Biosynthesis and Total Synthesis Studies on The Jadomycin Family of Natural Products

Reagent-controlled chemo/stereoselective glycosylation of ʟ-fucal to access rare deoxysugars

2,6-Dideoxy sugars constitute an important class of anticancer antibiotics natural products and serve as essential medicinal tools for carbohydrate-based drug discovery and vaccine development. In particular, 2-deoxy ʟ-fucose or ʟ-oliose is a rare sugar and vital structural motif of several potent antifungal and immunosuppressive bioactive molecules. Herein, we devised a reagent-controlled stereo and chemoselective activation of ʟ-fucal, enabling the distinctive glycosylation pathways to access the rare ʟ-oliose and 2,3-unsaturated ʟ-fucoside. The milder oxo-philic Bi(OTf)3 catalyst induced the direct 1,2-addition predominantly, whereas B(C6F5)3 promoted the allylic Ferrier-rearrangement of the enol-ether moiety in ʟ-fucal glycal donor, distinguishing the competitive mechanisms. The reagent-tunable modular approach is highly advantageous, employing greener catalysts and atom-economical transformations, expensive ligand/additive-free, and probed for a diverse range of substrates comprising monosaccharides, amino-acids, bioactive natural products, and drug scaffolds embedded with susceptible or labile functionalities.

Stereoselective Synthesis of 2-Deoxy Glycosides via Iron Catalysis

An Fe-catalyzed 2-deoxy glycosylation method was developed from 3,4-O-carbonate glycals directly at room temperature. This novel approach enabled facile access to alkyl and aryl 2-deoxy glycosides in high yields with exclusive α-stereoselectivity, tolerating various alcohols, phenols, and glycals. The synthetic utility and advantage of this strategy have been demonstrated by the modification of six natural products and the construction of a tetrasaccharide.

De novo asymmetric Achmatowicz approach to oligosaccharide natural products

The development and application of the asymmetric synthesis of oligosaccharides from achiral starting materials is reviewed. This de novo asymmetric approach centers around the use of asymmetric catalysis for the synthesis of optically pure furan alcohols in conjunction with Achmatowicz oxidative rearrangement for the synthesis of various pyranones. In addition, the use of a diastereoselective palladium-catalyzed glycosylation and subsequent diastereoselective post-glycosylation transformation was used for the synthesis of oligosaccharides. The application of this approach to oligosaccharide synthesis is discussed.

Nonenzymatic Reactions in Natural Product Formation

Biosynthetic mechanisms of natural products primarily depend on systems of protein catalysts. However, within the field of biosynthesis, there are cases in which the inherent chemical reactivity of metabolic intermediates and substrates evades the involvement of enzymes. These reactions are difficult to characterize based on their reactivity and occlusion within the milieu of the cellular environment. As we continue to build a strong foundation for how microbes and higher organisms produce natural products, therein lies a need for understanding how protein independent or nonenzymatic biosynthetic steps can occur. We have classified such reactions into four categories: intramolecular, multicomponent, tailoring, and light-induced reactions. Intramolecular reactions is one of the most well studied in the context of biomimetic synthesis, consisting of cyclizations and cycloadditions due to the innate reactivity of the intermediates. There are two subclasses that make up multicomponent reactions, one being homologous multicomponent reactions which results in dimeric and pseudodimeric natural products, and the other being heterologous multicomponent reactions, where two or more precursors from independent biosynthetic pathways undergo a variety of reactions to produce the mature natural product. The third type of reaction discussed are tailoring reactions, where postmodifications occur on the natural products after the biosynthetic machinery is completed. The last category consists of light-induced reactions involving ecologically relevant UV light rather than high intensity UV irradiation that is traditionally used in synthetic chemistry. This review will cover recent nonenzymatic biosynthetic mechanisms and include sources for those reviewed previously.

Himalaquinones A-G, Angucyclinone-Derived Metabolites Produced by the Himalayan Isolate Streptomyces sp. PU-MM59

Himalaquinones A-G, seven new anthraquinone-derived metabolites, were obtained from the Himalayan-based Streptomyces sp. PU-MM59. The chemical structures of the new compounds were identified based on cumulative analyses of HRESIMS and NMR spectra. Himalaquinones A-F were determined to be unique anthraquinones that contained unusual C-4a 3-methylbut-3-enoic acid aromatic substitutions, while himalaquinone G was identified as a new 5,6-dihydrodiol-bearing angucyclinone. Comparative bioactivity assessment (antimicrobial, cancer cell line cytotoxicity, impact on 4E-BP1 phosphorylation, and effect on axolotl embryo tail regeneration) revealed cytotoxic landomycin and saquayamycin analogues to inhibit 4E-BP1p and inhibit regeneration. In contrast, himalaquinone G, while also cytotoxic and a regeneration inhibitor, did not affect 4E-BP1p status at the doses tested. As such, this work implicates a unique mechanism for himalaquinone G and possibly other 5,6-dihydrodiol-bearing angucyclinones.

Targeted Genome Mining-From Compound Discovery to Biosynthetic Pathway Elucidation

Natural products are an important source of novel investigational compounds in drug discovery. Especially in the field of antibiotics, Actinobacteria have been proven to be a reliable source for lead structures. The discovery of these natural products with activity- and structure-guided screenings has been impeded by the constant rediscovery of previously identified compounds. Additionally, a large discrepancy between produced natural products and biosynthetic potential in Actinobacteria, including representatives of the order Pseudonocardiales, has been revealed using genome sequencing. To turn this genomic potential into novel natural products, we used an approach including the in-silico pre-selection of unique biosynthetic gene clusters followed by their systematic heterologous expression. As a proof of concept, fifteen Saccharothrixespanaensis genomic library clones covering predicted biosynthetic gene clusters were chosen for expression in two heterologous hosts, Streptomyceslividans and Streptomycesalbus. As a result, two novel natural products, an unusual angucyclinone pentangumycin and a new type II polyketide synthase shunt product SEK90, were identified. After purification and structure elucidation, the biosynthetic pathways leading to the formation of pentangumycin and SEK90 were deduced using mutational analysis of the biosynthetic gene cluster and feeding experiments with ¹³C-labelled precursors.

Biosynthesis, synthetic studies, and biological activities of the jadomycin alkaloids and related analogues

The jadomycins are an expanding class of compounds produced from Streptomyces venezuelae, by diverting the normal biosynthesis which provides the antibiotic chloramphenicol. In the presence of amino acids, and either by heat shock, supplementation with ethanol, or when phage SV1 is added to the culture, the formation of substituted jadomycins and benzo[b]phenanthridines can be achieved. The first part of this review provides details of intermediates involved in the biosynthesis of the jadomycins and the related benzo[b]phenanthridines. Both the jadomycins and the benzo[b]phenanthridines share biosynthetic pathways with a large class of naturally occurring compounds known as the angucyclines. The biosynthetic pathways diverge when it is postulated that an intermediate quinone, such as 3-(2-formyl-6-hydroxy-4-methylphenyl)-8-hydroxy-1,4-naphthoquinone-2-carboxylic acid is formed. The quinone then undergoes reactions with amino acids and derivatives in the culture medium to ultimately afford a library of jadomycins and a few benzo[b]phenanthridines. The second part of the review initially details synthetic efforts toward the synthesis of the naturally occurring benzo[b]phenanthridine, phenanthroviridin, and then outlines methods that have been used to assemble a selection of jadomycins. Total syntheses of jadomycin A and B, derived from l-isoleucine, are described. In addition, the synthesis of the aglycon of jadomycins M, W, S, and T is outlined. These four jadomycins were derived from l-methionine, l-tryptophan, l-serine and l-threonine respectively. As a result of these synthetic efforts, the structures of jadomycin S and T have been revised. The third part of the review describes the reported antibacterial and anticancer activities of both the jadomycins and some naturally occurring benzo[b]phenanthridines.

Gene Cluster Activation in a Bacterial Symbiont Leads to Halogenated Angucyclic Maduralactomycins and Spirocyclic Actinospirols

Growth from spores activated a biosynthetic gene cluster in Actinomadura sp. RB29, resulting in the identification of two novel groups of halogenated polyketide natural products, named maduralactomycins and actinospirols. The unique tetracyclic and spirocyclic structures were assigned based on a combination of NMR analysis, chemoinformatic calculations, X-ray crystallography, and ¹³C labeling studies. On the basis of HRMS² data, genome mining, and gene expression studies, we propose an underlying noncanonical angucycline biosynthesis and extensive post-polyketide synthase (PKS) oxidative modifications.

Isolation of a post-PKS C–C branching jadomycin from S. venezuelae ISP5230 in the presence of 8-aminooctanoic acid

The jadomycin family of natural products was first identified and characterized by Vining and co-workers at Dalhousie University in the 1990s. Herein, we report findings from a recently developed co-amino acid supplementation culture method with S. venezuelae ISP5230 using 8-aminooctanoic acid, where the major natural product was a jadomycin variant omitting an E-ring (1). These results reinforce that the 3a position is susceptible to nucleophilic addition by cellular metabolites in jadomycin biosynthesis when intramolecular cyclization is unfavorable. Further, the cytotoxicity data for several unsubstituted E-ring jadomycins are reported and discussed.

Engineered jadomycin analogues with altered sugar moieties revealing JadS as a substrate flexible O-glycosyltransferase

Glycosyltransferases (GTs)-mediated glycodiversification studies have drawn significant attention recently, with the goal of generating bioactive compounds with improved pharmacological properties by diversifying the appended sugars. The key to achieving glycodiversification is to identify natural and/or engineered flexible GTs capable of acting upon a broad range of substrates. Here, we report the use of a combinatorial biosynthetic approach to probe the substrate flexibility of JadS, the GT in jadomycin biosynthesis, towards different non-native NDP-sugar substrates, enabling us to identify six jadomycin B analogues with different sugar moieties. Further structural engineering by precursor-directed biosynthesis allowed us to obtain 11 new jadomycin analogues. Our results for the first time show that JadS is a flexible O-GT that can utilize both L- and D- sugars as donor substrates, and tolerate structural changes at the C2, C4 and C6 positions of the sugar moiety. JadS may be further exploited to generate novel glycosylated jadomycin molecules in future glycodiversification studies.

Ru-catalyzed sequence for the synthesis of cyclic amido-ethers

Efficient synthesis of versatile building blocks for enabling medicinal chemistry research has always challenged synthetic chemists to develop innovative methods. Of particular interest are the methods that are amenable to the synthesis of chemically distinct and diverse classes of pharmaceutically relevant motifs. Herein we report a general method for the one-pot synthesis of cyclic α-amido-ethers containing different amide functionalities including lactams, tetramic acids and amino acids. For the incorporation of the nucleotide bases, a chemo and regioselective palladium-catalyzed transformation has been developed, providing rapid access to nucleoside analogs.

A Survey of Recent Synthetic Applications of 2,3-Dideoxy-Hex-2-enopyranosides

Unsaturated carbohydrate derivatives are useful intermediates in synthetic transformations leading to a variety of compounds. The aim of this review is to highlight the rich chemistry of ∆-2,3 unsaturated pyranosides, emphasizing the variety of transformations that have been carried out in these substrates during the last decade.

De novo asymmetric synthesis of the pyranoses: from monosaccharides to oligosaccharides

The various methods for the de novo asymmetric synthesis of the pyranose sugars are surveyed. The presentation begins with the work of Masamune and Sharpless with the use of the Sharpless asymmetric epoxidation for the synthesis of all eight l-hexoses. The development of other asymmetric reactions and their application for the synthesis of specific hexopyranoses are further discussed. The broad application of the Achmatowicz rearrangement with asymmetric catalysis, for the synthesis of various pyranones and imino sugars, is also presented. Finally, the use of a diastereoselective palladium-catalyzed glycosylation with the Achmatowicz approach for the synthesis of oligosaccharides and applications to medicinal chemistry are discussed.