32–S35 and Tables S1–S4 ).…”

“…The IR spectrum showed absorption bands for amine (3346 cm −1 ), aromatic (1625, 1579 cm −1 ), and ether (1291 cm −1 ) groups. The general features of the 1 H and 13 C NMR spectra of 2e ( Table 1 , Supplementary Materials, Figures S18 and S19 ) resembled those of kumbicin A ( 2d ), petromurin D ( 2b ), and two bis -indolyl benzenoids isolated from the soil fungus Aspergillus kumbius [ 15 ]. The 13 C NMR spectrum ( Table 1 ) exhibited twelve carbon signals which, in combination with DEPTs and HSQC spectra, can be categorized in six quaternary sp 2 (δ C 153.1, 147.6, 131.0, 127.4, 122.0, 106.7), four methine sp 2 (δ C 126.0, 111.9, 110.9, 102.1), and two methoxyl (δ C 60.3, 55.3) carbon signals.…”

“…Frs 462–517 were combined (253.9 mg) and recrystallized in MeOH to give 31.4 mg of an orange solid of emodin ( 1b ) [ 14 ]. Frs 528–583 were combined (535.8 mg) and precipitated in MeOH to give 254.4 mg of kumbicin D ( 4 ) [ 15 ], and the mother liquor that was crystallized in MeOH to give 11.8 mg of β-ergosterol 5,8-endoperoxide [ 14 ]. Frs 586–588 were combined (1.16 g) and recrystallized in Me 2 CO to give 345.0 mg of white crystal of asterriquinol D dimethyl ether ( 2a ) [ 15 ].…”

“…Chromatographic fractionation and further purification of the crude ethyl acetate extract of the cultures of A. candidus KUFA 0062, furnished two previously undescribed compounds named candidusin D ( 2e ) and preussin C ( 5b ), as well as the previously reported chrysophanic acid ( 1a ) [ 13 ], emodin ( 1b ) [ 14 ], six bis -indolyl benzenoids including asterriquinol D dimethyl ether ( 2a ) [ 15 ], petromurin C ( 2b ) [ 16 ], kumbicin B ( 2c ) [ 15 ], kumbicin A ( 2d ) [ 15 ], 2″-oxoasterriquinol D methyl ether ( 3 ) [ 17 ], kumbicin D ( 4 ) [ 15 ], the hydroxypyrrolidine alkaloid preussin ( 5a ) [ 18 , 19 , 20 ], (3 S , 6 S )-3,6-dibenzylpiperazine-2,5-dione ( 6 ) [ 21 ], and 4-(acetylamino) benzoic acid ( 7 ) [ 22 ] ( Figure 1 ). Additionally, the common fungal metabolites, i.e., palmitic acid, clionasterol [ 23 ], and ergosterol 5,8-endoperoxides [ 14 ] were also isolated ( Supplementary Material, Figure S1 ).…”