Structure assignment, total synthesis, and antiviral evaluation of cycloviracin B1

Advancements in Enacyloxins Total Synthesis: Access to the Chlorinated Polyunsaturated Chain Peculiar to this Promising Family of Antibiotics

The first synthesis of the protected chain specific to the enacyloxin antibiotic family is reported. The noticeable features are (a) the construction of the chlorinated undecapentaenoic moiety implementing the sequence Tsuji's alkyne syn allyl-chlorination, E-selective Pd/Cu-catalyzed allene-alkyne coupling, Horner-Wadsworth-Emmons olefination, dehydration; (b) control of the C18 chlorinated stereogenic center by organo-catalyzed aldehyde α-chlorination; and (c) the assemblage of this aldehyde with the C1-C16 ketone using a highly diastereoselective Mukaiyama aldol.

Antiviral Activities of Streptomyces KSF 103 Methanolic Extracts against Dengue Virus Type-2

Dengue has long been a serious health burden to the global community, especially for those living in the tropics. Despite the availability of vaccines, effective treatment for the infection is still needed and currently remains absent. In the present study, the antiviral properties of the Streptomyces sp. KSF 103 methanolic extract (Streptomyces KSF 103 ME), which consists of a number of potential antiviral compounds, were investigated against dengue virus serotype 2 (DENV-2). The effects of this extract against DENV-2 replication were determined using the quantitative Real Time-Polymerase Chain Reaction (qRT-PCR). Findings from the study suggested that the Streptomyces KSF 103 ME showed maximum inhibitory properties toward the virus during the virus entry stage at concentrations of more than 12.5 µg/mL. Minimal antiviral activities were observed at other virus replication stages; adsorption (42% reduction at 50 µg/mL), post-adsorption (67.6% reduction at 50 µg/mL), prophylactic treatment (68.4% and 87.7% reductions at 50 µg/mL and 25 µg/mL, respectively), and direct virucidal assay (48% and 56.8% reductions at 50 µg/mL and 25 µg/mL, respectively). The Streptomyces KSF 103 ME inhibited dengue virus replication with a 50% inhibitory concentration (IC50) value of 20.3 µg/mL and an International System of Units (SI) value of 38.9. The Streptomyces KSF 103 ME showed potent antiviral properties against dengue virus (DENV) during the entry stage. Further studies will be needed to deduce the antiviral mechanisms of the Streptomyces KSF 103 ME against DENV.

An Orthogonal Dynamic Covalent Chemistry Tool for Ring-Opening Polymerization of Cyclic Oligochalcogenides on Detachable Helical Peptide Templates

A model system is introduced as a general tool to elaborate on orthogonal templation of dynamic covalent ring-opening polymerization (ODC-TROP). The tool consists of 310 helical peptides as unprecedented templates and semicarbazones as orthogonal dynamic covalent linkers. With difficult-to-control 1,2-dithiolanes, ODC-TROP on the level of short model oligomers occurs with high templation efficiency, increasing and diminishing upon helix stabilization and denaturation, respectively. Further, an anti-templated conjugate with mispositioned monomers gave reduced templation upon helix twisting. Even with the "unpolymerizable" 1,2-diselenolanes, initial studies already afford mild templation efficiency. These proof-of-principle results promise that the here introduced tool, recyclable and enabling late-stage side chain modification, will be useful to realize ODC-TROP of intractable or unknown cyclic dynamic covalent monomers for dynamer materials as well as cellular uptake and signaling applications.

Total Synthesis of Mycinamicin IV as Integral Part of a Collective Approach to Macrolide Antibiotics

The total synthesis of the 16-membered macrolide mycinamicin IV is outlined, which complements our previously disclosed, largely catalysis-based route to the aglycone. This work must also be seen in the context of our recent conquest of aldgamycin N, a related antibiotic featuring a similar core but a distinctly different functionalization pattern. Taken together, these projects prove that the underlying blueprint is integrative and hence qualifies for a collective approach to this prominent class of natural products. In both cases, the final glycosylation phase mandated close attention and was accomplished only after robust de novo syntheses of the (di)deoxy sugars of the desosamine, chalcose, mycinose and aldgarose types had been established. Systematic screening of the glycosidation promoter was also critically important for success.

Novel Galactosyl Moiety-Conjugated Furylchalcones Synthesized Facilely Display Significant Regulatory Effect on Plant Growth

The expansion of weed infestation has increased the demand on new herbicides. A series of novel galactosyl moiety-conjugated furylchalcones was facilely synthesized in which the furyl group (A ring) was combined with the substituted benzene group (B ring), and a galactosyl moiety was introduced. All these galactosyl furylchalcones were predicted to be phloem-mobile. Most of the galactosyl furylchalcones significantly promoted early seedling growth of sorghum and barnyardgrass under dark conditions, but all of them revealed considerable anti-growth ability on illuminated pot plants; especially, 1-(3'-(4″-O-β-d-galactopyranosyl)furyl)-3-(4″-nitrophenyl)-2-en-1-one (B₁₁) had a better herbicidal activity against rapeseed and Chinese amaranth than haloxyfop-R-methyl. The median efficient concentrations (EC₅₀) of compound B₁₁ against cucumber and wheat were 9.55 and 26.97 mg/L, respectively, also showing a stronger suppressing capacity than 2,4-D. Molecular docking with phosphoenolpyruvate carboxylase protein showed a stable binding conformation in which the galactosyl group interacted with LYS363 and GLU369, the furan ring and carbonyl bound with ARG184, and the crosslink of the nitro group with GLU240 formed a salt bridge. The results demonstrate that galactosyl furylchalcones possess the great potential as new herbicides for weed management, and further evaluations on more weeds are required for practical application.

Total synthesis, isolation, surfactant properties, and biological evaluation of ananatosides and related macrodilactone-containing rhamnolipids

Rhamnolipids are a specific class of microbial surfactants, which hold great biotechnological and therapeutic potential. However, their exploitation at the industrial level is hampered because they are mainly produced by the opportunistic pathogen Pseudomonas aeruginosa. The non-human pathogenic bacterium Pantoea ananatis is an alternative producer of rhamnolipid-like metabolites containing glucose instead of rhamnose residues. Herein, we present the isolation, structural characterization, and total synthesis of ananatoside A, a 15-membered macrodilactone-containing glucolipid, and ananatoside B, its open-chain congener, from organic extracts of P. ananatis. Ananatoside A was synthesized through three alternative pathways involving either an intramolecular glycosylation, a chemical macrolactonization or a direct enzymatic transformation from ananatoside B. A series of diasteroisomerically pure (1→2), (1→3), and (1→4)-macrolactonized rhamnolipids were also synthesized through intramolecular glycosylation and their anomeric configurations as well as ring conformations were solved using molecular modeling in tandem with NMR studies. We show that ananatoside B is a more potent surfactant than its macrolide counterpart. We present evidence that macrolactonization of rhamnolipids enhances their cytotoxic and hemolytic potential, pointing towards a mechanism involving the formation of pores into the lipidic cell membrane. Lastly, we demonstrate that ananatoside A and ananatoside B as well as synthetic macrolactonized rhamnolipids can be perceived by the plant immune system, and that this sensing is more pronounced for a macrolide featuring a rhamnose moiety in its native 1 C 4 conformation. Altogether our results suggest that macrolactonization of glycolipids can dramatically interfere with their surfactant properties and biological activity.

Scalable De Novo Synthesis of Aldgarose and Total Synthesis of Aldgamycin N

Since the accompanying study had shown that the introduction of the eponymous aldgarose sugar to the C5-OH group of the macrocyclic aglycone of aldgamycin N is most difficult, if not even impossible, the synthesis route was revised and the glycosidation performed at an earlier stage. To mitigate the "cost" of this strategic amendment, a practical and scalable de novo synthesis of this branched octose was developed. The glycoside formation required mild conditions; it commenced with the reaction of the aglycone with the trichloroacetimidate donor to give a transient orthoester, which slowly rearranged to the desired aldgaropyranoside. The presence of the polar peripheral groups in the product did not impede the selective late-stage functionalization of the macrolide ring itself: the contained propargylic alcohol entity was readily transformed into the characteristic acyloin motif of the target by a ruthenium-catalyzed trans-hydrostannation followed by a modified Chan-Lam-type coupling.

An Overview on Glyco-Macrocycles: Potential New Lead and their Future in Medicinal Chemistry

Macrocycles cover a small segment of molecules with a vast range of biological activity in the chemotherapeutic world. Primarily, the natural sources derived from macrocyclic drug candidates with a wide range of biological activities are known. Further evolutions of the medicinal chemistry towards macrocycle-based chemotherapeutics involve the functionalization of the natural product by hemisynthesis. More recently, macrocycles based on carbohydrates have evolved a considerable interest among the medicinal chemists worldwide. Carbohydrates provide an ideal scaffold to generate chiral macrocycles with well-defined pharmacophores in a decorated fashion to achieve the desired biological activity. We have given an overview on carbohydrate-derived macrocycle involving their synthesis in drug design and discovery and potential role in medicinal chemistry.

Chagosensine: A Riddle Wrapped in a Mystery Inside an Enigma

The marine macrolide chagosensine is supposedly distinguished by a (Z,Z)-configured 1,3-chlorodiene contained within a highly strained 16-membered lactone ring, which also incorporates two trans-2,5-disubstituted tetrahydrofuran (THF) rings; this array is unique. After our initial synthesis campaign had shown that the originally proposed structure is incorrect, the published data set was critically revisited to identify potential mis-assignments. The "northern" THF ring and the anti-configured diol in the "southern" sector both seemed to be sites of concern, thus making it plausible that a panel of eight diastereomeric chagosensine-like compounds would allow the puzzle to be solved. To meet the challenge, the preparation of the required building blocks was optimized, and a convergent strategy for their assembly was developed. A key role was played by the cobalt-catalyzed oxidative cyclization of alken-5-ol derivatives ("Mukaiyama cyclization"), which is shown to be exquisitely chemoselective for terminal alkenes, leaving even terminal alkynes (and other sites of unsaturation) untouched. Likewise, a palladium-catalyzed alkyne alkoxycarbonylation reaction with formation of an α-methylene-γ-lactone proved instrumental, which had not found application in natural product synthesis before. Further enabling steps were a nickel-catalyzed "Tamaru-type" homocrotylation, stereodivergent aldehyde homologations, radical hydroindation, and palladium-catalyzed alkyne-1,2-bis-stannation. The different building blocks were assembled in a serial fashion to give the idiosyncratic chlorodienes by an unprecedented site-selective Stille coupling followed by copper-mediated tin/chlorine exchange. The macrolactones were closed under forcing Yamaguchi conditions, and the resulting products were elaborated into the targeted compound library. Yet, only one of the eight diastereomers turned out to be stable in the solvent mixture that had been used to analyze the natural product; all other isomers were prone to ring opening and/or ring expansion. In addition to this stability issue, our self-consistent data set suggests that chagosensine has almost certainly little to do with the structure originally proposed by the isolation team.

Making natural products from renewable feedstocks: back to the roots?

This review highlights the utilization of biomass-derived building blocks in the total synthesis of natural products.

Synthesis and Preliminary Evaluation of Biological Activity of Glycoconjugates Analogues of Acyclic Uridine Derivatives

Herein we present the methodology for obtaining glycosyltransferase inhibitors, analogues of natural enzyme substrates of donor-type: UDP-glucose and UDP-galactose. The synthesis concerned glycoconjugates, nucleoside analogues containing an acyclic ribose mimetic linked to a uracil moiety in their structure. The biological activity of the synthesised compounds was determined on the basis of their ability to inhibit the model enzyme action of β-1,4-galactosyltransferase from bovine milk. The obtained results allowed to expand and supplement the existing library of synthetic compounds that are able to regulate the biological activity of enzymes from the GT class.

Readily Available Highly Active [Ti]-Adamantyl-BINOL Catalysts for the Enantioselective Alkylation of Aldehydes

A series of enantiopure (R)-adamantyl 1,1'-binaphthalene-2,2'-diols were successfully synthesized in a straightforward one-step reaction from commercial products with excellent overall yields. The activity of chiral titanium catalyst derived from (R)-3,6,6'-tri(adamantan-1-yl)-[1,1'-binaphthalene]-2,2'-diol ligand, (R)-(2)-Ti, in the enantioselective asymmetric alkylation of aldehydes is significantly enhanced (up to 98% yield and 99% ee).

An eco-compatible strategy for the diversity-oriented synthesis of macrocycles exploiting carbohydrate-derived building blocks

An efficient, eco-compatible diversity-oriented synthesis (DOS) approach for the generation of library of sugar embedded macrocyclic compounds with various ring size containing 1,2,3-triazole has been developed. This concise strategy involves the iterative use of readily available sugar-derived alkyne/azide-alkene building blocks coupled through copper catalyzed azide-alkyne cycloaddition (CuAAC) reaction followed by pairing of the linear cyclo-adduct using greener reaction conditions. The eco-compatibility, mild reaction conditions, greener solvents, easy purification and avoidance of hazards and toxic solvents are advantages of this protocol to access this important structural class. The diversity of the macrocycles synthesized (in total we have synthesized 13 macrocycles) using a set of standard reaction protocols demonstrate the potential of the new eco-compatible approach for the macrocyclic library generation.

Advances in the synthesis of glycosidic macrolides: clavosolides A-D and cyanolide A

Covering: 2005 to 2016Clavosolides A-D and cyanolide A are glycosidic macrolides and represent a new family of marine natural products. They possess a number of unusual structural features and have attracted considerable interest from the synthetic community. This review presents a comprehensive survey of all aspects of the clavosolides A-D and cyanolide A. Specific topics include isolation, structure determination, biological activity, and synthetic approaches.

Octanoylation of early intermediates of mycobacterial methylglucose lipopolysaccharides

Mycobacteria synthesize unique intracellular methylglucose lipopolysaccharides (MGLP) proposed to modulate fatty acid metabolism. In addition to the partial esterification of glucose or methylglucose units with short-chain fatty acids, octanoate was invariably detected on the MGLP reducing end. We have identified a novel sugar octanoyltransferase (OctT) that efficiently transfers octanoate to glucosylglycerate (GG) and diglucosylglycerate (DGG), the earliest intermediates in MGLP biosynthesis. Enzymatic studies, synthetic chemistry, NMR spectroscopy and mass spectrometry approaches suggest that, in contrast to the prevailing consensus, octanoate is not esterified to the primary hydroxyl group of glycerate but instead to the C6 OH of the second glucose in DGG. These observations raise important new questions about the MGLP reducing end architecture and about subsequent biosynthetic steps. Functional characterization of this unique octanoyltransferase, whose gene has been proposed to be essential for M. tuberculosis growth, adds new insights into a vital mycobacterial pathway, which may inspire new drug discovery strategies.

Racemic total synthesis of dactyloidin and demethyldactyloidin through the dl-proline-catalyzed Knoevenagel condensation/[4 + 2] cycloaddition cascade

An efficient approach towards the first racemic total synthesis of dactyloidin (2) and demethyldactyloidin (3) is described.

A total synthesis of spirastrellolide A methyl ester

A concise total synthesis of spirastrellolide A methyl ester (1 a, R(1) =Me) as the parent compound of a series of highly cytotoxic marine macrolides is disclosed, which exploits and expands the flexibility of a synthesis plan previously developed by our group en route to the sister compound spirastrellolide F methyl ester (6 a, R(1) =Me). Key to success was the masking of the signature Δ(15,16) -bond of 1 a as a C16-carbonyl group until after the stereogenic center at C24 had been properly set by a highly selective hydrogenation of the C24 exo-methylene precursor 66. Conformational control over the macrocyclic frame allowed the proper stereochemical course to be dialed into this reduction process. The elaboration of the C16 ketone to the C15-C16 double bond was accomplished by a chemoselective alkenyl triflate formation followed by a palladium-catalyzed hydride delivery. The role of the ketone at C16 as a strategic design element is also evident up-stream of the key intermediate 66, the assembly of which hinged upon the addition of the polyfunctionalized dithiane 37 to the similarly elaborate aldehyde fragment 46. Other crucial steps of the total synthesis were an alkyl-Suzuki coupling and a Yamaguchi lactonization that allowed the Northern and the Southern sector of the target to be stitched together and the macrocyclic perimeter to be forged. The lateral chain comprising the remote C46 stereocenter was finally attached to the core region by a modified Julia-Kocienski olefination. The preparation of the individual building blocks led to some methodological spin-offs, amongst which the improved procedure for the N-O-bond cleavage of isoxazolines by zero-valent molybdenum and the ozonolysis of a double bond in the presence of other oxidation-prone functionality are most noteworthy. Preliminary biological data suggest that the entire carbon framework, that is the macrocyclic core plus the lateral chain, might be necessary for high cytotoxicity.

Total synthesis of (+)-SCH 351448

A convergent synthesis of (+)-SCH 351448 (1), a monosodium salt of a C(2)-symmetric macrodiolide, is described. Our approach is based on a [4 + 2] annulation with a chiral allyl silane (anti-5c) to assemble the pyran subunits. Homodimerization was carried out in a stepwise fashion; initial esterification at C29' followed by macrocyclization at C29 afforded the desired macrodiolide.