Isolation of (-)-cryptosporiospin, a chlorinated cyclopentenone fungitoxic metabolite from Phialophora asteris f. sp. helianthi

Secondary metabolites from Aspergillus terreus F6-3, a marine fungus associated with Johnius belengerii

Two new terrein derivatives asperterreinones A-B (1-2), one new octahydrocoumarin derivative (±)-asperterreinin A (6), along with seventeen known compounds, were isolated from Aspergillus terreus F6-3, a marine fungus associated with Johnius belengerii. The structures of 1, 2, and 6 were established on the basis of 1D and 2D NMR, mass spectroscopy, comparative electronic circular dichroism (ECD) spectra analysis, density functional theory calculation of 13C NMR, and DP4+ probability analysis. Among all the isolates, eurylene (7), a constituent of the Malaysian medicinal plant Eurycoma longifolia, was obtained from a microbial source for first time. In the in vitro bioassay, 11 and 13 showed potent inhibitory activity against the Escherichia coli β-glucuronidase (EcGUS) with IC50 values of 27.75 ± 0.73 and 17.73 ± 0.81 μM, respectively. It was the first time that questinol (11) and (±)-aspertertone B (13) were reported as potent EcGUS inhibitors.

Early Oxidative Transformations During the Biosynthesis of Terrein and Related Natural Products

The mycotoxin terrein is derived from the C₁₀‐precursor 6‐hydroxymellein (6‐HM) via an oxidative ring contraction. Although the corresponding biosynthetic gene cluster (BGC) has been identified, details of the enzymatic oxidative transformations are lacking. Combining heterologous expression and in vitro studies we show that the flavin‐dependent monooxygenase (FMO) TerC catalyzes the initial oxidative decarboxylation of 6‐HM. The reactive intermediate is further hydroxylated by the second FMO TerD to yield a highly oxygenated aromatic species, but further reconstitution of the pathway was hampered. A related BGC was identified in the marine‐derived Roussoella sp. DLM33 and confirmed by heterologous expression. These studies demonstrate that the biosynthetic pathways of terrein and related (polychlorinated) congeners diverge after oxidative decarboxylation of the lactone precursor that is catalyzed by a conserved FMO and further indicate that early dehydration of the side chain is an essential step.

Biosynthetic Machinery of 6-Hydroxymellein Derivatives Leading to Cyclohelminthols and Palmaenones

Oxygenated cyclopentene systems are unique structural motifs found in fungal polyketides such as terrein, cyclohelminthols, and palmaenones. Here we report the identification of the biosynthetic gene clusters for cyclohelminthols and palmaenones and the functional characterization of the polyketide synthases and halogenases involved in the construction of 6-hydroxymellein derivatives. Heterologous expression in Aspergillus oryzae demonstrated that 6-hydroxymellein is a common biosynthetic intermediate and that chlorination occurs in the early stages of its products' biosynthesis. This was further confirmed by in vitro enzymatic reactions conducted in the presence of recombinant proteins. Plausible means of biogenesis of fungal polyketides from 6-hydroxymellein derivatives, additionally supported by the reported labeling patterns of terrein and structurally related fungal polyketides, are also discussed. This study sets the stage for elucidation of the biosynthetic machinery of fungal polyketides of this type.

Xanthocidin Derivatives from the Endophytic Streptomyces sp. AcE210 Provide Insight into Xanthocidin Biosynthesis

Xanthocidin and six new derivatives were isolated from the endophytic Streptomyces sp. AcE210. Their planar structures were elucidated by 1D and 2D NMR spectroscopy as well as by HRMS. The absolute configuration of one compound was determined by using vibrational circular dichroism spectroscopy (VCD). The structural similarities of xanthocidin and some of the isolated xanthocidin congeners to the methylenomycins A, B, and C suggested that the biosynthesis of these compounds might follow a similar route. Feeding studies with isotopically labelled [¹³ C₅ ]-l-valine showed that instead of utilizing acetyl-CoA as starter unit, which has been proposed for the methylenomycin biosynthesis, Streptomyces sp. AcE210 employs an isobutyryl-CoA starter unit, resulting in a branched side chain in xanthocidin. Further evidence for a comparable biosynthesis was given by the analysis of the genome sequence of Streptomyces sp. AcE210 that revealed a cluster of homologues to the mmy genes involved in methylenomycin biosynthesis.

Structure and Biogenesis of Roussoellatide, a Dichlorinated Polyketide from the Marine-Derived Fungus Roussoella sp. DLM33

The structure of the fungal metabolite roussoellatide (1) has been established by spectroscopic and X-ray diffraction analyses. Results from feeding experiments with [1-(13)C]acetate, [2-(13)C]acetate, and [1,2-(13)C]acetate were consistent with a biosynthetic pathway to the unprecedented skeleton of 1 involving Favorskii rearrangements in separate pentaketides, subsequently joined via an intermolecular Diels-Alder reaction.

Asymmetric formal synthesis of (–)-cryptosporiopsin, a metabolite of Phialophora asterris, and its 3-deschloro congener

An asymmetric formal synthesis of the fungal metabolites (–)-cryptosporiopsin and (–)-3-deschloro crypto sporiopsin has been accomplished. The asymmetry was introduced via a diastereoselective [2+2]-cycloaddition between an enoxy-ester, bearing a bulky chiral auxiliary, and dichloroketene. Diastereomeric ratios of 10:1 were obtained from the reaction mixture, but this could be increased by simple crystallization. Completion of the synthesis was only accomplished after the serendipitous discovery of a base-catalyzed rearrangement to furnish the requisite 3-chloroenone moiety.Key words: diastereoselective [2+2]-cycloaddition, rearrangement, chiral auxiliary.