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.

Bioconversion of 6-epi-Notoamide T Produces Metabolites of Unprecedented Structures in a Marine-derived Aspergillus sp.

We previously described the bioconversion of Notoamide T into (+)-Stephacidin A and (-)-Notoamide B, which suggested that Versicolamide B (8) is biosynthesized from 6-epi-Notoamide T (10) via 6-epi-Stephacidin A. Here we report that [13C]2-10 was incorporated into isotopically enriched 8 and seven new metabolites, which were not produced under normal culture conditions. The results suggest that the addition of excess precursor activated the expression of dormant tailoring genes giving rise to these structurally unprecedented metabolites.

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.

Fungal origins of the bicyclo[2.2.2]diazaoctane ring system of prenylated indole alkaloids.

Over eight different families of natural products consisting of nearly 70 secondary metabolites that contain the bicyclo[2.2.2]diazaoctane ring system have been isolated from various Aspergillus, Penicillium, and Malbranchea species. Since 1968, these secondary metabolites have been the focus of numerous biogenetic, synthetic, taxonomic, and biological studies and, as such, have made a lasting impact across multiple scientific disciplines. This review covers the isolation, biosynthesis, and biological activity of these unique secondary metabolites containing the bridging bicyclo[2.2.2]diazaoctane ring system. Furthermore, the diverse fungal origin of these natural products is closely examined and, in many cases, updated to reflect the currently accepted fungal taxonomy.

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.

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

6-Hydroxydeoxybrevianamide E is proposed as a biosynthetic precursor to several advanced metabolites isolated from both marine-derived Aspergillus sp. and a terrestrial-derived Aspergillus versicolor. To verify the role of this reverse-prenylated indole alkaloid as an intermediate along the biosynthetic pathway, [(13)C](2)-[(15)N]-6-hydroxydeoxybrevianamide E was synthesized and fed to Aspergillus versicolor. Analysis of the metabolites showed incorporation of the intermediate only into the natural product notoamide J.

Notoamide E: Biosynthetic incorporation into notoamides C and D in cultures of Aspergillus versicolor NRRL 35600.

Notoamide E, a short-lived secondary metabolite, has been proposed as a biosynthetic intermediate to several advanced metabolites isolated from Aspergillus versicolor. In order to verify the role of this indole alkaloid along the biosynthetic pathway, synthetic doubly (13)C-labeled notoamide E was fed to Aspergillus versicolor. Analysis of the metabolites showed significant incorporation of notoamide E into the natural products notoamides C and D.

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.

Genome-based characterization of two prenylation steps in the assembly of the stephacidin and notoamide anticancer agents in a marine-derived Aspergillus sp.

Stephacidin and notoamide natural products belong to a group of prenylated indole alkaloids containing a core bicyclo[2.2.2]diazaoctane ring system. These bioactive fungal secondary metabolites have a range of unusual structural and stereochemical features but their biosynthesis has remained uncharacterized. Herein, we report the first biosynthetic gene cluster for this class of fungal alkaloids based on whole genome sequencing of a marine-derived Aspergillus sp. Two central pathway enzymes catalyzing both normal and reverse prenyltransfer reactions were characterized in detail. Our results establish the early steps for creation of the prenylated indole alkaloid structure and suggest a scheme for the biosynthesis of stephacidin and notoamide metabolites. The work provides the first genetic and biochemical insights for understanding the structural diversity of this important family of fungal alkaloids.

Synthesis of notoamide J: a potentially pivotal intermediate in the biosynthesis of several prenylated indole alkaloids.

An efficient total synthesis of notoamide J, a new prenylated indole alkaloid and potential biosynthetic precursor, is described herein. Starting from L-proline and a substituted tryptophan derivative, this synthesis also employs an oxidation and pinacol rearrangement for the formation of the oxindole in the final step.

Detection of VM55599 and preparaherquamide from Aspergillus japonicus and Penicillium fellutanum: biosynthetic implications.

The secondary metabolites VM55599 (4) and preparaherquamide (5) have been identified by LC-MS(n) analysis as natural metabolites in cultures of Penicillium fellutanum, whereas preparaherquamide has been identified only in cultures of Aspergillus japonicus. In accord with a previous proposal, the identification of both metabolites, which have a diastereomeric relationship, provides indirect support for a unified biosynthetic scheme.

Synthesis and bioconversions of notoamide T: a biosynthetic precursor to stephacidin A and notoamide B.

In an effort to further elucidate the biogenesis of the stephacidin and notoamide families of natural products, notoamide T has been identified as the likely precursor to stephacidin A. The total synthesis of notoamide T is described along with it is C-6-epimer, 6-epi-notoamide T. The chemical conversion of stephacidin A to notoamide T by reductive ring opening is described as well as the oxidative conversion of notoamide T to stephacidin A. Furthermore, [(13)C](2)-notoamide T was synthesized and provided to Aspergillus versicolor and Aspergillus sp. MF297-2, in which significant incorporation was observed in the advanced metabolite, notoamide B.

Comparative analysis of the biosynthetic systems for fungal bicyclo[2.2.2]diazaoctane indole alkaloids: the (+)/(-)-notoamide, paraherquamide and malbrancheamide pathways.

The biosynthesis of fungal bicyclo[2.2.2]diazaoctane indole alkaloids with a wide spectrum of biological activities have attracted increasing interest. Their intriguing mode of assembly has long been proposed to feature a non-ribosomal peptide synthetase, a presumed intramolecular Diels-Alderase, a variant number of prenyltransferases, and a series of oxidases responsible for the diverse tailoring modifications of their cyclodipeptide-based structural core. Until recently, the details of these biosynthetic pathways have remained largely unknown due to lack of information on the fungal derived biosynthetic gene clusters. Herein, we report a comparative analysis of four natural product metabolic systems of a select group of bicyclo[2.2.2]diazaoctane indole alkaloids including (+)/(-)-notoamide, paraherquamide and malbrancheamide, in which we propose an enzyme for each step in the biosynthetic pathway based on deep annotation and on-going biochemical studies.

Biosynthetic studies of the notoamides: isotopic synthesis of stephacidin A and incorporation into notoamide B and sclerotiamide.

The advanced natural product stephacidin A is proposed as a biosynthetic precursor to notoamide B in various Aspergillus species. Doubly (13)C-labeled racemic stephacidin A was synthesized and fed to cultures of the terrestrial-derived fungus, Aspergillus versicolor NRRL 35600, and the marine-derived fungus, Aspergillus sp. MF297-2. Analysis of the metabolites revealed enantiospecific incorporation of intact (-)-stephacidin A into (+)-notoamide B in Aspergillus versicolor and (+)-stephacidin A into (-)-notoamide B in Aspergillus sp. MF297-2. (13)C-Labeled sclerotiamide was also isolated from both fungal cultures.

Intramolecular chiral relay at stereogenic nitrogen. Synthesis and application of a new chiral auxiliary derived from (1R,2S)-norephedrine and acetone.

(1R,2S)-Norephedrine has been employed in the synthesis of a novel 3,4,5,6-tetrahydro-2H-1,3,4-oxadiazin-2-one via reductive alkylation with acetone, N-nitrosation, reduction, and cyclization. The oxadiazinone was treated with either propionyl chloride or 3-thiophenylpropionyl chloride to afford the corresponding N(3)-acylated oxadiazinones 9a and 9b, respectively. X-ray crystallographic analysis of the N(3)-thiophenylpropionyl oxadiazinone 9b revealed that the C(2)-urethane carbonyl and the N(3)-carbonyl are arranged in an anti-periplanar conformation. The oxadiazinones were subsequently applied in the titanium-mediated asymmetric aldol addition reaction by treatment with titanium tetrachloride, triethylamine, and a variety of aldehydes at 0 degrees C. The aldol adducts 10a-i and 11a,b were found to have diastereoselectivities ranging from 8:1 to >99:1 favoring the formation of the non-Evans syn configuration. The absolute stereochemistry of the adduct 10a was determined by acid hydrolysis. This process afforded the N(4)-isopropyloxadiazinone 8 and (2S,3S)-3-hydroxy-2-methyl-3-phenylpropanoic acid 14 in >/=95% enantiomeric excess.