Studies on the biosynthesis of the notoamides: synthesis of an isotopomer of 6-hydroxydeoxybrevianamide E and biosynthetic incorporation into notoamide J

New Prenylated Indole Homodimeric and Pteridine Alkaloids from the Marine-Derived Fungus Aspergillus austroafricanus Y32-2

Chemical investigation of secondary metabolites from the marine-derived fungus Aspergillus austroafricanus Y32-2 resulted in the isolation of two new prenylated indole alkaloid homodimers, di-6-hydroxydeoxybrevianamide E (1) and dinotoamide J (2), one new pteridine alkaloid asperpteridinate A (3), with eleven known compounds (4-14). Their structures were elucidated by various spectroscopic methods including HRESIMS and NMR, while their absolute configurations were determined by ECD calculations. Each compound was evaluated for pro-angiogenic, anti-inflammatory effects in zebrafish models and cytotoxicity for HepG2 human liver carcinoma cells. As a result, compounds 2, 4, 5, 7, 10 exhibited pro-angiogenic activity in a PTK787-induced vascular injury zebrafish model in a dose-dependent manner, compounds 7, 8, 10, 11 displayed anti-inflammatory activity in a CuSO₄-induced zebrafish inflammation model, and compound 6 showed significant cytotoxicity against HepG2 cells with an IC₅₀ value of 30 µg/mL.

Flavin-Dependent Monooxygenases NotI and NotI' Mediate Spiro-Oxindole Formation in Biosynthesis of the Notoamides

The fungal indole alkaloids are a unique class of complex molecules that have a characteristic bicyclo[2.2.2]diazaoctane ring and frequently contain a spiro-oxindole moiety. While various strains produce these compounds, an intriguing case involves the formation of individual antipodes by two unique species of fungi in the generation of the potent anticancer agents (+)- and (-)-notoamide A. NotI and NotI' have been characterized as flavin-dependent monooxygenases that catalyze epoxidation and semi-pinacol rearrangement to form the spiro-oxindole center within these molecules. This work elucidates a key step in the biosynthesis of the notoamides and provides an evolutionary hypothesis regarding a common ancestor for production of enantiopure notoamides.

Structural and stereochemical diversity in prenylated indole alkaloids containing the bicyclo[2.2.2]diazaoctane ring system from marine and terrestrial fungi

Covering: up to February 2017 Various fungi of the genera Aspergillus, Penicillium, and Malbranchea produce prenylated indole alkaloids possessing a bicyclo[2.2.2]diazaoctane ring system. After the discovery of distinct enantiomers of the natural alkaloids stephacidin A and notoamide B, from A. protuberus MF297-2 and A. amoenus NRRL 35660, another fungi, A. taichungensis, was found to produce their diastereomers, 6-epi-stephacidin A and versicolamide B, as major metabolites. Distinct enantiomers of stephacidin A and 6-epi-stephacidin A may be derived from a common precursor, notoamide S, by enzymes that form a bicyclo[2.2.2]diazaoctane core via a putative intramolecular hetero-Diels-Alder cycloaddition. This review provides our current understanding of the structural and stereochemical homologies and disparities of these alkaloids. Through the deployment of biomimetic syntheses, whole-genome sequencing, and biochemical studies, a unified biogenesis of both the dioxopiperazine and the monooxopiperazine families of prenylated indole alkaloids constituted of bicyclo[2.2.2]diazaoctane ring systems is presented.

Isolation of a new indoxyl alkaloid, Amoenamide B, from Aspergillus amoenus NRRL 35600: biosynthetic implications and correction of the structure of Speramide B

A new prenylated indoxyl alkaloid, Amoenamide B (1), was isolated from Aspergillus amoenus NRRL 35600 along with Asperochramide A (2). Although many prenylated oxyindole alkaloids, containing bicyclo[2.2.2]diazaoctane cores, have been isolated from the fungus of the genera Aspergillus and Penicillium to date, 1 is the fourth compound with the indoxyl unit containing the cores. During the structure elucidation of 1, we found that the planar structure matched to that of Speramide A (3), isolated from A. ochraceus KM007, but the reported structure of 3 was incorrect and turned out to be that of Taichunamide H (4), recently isolated from A. versicolor HDN11-84.

Enantioselective inhibitory abilities of enantiomers of notoamides against RANKL-induced formation of multinuclear osteoclasts

The marine-derived Aspergillus protuberus MF297-2 and the terrestrial A. amoenus NRRL 35600 produce enantiomeric prenylated indole alkaloids. Investigation of biological activities of the natural and synthetic derivatives revealed that (-)-enantiomers of notoamides A and B, 6-epi-notoamide T, and stephacidin A inhibited receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL)-induced osteoclastogenic differentiation of murine RAW264 cells more strongly than their respective (+)-enantiomers. Among them, (-)-6-epi-notoamide T was the most potent inhibitor with an IC₅₀ value of 1.7μM.

Isolation of amoenamide A and five antipodal prenylated alkaloids from Aspergillus amoenus NRRL 35600

A new prenylated alkaloid, Amoenamide A (6), was isolated from the fungus Aspergillus amoenus NRRL 35600. Previously, 6 was postulated to be a precursor of Notoamide E4 (21) converted from Notoamide E (16), which was a key precursor of the prenylated indole alkaloids in the fungi of the genus Aspergillus. We previously succeeded in the isolation of two pairs of antipodes, Stephacidin A (1) and Notoamide B (2), from A. amoenus and A. protuberus MF297-2 and expected the presence of other antipodes in the culture of A. amoenus. We here report five new antipodes (7-11) along with a new metabolite (12), which was isolated as a natural compound for the first time, from A. amoenus.

Biosynthesis of fungal indole alkaloids

This review provides a summary of recent research advances in elucidating the biosynthesis of fungal indole alkaloids. The different strategies used to incorporate and derivatize the indole/indoline moieties in various families of fungal indole alkaloids will be discussed, including tryptophan-containing nonribosomal peptides, polyketide-nonribosomal peptide hybrids, and alkaloids derived from other indole building blocks. This review also includes a discussion regarding the downstream modifications that generate chemical and structural diversity among indole alkaloids.

Shornephine A: structure, chemical stability, and P-glycoprotein inhibitory properties of a rare diketomorpholine from an Australian marine-derived Aspergillus sp

Chemical analysis of an Australian marine sediment-derived Aspergillus sp. (CMB-M081F) yielded the new diketomorpholine (DKM) shornephine A (1) together with two known and one new diketopiperazine (DKP), 15b-β-hydroxy-5-N-acetyladreemin (2), 5-N-acetyladreemin (3), and 15b-β-methoxy-5-N-acetyladreemin (4), respectively. Structure elucidation of 1–4 was achieved by detailed spectroscopic analysis, supported by chemical degradation and derivatization, and biosynthetic considerations. The DKM (1) underwent a facile (auto) acid-mediated methanolysis to yield seco-shornephine A methyl ester (1a). Our mechanistic explanation of this transformation prompted us to demonstrate that the acid-labile and solvolytically unstable DKM scaffold can be stabilized by N-alkylation. Furthermore, we demonstrate that at 20 μM shornephine A (1) is a noncytotoxic inhibitor of P-glycoprotein-mediated drug efflux in multidrug-resistant human colon cancer cells.

Palladium-catalyzed synthesis of N-tert-prenylindoles

Palladium-catalyzed N-tert-prenylations of indoles, tricarbonylchromium-activated indoles, and indolines that occur in high yields (up to 94%) with high tert-prenyl-to-n-prenyl selectivity (up to 12:1) are reported.

Marine natural products

This review covers the literature published in 2011 for marine natural products, with 870 citations (558 for the period January to December 2011) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1152 for 2011), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Aspergillus section Versicolores: nine new species and multilocus DNA sequence based phylogeny

β-tubulin, calmodulin, internal transcribed spacer and partial lsu-rDNA, RNA polymerase 2, DNA replication licensing factor Mcm7, and pre-rRNA processing protein Tsr1 were amplified and sequenced from numerous isolates belonging to Aspergillus sect. versicolor. The isolates were analyzed phylogenetically using the concordance model to establish species boundaries. Aspergillus austroafricanus, A. creber, A. cvjetkovicii, A. fructus, A. jensenii, A. puulaauensis, A. subversicolor, A. tennesseensis and A. venenatus are described as new species and A. amoenus, A. protuberus,A. sydowii, A. tabacinus and A. versicolor are accepted as distinct species on the basis of molecular and phenotypic differences. PCR primer pairs used to detect A. versicolor in sick building syndrome studies have a positive reaction for all of the newly described species except A. subversicolor.

Enantiomeric natural products: occurrence and biogenesis

In nature, chiral natural products are usually produced in optically pure form-however, occasionally both enantiomers are formed. These enantiomeric natural products can arise from a single species or from different genera and/or species. Extensive research has been carried out over the years in an attempt to understand the biogenesis of naturally occurring enantiomers; however, many fascinating puzzles and stereochemical anomalies still remain.

Biochemical characterization of NotB as an FAD-dependent oxidase in the biosynthesis of notoamide indole alkaloids

Notoamides produced by Aspergillus spp. bearing the bicyclo[2.2.2]diazaoctane core structure with unusual structural diversity represent a compelling system to understand the biosynthesis of fungal prenylated indole alkaloids. Herein, we report the in vitro characterization of NotB, which catalyzes the indole 2,3-oxidation of notoamide E (13), leading to notoamides C (11) and D (12) through an apparent pinacol-like rearrangement. This unique enzymatic reaction with high substrate specificity, together with the information derived from precursor incorporation experiments using [(13)C](2)-[(15)N](2) quadruply labeled notoamide S (10), demonstrates 10 as a pivotal branching point in notoamide biosynthesis.

Study on the biosynthesis of the notoamides: Pinacol-type rearrangement of the isoprenyl unit in deoxybrevianamide E and 6-hydroxydeoxybrevianamide E

Two reverse-prenylated indole alkaloids, deoxybrevianamide E and 6-hydroxydeoxybrevianamide E, are proposed as biosynthetic precursors for advanced metabolites isolated from the marine-derived Aspergillus sp. In order to uncover the role of the alkaloids in the biosynthetic pathway, the feeding experiments of the [(13)C](2)-[(15)N]-labeled deoxybrevianamide E and 6-hydroxydeoxybrevianamide E were performed to afford the metabolites, which were produced by oxidation and successive pinacol-type rearrangement of the isoprenyl units.