Isolation and Structure Elucidation of Some Components of the Antitumor Antibiotic Mixture ‘Rubiflavin’

Bioprospecting of Actinobacterial Diversity and Antibacterial Secondary Metabolites from the Sediments of Four Saline Lakes on the Northern Tibetan Plateau

The Tibetan Plateau, known as the "Roof of the World" and "The Third Pole", harbors numerous saline lakes primarily distributed in the Northern Tibetan Plateau. However, the challenging conditions of high altitude, low oxygen level, and harsh climate have limited investigations into the actinobacteria from these saline lakes. This study focuses on investigating the biodiversity and bioactive secondary metabolites of cultivable actinobacteria isolated from the sediments of four saline lakes on the Northern Tibetan Plateau. A total of 255 actinobacterial strains affiliated with 21 genera in 12 families of 7 orders were recovered by using the pure culture technique and 16S rRNA gene phylogenetic analysis. To facilitate a high-throughput bioactivity evaluation, 192 isolates underwent OSMAC cultivation in a miniaturized 24-well microbioreactor system (MATRIX cultivation). The antibacterial activity of crude extracts was then evaluated in a 96-well plate antibacterial assay. Forty-six strains demonstrated antagonistic effects against at least one tested pathogen, and their underlying antibacterial mechanisms were further investigated through a dual-fluorescent reporter assay (pDualrep2). Two Streptomyces strains (378 and 549) that produce compounds triggering DNA damage were prioritized for subsequent chemical investigations. Metabolomics profiling involving HPLC-UV/vis, UPLC-QTOF-MS/MS, and molecular networking identified three types of bioactive metabolites belonging to the aromatic polyketide family, i.e., cosmomycin, kidamycin, and hedamycin. In-depth analysis of the metabolomic data unveiled some potentially novel anthracycline compounds. A genome mining study based on the whole-genome sequences of strains 378 and 549 identified gene clusters potentially responsible for cosmomycin and kidamycin biosynthesis. This work highlights the effectiveness of combining metabolomic and genomic approaches to rapidly identify bioactive chemicals within microbial extracts. The saline lakes on the Northern Tibetan Plateau present prospective sources for discovering novel actinobacteria and biologically active compounds.

Naturally Occurring Chromone Glycosides: Sources, Bioactivities, and Spectroscopic Features

Chromone glycosides comprise an important group of secondary metabolites. They are widely distributed in plants and, to a lesser extent, in fungi and bacteria. Significant biological activities, including antiviral, anti-inflammatory, antitumor, antimicrobial, etc., have been discovered for chromone glycosides, suggesting their potential as drug leads. This review compiles 192 naturally occurring chromone glycosides along with their sources, classification, biological activities, and spectroscopic features. Detailed biosynthetic pathways and chemotaxonomic studies are also described. Extensive spectroscopic features for this class of compounds have been thoroughly discussed, and detailed 13C-NMR data of compounds 1-192, have been added, except for those that have no reported 13C-NMR data.

A comprehensive review of glycosylated bacterial natural products

A systematic analysis of all naturally-occurring glycosylated bacterial secondary metabolites reported in the scientific literature up through early 2013 is presented. This comprehensive analysis of 15 940 bacterial natural products revealed 3426 glycosides containing 344 distinct appended carbohydrates and highlights a range of unique opportunities for future biosynthetic study and glycodiversification efforts.

First Enantioselective Total Synthesis and Structure Determination of the Anthrapyran Metabolite γ-Indomycinone

The first total synthesis of (R)-gamma-indomycinone has been achieved which allowed the determination of the configuration of the stereogenic center of natural gamma-indomycinone as (S). The approach stands out for its generality and efficiency. [reaction: see text]

2′-Carboxybenzyl glycosides: glycosyl donors for C-glycosylation and conversion into other glycosyl donors

Glycosylation of various glycosyl acceptors with 2'-carboxybenzyl (CB) 2,3,4,6-tetra-O-benzyl-beta-D-glucopyranoside and CB 2,3,4,6-tetra-O-benzyl-alpha-D-mannopyranoside as glycosyl donors afforded alpha-C-glycosides exclusively or predominantly in good yields. CB glycosides were also converted to other well-known glycosyl donors, the corresponding phenyl thioglycoside and the glycosyl fluoride derivatives.

A Synthesis of l-Vancosamine Derivatives from Non-Carbohydrate Precursors by a Short Sequence Based on the Marshall, McDonald, and Du Bois Reactions

[reaction: see text] The carbamate-protected l-vancosamine glycal, viewed as a universal precursor for vancosamine derivatives, was prepared by a short scheme based on diastereoselective addition of an allenyl stannane to a lactaldehyde ether, the tungsten-catalyzed alkynol cycloisomerization, and the rhodium-catalyzed C-H insertion of a carbamate nitrogen. This sequence is a prototype for a new and efficient strategy for the synthesis of 3-amino sugar derivatives. The key intermediate was elaborated to the silyl ether of N,N-dimethyl vancosamine glycal.

Umpolung strategy for the synthesis of 2-deoxy-C-aryl glycosides: a serendipitous, efficient route for C-furanoside analogues

[reaction: see text] 2-Deoxy-C-aryl glycosides are potential synthetic targets as they form a very vital moiety of several biologically active natural products. This paper describes a synthetic route using an umpolung strategy, which has not been explored till date. Our synthetic endeavor led to a versatile intermediate aryl ketone 10, which has paved the way for two important classes of C-glycosides, viz., C-alkyl furanosides 12 and methyl 2-deoxy-C-aryl pyranosides 14.

A de novo synthesis of 2,6-dideoxy-C-aryl glycosides based on ring closing metathesis and diastereoselective epoxide cleavage/anomerization reactions

[formula: see text] This paper describes a synthesis of enantiomerically pure 2,6-dideoxy-C-aryl glycosides, starting from non-carbohydrate precursors. The synthesis starts from homoallylic alcohols (obtained in enantiomerically pure form by enzymatic resolution), which are elaborated to dihydropyrans using ring closing metathesis as the key step. Epoxidation and epoxide cleavage complete the synthesis. The stereochemical outcome of the sequence depends on the conditions of the epoxide cleavage reaction.

Hedamycin intercalates the DNA helix and, through carbohydrate-mediated recognition in the minor groove, directs N7-alkylation of guanine in the major groove in a sequence-specific manner

BACKGROUND

The pluramycins are a class of antitumor antibiotics that exert their biological activity through interaction with DNA. Recent studies with the analog altromycin B have determined that these agents intercalate into the DNA molecule, position carbohydrate substituents into both major and minor grooves, and alkylate the DNA molecule by epoxide-mediated electrophilic attack on N7 of guanine located to the 3' side of the drug molecule. Alkylation is sequence dependent and appears to be modulated by glycoside substituents attached at the corners of a planar chromophore. The altromycin B-like analogs preferentially alkylate 5'AG sequences; hedamycin-like analogs prefer 5'TG and 5'CG sequences. Although the mechanism of guanine modification by altromycin B has been extensively studied, the mechanism of action of hedamycin has not been previously determined.

RESULTS

Using high-field NMR, we have shown that hedamycin stacks to the 5' side of the guanine nucleotide at the site of intercalation in a DNA decamer, positioning both aminosaccharides into the minor groove to direct alkylation by the epoxide moiety on N7 of guanine. The C10 linked N,N-dimethylvancosamine sugar moiety interacts to the 5' side of the intercalation site, while the C8 linked anglosamine moiety interacts to the 3' side. The binding interactions of the two aminosugars steer the C2 double epoxide located in the major groove into the proximity of N7 of guanine. Unexpectedly, it is not the first epoxide that undergoes electrophilic addition to N7 of guanine, which would correspond to altromycin B, but the second, terminal epoxide.

CONCLUSIONS

We have used two-dimensional NMR to elucidate the sequence-selective recognition of DNA by hedamycin and the mechanism of covalent modification of guanine by this antibiotic. Characterization of the intermolecular interactions between both hedamycin and altromycin B and their targeted DNA sequences has yielded a better understanding of the reasons for variations in sequence selectivity and alkylation reactivity among the pluramycin compounds.