Purification and Preparation of Moenomycin A from Fermentation Broth by Multidimensional Chromatography

Based on various adsorption characteristics of resins, a novel method for purifying and preparing moenomycin A from fermentation broth was established by combining different chromatographic modes into a three-step preparative chromatography process. Fermentation broth of moenomycins was firstly prepurified by macroporous adsorbent XAD7HP to remove most strong polar impurities, then further purified by anion exchange resin FPA98Cl, and finally refined moenomycin A was obtained by use of semi-preparative reversed-phase chromatographic column packed with Chromtorex C8 silica gel. As the main indicators, purity and yield of moenomycin A were examined in order to optimise the chromatographic process for each step. Under optimized chromatographic conditions, the purity and total yield of moenomycin A were 95.0 and 22.2 %, and the biological potency of moenomycin A was 2232 U mg⁻¹, significantly higher than 1395 U mg⁻¹, which is the potency of the standard from Agriculture Ministry of China. Three-step preparative chromatographic mode could gradually and effectively remove impurities. The present method is practical, easy to be operated with less solvent consumption, and provides a new idea for the preparation of moenomycin A with high purity.

     

Modular synthesis of diphospholipid oligosaccharide fragments of the bacterial cell wall and their use to study the mechanism of moenomycin and other antibiotics

We present a flexible, modular route to GlcNAc-MurNAc-oligosaccharides that can be readily converted into peptidoglycan (PG) fragments to serve as reagents for the study of bacterial enzymes that are targets for antibiotics. Demonstrating the utility of these synthetic PG substrates, we show that the tetrasaccharide substrate lipid IV (3), but not the disaccharide substrate lipid II (2), significantly increases the concentration of moenomycin A required to inhibit a prototypical PG-glycosyltransferase (PGT). These results imply that lipid IV and moenomycin A bind to the same site on the enzyme. We also show the moenomycin A inhibits the formation of elongated polysaccharide product but does not affect length distribution. We conclude that moenomycin A blocks PG-strand initiation rather than elongation or chain termination. Synthetic access to diphospholipid oligosaccharides will enable further studies of bacterial cell wall synthesis with the long-term goal of identifying novel antibiotics.     

Beta-selective glycosylations with masked D-mycosamine precursors

[reaction: see text]. Both an intramolecular aglycon delivery (IAD) method and an intermolecular S(N)2 displacement method were examined for beta-selective glycosylations of cholesterol with D-mycosamine. An anomeric sulfoxide, sulfide, selenide, and fluoride were all successfully used as glycosyl donors in IAD reactions. The alpha-bromo ketone 19 was synthesized from protected mycosamine and employed in an intermolecular S(N)2 glycosylation reaction. Both routes were successful for the model alcohol, cholesterol.     

Characterization of moenomycin antibiotic complex by multistage MALDI-IT/RTOF-MS and ESI-IT-MS

Flavomycin is a commercially available antimicrobial growth promoter and an authorized additive for feeding stuffs in the EU and in the USA. As most antibiotically active products biosynthesized by microorganisms, it contains not only a single active compound but is a complex mixture of structurally closely related substances. Multistage matrix-assisted laser desorption/ionization-ion trap/reflectron time-of-flight mass spectrometry (MALDI-IT/RTOF-MS) and liquid chromatography-electrospray ionization-ion trap-mass spectrometry (LC-ESI-IT-MS) were utilized for a detailed analysis of the constituents of the Flavomycin complex based on low-energy collision induced dissociation (CID). An optimal sample preparation for negative ion vacuum MALDI-MS for this compound class was developed. The MALDI-IT/RTOF-MS2 and -MS3 analysis starting with the precursor [M - H]- ions of these interesting phosphoglycolipids, named moenomycins, yielded a large variety of product ions that facilitated the structural characterization of this class of compounds. Based on the derived CID fragmentation pathway of the five known major constituents, namely moenomycin A, moenomycin A12, moenomycin C4, moenomycin C3. and moenomycin C1, four not yet described moenomycin-type constituents could be characterized. They were assigned as 4F-demethyl-6E-O-de-beta-D-glucopyranosyl-moenomycin A, 6B-N-de(2-hydroxy-5-oxo-1-cyclopenten-1-yl)-moenomycin A, 6B-hydroxy-6B-de[N-(2-hydroxy-5-oxo-1-cyclopenten-1-yl)amino]-moenomycin A, and 6C-hydroxy-moenomycin A. In addition, a moenomycin A carrying an oxygen in the moenocinol-group was found, which is most probably a chemical degradation product. These new compounds were verified by LC-ESI-IT-MS.     

On Penicillin‐Binding Protein 1b Affinity‐Labeling Reagents

The synthesis of some 3‐aryl‐3‐(trifluoromethyl)3H‐diazirine and benzophenone‐based photoaffinity labels is reported. The photolabile group is bound to a scaffold that also accommodates functional groups to which an indicator unit (biotin) and the bioactive ligand can be attached orthogonally. To three of the labels, moenomycin was conjugated with the aim to provide tools for the identification of the moenomycin binding site within the transglycosylase domain of the enzyme PBP 1b. Some preliminary photoaffinity‐labeling experiments were carried out.     

Hydrogen Bond Mediated Aglycone Delivery: Synthesis of Linear and Branched α‐Glucans

A Hydrogen bond mediated aglycone delivery (HAD) method was applied to the synthesis of α‐glucans, which are abundant in nature, but as targets represent a notable challenge to chemists. The synthesis of linear oligosaccharide sequences was accomplished in complete stereoselectivity in all glycosylations. The efficacy of HAD may diminish with the increased bulk of the glycosyl acceptor, and may be an important factor for the syntheses of oligomers beyond pentasaccharides. The synthesis of a branched structure proved more challenging, particularly with bulky trisaccharide acceptors.     

4,6‐O‐Benzylidene Directed β‐Mannosylation Without Intermediate Triflate Formation? Comparison of Trichloroacetimidate and DISAL Donors in Microwave‐Promoted Glycosylations under Neutral Conditions

4,6‐Benzylidene‐protected mannosyl donors have emerged as efficient tools for the formation of 1,2‐cis β‐mannosides, which otherwise are difficult to access. Previously studied sulfoxide and trichloroacetimidate mannosyl donors were activated with strong Lewis acids at low temperature and the glycosylations are believed to proceed through intermediate formation of an α‐triflate. This paper describes the synthesis of new benzylidene‐protected glucosyl and mannosyl methyl 3,5‐dinitrosalicylate (DISAL) donors, their application in O‐glycosylations, and comparison with a mannosyl trichloroacetimidate donor. In contrast to previous reports on torsionally “disarmed” donors, these glycosylations were performed in the absence of strong Lewis acids, but in the presence of lithium perchlorate or triflate, using either conventional heating to 40 to 60°C or precise microwave heating to 100 to 150°C. This approach aimed at addressing the question of whether mannosyl triflate intermediates are essential for high β‐selectivity in 4,6‐O‐benzylidene directed mannosylations. We find, again, that precise microwave heating promoted glycosylations under very mild conditions. While a DISAL mannosyl donor gave higher β‐selectivity in the presence of LiOTf than with LiClO₄, the corresponding trichloroacetimidate did not give any β‐selectivity with LiOTf. Thus, under these conditions, the nature of the original leaving group, trichloroacetimidate vs. DISAL, still is important. However, our results point to the possibility of intermediate formation of a mannosyl triflate from a DISAL donor.     

Preparation of vinylglycines by thermolysis of homocysteine sulfoxides

The synthesis and efficacy of preparing Cbz-VG-OMe (1) by thermolysis of alkyl and aryl homocysteine sulfoxides were surveyed. This investigation determined that aryl sulfoxide analogs were more effective for the reaction and that the 2-nitrophenyl analog 10f possessed a unique ability to syn eliminate at temperatures as low as 100 °C. The thermolysis of sulfoxide 10f was additionally discovered to occur under toluene reflux and when sodium acetate was added, Cbz-VG-OMe (1) could be obtained in high purity by simple filtration of the precipitated sulfenic acid byproduct 12. This mild protocol which was also applied in the synthesis of VG dipeptide 13 would have utility in the general synthesis of olefins and alkenes from 2-nitrophenylsulfoxides.     

Single-Step Glycosylations with 13C-Labelled Sulfoxide Donors: A Low-Temperature NMR Cartography of the Distinguishing Mechanistic Intermediates

Glycosyl sulfoxides have gained recognition in the total synthesis of complex oligosaccharides and as model substrates for dissecting the mechanisms involved. Reactions of these donors are usually performed under pre-activation conditions, but an experimentally more convenient single-step protocol has also been reported, whereby activation is performed in the presence of the acceptor alcohol; yet, the nature and prevalence of the reaction intermediates formed in this more complex scenario have comparatively received minimal attention. Herein, a systematic NMR-based study employing both ¹³C-labelled and unlabelled glycosyl sulfoxide donors for the detection and monitoring of marginally populated intermediates is reported. The results conclusively show that glycosyl triflates play a key role in these glycosylations despite the presence of the acceptor alcohol. Importantly, the formation of covalent donor/acceptor sulfonium adducts was identified as the main competing reaction, and thus a non-productive consumption of the acceptor that could limit the reaction yield was revealed.     

Moenuronic acid: synthetic studies and absolute configuration

The absolute configuration of the moenuronic acid moiety of the antibiotic moenomycin A has been established by a partial synthesis starting from D-galactose. The relation between the absolute conformation of α-hydroxy lactones and their chiroptical properties is discussed in detail.     

Studies on the Synthesis of Di‐ and Trisaccharide Analogues of Moenomycin A. Modifications in Unit E and in the Lipid Part

Routes allowing the synthesis of moenomycin analogues with one modified sugar component and with new lipid parts were developed (see 10c, 12c, 16b , and 20b in Schemes 2–4). It is anticipated that such analogues will be useful for studying the mode of action of the moenomycin‐type transglycosylase inhibitors in detail and for preparing analogues with improved pharmacokinetic properties.     

Preparation of tri- and difluoromethylsilanes via an unusual magnesium metal-mediated reductive tri- and difluoromethylation of chlorosilanes using tri- and difluoromethyl sulfides,sulfoxides, and sulfones

A new and efficient method for the preparation of tri- and difluoromethylsilanes using magnesium metal-mediated reductive tri- and difluoromethylation of chlorosilanes is reported using tri- and difluoromethyl sulfides, sulfoxides, and sulfones. The byproduct of the process is diphenyl disulfide. Since phenyl trifluoromethyl sulfone, sulfoxide, and sulfide are readily prepared from trifluoromethane (CF(3)H) and diphenyl disulfide, the method can be considered to be catalytic in diphenyl disulfide for the preparation of (trifluoromethyl)trimethylsilane (TMS-CF(3)) from non-ozone-depleting trifluoromethane.     

Stereoselective Synthesis of the O-antigen of A. baumannii ATCC 17961 Using Long-Range Levulinoyl Group Participation

Herein, we described the first synthesis of the pentasaccharide and decasaccharide of the A. baumannii ATCC 17961 O-antigen for developing a synthetic carbohydrate-based vaccine against A. baumannii infection. The efficient synthesis of the rare sugar 2,3-diacetamido-glucuronate was achieved using our recently introduced organocatalytic glycosylation method. We found, for the first time, that long-range levulinoyl group participation via a hydrogen bond can result in a significantly improved β-selectivity in glycosylations. This solves the stereoselectivity problem of highly branched galactose acceptors. The proposed mechanism was supported by control experiments and DFT computations. Benefiting from the long-range levulinoyl group participation strategy, the pentasaccharide donor and acceptor were obtained via an efficient [2+1+2] one-pot glycosylation method and were used for the target decasaccharide synthesis.     

A new glycosylation method part 3: study of microwave effects at low temperatures to control reaction pathways and reduce byproducts

The efficiency of microwave irradiation at low temperature for glycosylations is described. Although oligosaccharide synthesis usually requires reactive donors for glycosylations, which have leaving groups on the anomer positions, i.e., trichloroacetoimidates, halogenates, thioalkyl glycosides, etc., the suitable donors in our microwave supported synthesis of Lewis X oligosaccharide were very stable acetate derivatives. Regarding glycosylation with a fucosyl acetate donor and a glucosamine acceptor, microwave irradiation with simultaneous cooling improved yields. Moreover, further synthesis to Lewis X derivatives was achieved only with microwave irradiation at low temperatures. Without microwave irradiation, we could only obtain byproducts and none of the designed product at any reaction temperature.     

Total Synthesis and Biological Evaluation of the Glycosylated Macrocyclic Antibiotic Mangrolide A

The macrocyclic antibiotic mangrolide A has been described to exhibit potent activity against a number of clinically important Gram‐negative pathogens. Reported is the first enantioselective total synthesis of mangrolide A and derivatives. Salient features of this synthesis include a highly convergent macrocycle preparation, stereoselective synthesis of the disaccharide moiety, and two β‐selective glycosylations. The synthesis of mangrolide A and its analogues enabled the re‐examination of its activity against bacterial pathogens, and only minimal activity was observed.     

Moenomycin a: New chemistry that allows to attach the antibiotic to reporter groups, solid supports, and proteins

Moenomycin A (18) on reaction with the diazonium salt derived from bifunctional (protected) 15 yields the coupling product 19 which on reduction is converted into the moenomycin thiol derivative 21. Thiol 21 has been used to prepare selectively moenomycin dansyl and biotin adducts 26 and 28, respectively. This work was performed with the aim to use moenomycin as a tool for studies of the transglycosylation step in peptidoglycan biosynthesis.     

Stereoselective assembly of complex oligosaccharides using anomeric sulfonium ions as glycosyl donors

The development of selectively protected monosaccharide building blocks that can reliably be glycosylated with a wide variety of acceptors is expected to make oligosaccharide synthesis a more routine operation. In particular, there is an urgent need for the development of modular building blocks that can readily be converted into glycosyl donors for glycosylations that give reliably high 1,2-cis-anomeric selectivity. We report here that 1,2-oxathiane ethers are stable under acidic, basic, and reductive conditions making it possible to conduct a wide range of protecting group manipulations and install selectively removable protecting groups such as levulinoyl (Lev) ester, fluorenylmethyloxy (Fmoc)- and allyloxy (Alloc)-carbonates, and 2-methyl naphthyl ethers (Nap). The 1,2-oxathiane ethers could easily be converted into bicyclic anomeric sulfonium ions by oxidization to sulfoxides and arylated with 1,3,5-trimethoxybenzene. The resulting sulfonium ions gave high 1,2-cis-anomeric selectivity when glycosylated with a wide variety of glycosyl acceptors including properly protected amino acids, primary and secondary sugar alcohols and partially protected thioglycosides. The selective protected 1,2-oxathianes were successfully employed in the preparation of a branched glucoside derived from a glycogen-like polysaccharide isolated form the fungus Pseudallescheria boydii , which is involved in fungal phagocytosis and activation of innate immune responses. The compound was assembled by a latent-active glycosylation strategy in which an oxathiane was employed as an acceptor in a glycosylation with a sulfoxide donor. The product of such a glycosylation was oxidized to a sulfoxide for a subsequent glycosylation. The use of Nap and Fmoc as temporary protecting groups made it possible to install branching points.     

Convergent Synthesis of a Bisecting N‐Acetylglucosamine (GlcNAc)‐Containing N‐Glycan

The chemical synthesis of a bisecting N‐acetylglucosamine (GlcNAc)‐containing N‐glycan was achieved by a convergent synthetic route through [4+2] and [6+2] glycosylations. This synthetic route reduced the number of reaction steps, although the key glycosylations were challenging in terms of yields and selectivities owing to steric hindrance at the glycosylation site and a lack of neighboring group participation. The yields of these glycosylations were enhanced by stabilizing the oxocarbenium ion intermediate through ether coordination. Glycosyl donor protecting groups were explored in an effort to realize perfect α selectivity by manipulating remote participation. The simultaneous glycosylations of a tetrasaccharide with two disaccharides was investigated to efficiently construct a bisecting GlcNAc‐containing N‐glycan.     

A simple method for the preparation of N-sulfonylsulfilimines from sulfides.

While excellent methods exist for the oxidation of sulfides to sulfoxides R1R2S-->R1R2SO, the azaversion of this atom transfer redox process, i.e., R1R2S-->R1R2S=N-SO2R3, has been less reliable. In sulfilimine synthesis, sulfoxide has been an inevitable byproduct in all cases to date, and the yields of sulfilimine have varied widely. A nearly ideal procedure for the sulfide to sulfonyl sulfilimine transformation is described. Almost quantitative yields are achieved from a diverse set of sulfides and a broad range of the readily available sulfonyl nitrenoid sources known as chloramine salts (R3SO2NClNa), essentially by simply stirring them together in acetonitrile.     

Reconsidering glycosylations at high temperature: precise microwave heating

Current methods for glycosylation of complex alcohols, e.g. with glycosyl trichloroacetimidates, generally occur in the presence of a strong Lewis acid 'promoter', and at sub-ambient temperatures. However, the older literature reports high-temperature glycosylations, especially of phenols. We have described an efficient method for glycosylation of alcohols under neutral conditions, using as anomeric leaving group methyl 3,5-dinitrosalicylate (DISAL). Only a very few reports have described the use of microwaves to promote glycosylations, mainly of simple alcohols. Here we describe fast, high-temperature glycosylations using precise microwave heating in the synthesis of oligosaccharides, with both DISAL and widely used trichloroacetimidate glycosyl donors in the absence of strong Lewis acids. Also, we have applied microwave heating as a general protocol for evaluating new, potential glycosyl donors.