Synthetic routes toward pentasaccharide repeating unit corresponding to the O-antigen of Escherichia coli O181

An efficient synthetic strategy has been developed for the synthesis of the pentasaccharide repeating unit corresponding to the O-antigen of Escherichia coli O181. A one-pot, two step iterative glycosylation and [2?+?3] block glycosylation strategy have been adopted for the construction of the pentasaccharide derivative 2, which was then transformed into target compound 1 after a series of functional group transformations. Here H₂SO₄-silica has been used successfully as a promoter for all glycosylation reaction. The stereoselective outcomes of all glycosylation reactions were very good. The 2-acetamido-2,6-dideoxy-l-glucose (l-QuipNAc) building block was obtained from known carbohydrate l-rhamnose precursors.     

Dehydrative Glycosylation Enabled by a Comproportionation Reaction of 2‐Aryl‐1,3‐dithiane 1‐Oxide†

Summaryof main observation and conclusion A new dehydrativeglycosylation reaction has been established by capitalizing on the compropor-tionation reaction of 2-aryl-1,3-dithiane 1-oxides promoted by triflic anhydride(Tf2O).By wedding the high potency of thiophilic promoter system with the step efficiency of dehydrative glycosylation,this reagentunderwent facile intermolecular oxothio acetalization with C1-hemiacetal donor to install a temporary leaving group,rendering a transient electrophilic center at the remote site to the anomeric position.The sulfenyltriflate tethered at the ter-minus concomitantly activated the sulfide intramolecularly to afford the oxocarbenium ion,thereby facilitating the title glycosylation.Aside from accom-modating broad range functional groups and inactive hemiacetal substrates,the present activation protocol also proved expedient for 1,3-diol protection.Most importantly,this method further provided a fresh perspective for the application of sulfur chemistry to carbohydrate chemistry.     

Oligosaccharide Synthesis via Electrophile-Induced Activation of Glycosyl-N-Allylcarbamates 1

Abstract

Glycosyl-N-allyl carbamates, obtained by reaction of anomerically unprotected saccharides with allyl isocyanate, can be activated by an electrophile-induced cyclisation and reacted with glycosyl acceptors to form the corresponding oligosaccharides By this method the mucin core 2 trisaccharide² has successfully been synthesized. Due to the mild glycosylation conditions even 1-O-acetyl protected glycosyl acceptors can be used. This was demonstrated in the synthesis of a 1,6-linked glucosyl trisaccharide whereby a reptitious glycosylation strategy could be applied.

     

Stereoselective direct glycosylation with anomeric hydroxy sugars by activation with phthalic anhydride and trifluoromethanesulfonic anhydride involving glycosyl phthalate intermediates

An efficient direct one-pot glycosylation method with anomeric hydroxy sugars as glycosyl donors employing phthalic anhydride and triflic anhydride as activating agents has been developed. Thus, highly stereoselective beta-mannopyranosylations were achieved by the reaction of 2,3-di-O-benzyl-4,6-O-benzylidene-D-mannopyranose (2) with phthalic anhydride in the presence of DBU at room temperature followed by sequential addition of DTBMP and Tf2O and glycosyl acceptors to the reaction mixture at -78 degrees C in one-pot. Stereoselective alpha-glucopyranosylations with 2,3-di-O-benzyl-4,6-O-benzylidene-D-glucopyranose (25) and other glycosylations with glucopyranoses and mannopyranoses having tetra-O-benzyl- and tetra-O-benzoyl protecting groups were also possible by utilizing the present one-pot glycosylation protocol. The possible mechanism for the beta-mannosylation with 2 was proposed based on the NMR study, in which alpha-mannosyl phthalate 55alpha and alpha-mannosyl triflate 59 were detected as intermediates. The versatility and efficiency of the present glycosylation methodology, especially those of the beta-mannopyranosylation protocol, were readily demonstrated by the efficient synthesis of protected beta-(1-->4)-D-mannotriose 62 and beta-(1-->4)-D-mannotetraose 67 with perfect beta-stereoselectivity.     

Microwave-accelerated Fischer glycosylation

Fischer glycosylation has been used for decades for the synthesis of simple alkyl and aryl glycosides from free sugars. The reaction proceeds under reflux in the presence of catalytic acid with the alcohol as solvent. The main deficiency of this reaction is the long reaction time required. In this study microwave heating has been utilised for the Fischer glycosylation reaction of N-acetyl-d-glucosamine, N-acetyl-d-galactosamine, d-glucose, d-galactose and d-mannose with a variety of alcohols (methanol, ethanol, benzyl alcohol and allyl alcohol). Remarkable acceleration of the glycosylation reactions (minutes compared to hours) over conventional reflux heating was observed with good yields and production of the α-glycoside as the dominant product.     

Towards the preparation of 2″-deoxy-2″-fluoro-adenophostin A. Study of the glycosylation reaction

The synthesis of 2″-deoxy-2″-fluoro-adenophostin A framework starting from tri-O-acetylglucal and adenosine is described. The key steps are the formation of the 2-deoxy-2-fluoroglycosyl donor by electrophilic fluorination of tri-O-acetylglucal and the stereoselective glycosylation of a suitable adenosine derivative. The glycosylation reaction was optimized affording the desired 2″-deoxy-2″-fluoroglycoside with excellent α-stereoselectivity and in good yields, taking into account that glycosylations using nucleosides as glycosyl acceptors do not usually give excellent results. In that sense, an improvement of the glycosylation step with respect to that of the reported adenophostin synthesis, using adenosine derivatives as glycosyl donors, has been made.     

Concise synthesis of a pentasaccharide repeating unit corresponding to the O-antigen of Escherichia coli O102

An efficient synthetic strategy has been developed for the synthesis of a pentasaccharide repeating unit of the O-antigen of Escherichia coli O102 strain. The target pentasaccharide 1 has been synthesized using a [2+3] block glycosylation strategy. All glycosylation steps are highly stereoselective and high yielding. Concept of armed-disarmed and orthogonal glycosylation strategies has been applied during the synthesis. The target compound has been synthesized using the minimum number of steps.     

Glycosylative transcarbamylation: efficient transformation of tert-butyl carbamates to novel glycoconjugates

Reaction of a variety of tert-butyl carbamates under glycosylation conditions gives rise to anomeric 2-deoxy-2-amino sugar carbamates in a good to excellent yields. The reaction exhibits good tolerance to several common protecting groups, and has been used to generate unnatural glycopeptide building blocks.     

Efficient synthesis of the functional central core lactides, a constituent of antibiotic fattiviracins

An efficient convergent synthetic method has been developed for preparation of various stereoisomers and derivatives of fattiviracins via a common lactide. The synthetic route comprises seco acids prepared by the β-selective glycosylation of chiral 3-hydroxy-4-pentanoate obtained by enzymatic kinetic resolution. The regioselective protection of four individual hydroxy groups was achieved via the 4,6-O-benzylidenation of the glucose moiety from its TMS ethers. The dimeric cyclization of the seco acids under control of reaction concentration afforded the desired lactide without using KH. Our convergent synthetic method was successfully applied to direct installation of side chains to the lactide by cross metathesis to synthesize fattiviracin derivatives. We achieved improvements to the reported method with respect to: (1) synthesis of a convergent synthetic intermediate; (2) stereoselectivity in glycosylation; and (3) establishment of a low cost route suitable for large scale synthesis.     

Regio- and Stereoselective Synthesis of Thioglycosides from 4,5-Diphenyl- and 3,4,5-Triphenylimidazole-2-thione

Six new thioglycosides incorporating the 4,5-diphenyl- and 3,4,5-triphenylimidazole moiety have been successfully synthesized under both conventional and microwave conditions by reaction of the corresponding thiones with aceto-bromosugars in presence of triethylamine as base. Attempted preparation of the bis(glycosyl) derivatives from 4,5-diphenylimidazole-2-thione in the presence of different bases was unsuccessful. Evaluation of the glycosylthioimidazoles as disarmed donors has been investigated using different promoters; NBS/TMSOTf has been found to be an effective promoter for the activation of the anomeric center towards glycosylation reaction.     

7-Functionalized 7-deazapurine ribonucleosides related to 2-aminoadenosine, guanosine, and xanthosine: glycosylation of pyrrolo[2,3-d]pyrimidines with 1-O-acetyl-2,3,5-tri-O-benzoyl-D-ribofuranose

[reaction: see text] The Silyl-Hilbert-Johnson reaction as well as the nucleobase-anion glycosylation of a series of 7-deazapurines has been investigated, and the 7-functionalized 7-deazapurine ribonucleosides were prepared. Glycosylation of the 7-halogenated 6-chloro-2-pivaloylamino-7-deazapurines 9b-d with 1-O-acetyl-2,3,5-tri-O-benzoyl-D-ribofuranose (5) gave the beta-D-nucleosides 11b-d (73-75% yield), which were transformed to a number of novel 7-halogenated 7-deazapurine ribonucleosides (2b-d, 3b-d, and 4b-d) related to guanosine, 2-aminoadenosine, and xanthosine. 7-Alkynyl derivatives (2e-i, 3e-h, or 4g) have been prepared from the corresponding 7-iodonucleosides 2d, 3d, or 4d employing the palladium-catalyzed Sonogashira cross-coupling reaction. The 7-halogenated 2-amino-7-deazapurine ribonucleosides with a reactive 6-chloro substituent (18b-d) were synthesized in an alternative way using nucleobase-anion glycosylation performed on the 7-halogenated 2-amino-6-chloro-7-deazapurines 13b-d with 5-O-[(1,1-dimethylethyl)dimethylsilyl]-2,3-O-(1-methylethylidene)-alpha-D-ribofuranosyl chloride (17). Compounds 18b-d have been converted to the nucleosides 19b-d carrying reactive substituents in the pyrimidine moiety. Conformational analysis of selected nucleosides on the basis of proton coupling constants and using the program PSEUROT showed that these ribonucleosides exist in a preferred S conformation in solution.     

Isoquinoline‐1‐Carboxylate as a Traceless Leaving Group for Chelation‐Assisted Glycosylation under Mild and Neutral Reaction Conditions

Glycosyl isoquinoline‐1‐carboxylate was developed as a novel benchtop stable and readily available glycosyl donor. The glycosylation reaction was promoted by the inexpensive Cu(OTf)₂ salt under mild reaction conditions. The copper isoquinoline‐1‐carboxylate salt was precipitated from the solution and thus rendered a traceless leaving group. Surprisingly, the proton from the acceptor was absorbed by the precipitated metal complex and the reaction mixture remained at neutral pH. The copper‐promoted glycosylation was also proven to be completely orthogonal to the gold‐promoted glycosylation, and an iterative synthesis of oligosaccharides from benchtop stable anomeric ester building blocks becomes possible under mild reaction conditions.     

Synthesis and glycosylation of a series of 6-mono-, di-, and trifluoro S-phenyl 2,3,4-tri-O-benzyl-thiorhamnopyranosides. Effect of the fluorine substituents on glycosylation stereoselectivity

A series of 6-mono-, di-, and trifluoro analogs of S-phenyl 2,3,4-tri-O-benzyl-D- or L-thiorhamnopyranoside has been synthesized and used as donors in glycosylation reactions, with activation by the 1-benzenesulfinyl piperidine/triflic anhydride system. The stereochemical outcome of the glycosylation reactions was found to depend on the electron-withdrawing capability of the disarming substituent at the 6-position, i.e., on the number of fluorine atoms present. The results are explained with regard to the increased stability of the glycosyl triflates, shown to be intermediates in the reaction by low-temperature 1H NMR experiments, with increased fluorine content.     

Activation de la Ponction Hydroxyle Anomere via les Sels d'Iminium. O-Glycosylation des Tri-O-benzyl-2,3,5-D-arabino et -ribotorannoses

Abstract

The glycosylation reaction based upon anomeric free hydroxyl group activation by its transformation into an alkcxyiminium salt by the action of the N,N-dimethylformamide/phosgene complex has been extended to the pentofuranose series. The mechanism of this glycosylation is discussed. Nine glycosides have been prepared.     

A Highly Efficient Glycosidation of Glycosyl Chlorides by Using Cooperative Silver(I) Oxide–Triflic Acid Catalysis

Following our discovery that silver(I) oxide-promoted glycosylation with glycosyl bromides can be greatly accelerated in the presence of catalytic TMSOTf or TfOH, we report herein a new discovery that glycosyl chlorides are even more effective glycosyl donors under these reaction conditions. The developed reaction conditions work well with a variety of glycosyl chlorides. Both benzoylated and benzylated chlorides have been successfully glycosidated, and these reaction conditions proved to be effective in coupling substrates containing nitrogen and sulfur atoms. Another convenient feature of this glycosylation is that the progress of the reaction can be monitored visually; its completion can be judged by the disappearance of the characteristic dark color of Ag₂O.     

Efficient Synthesis of the Disialylated Tetrasaccharide Motif in N‐Glycans through an Amide‐Protection Strategy

A disialylated tetrasaccharide, Neu5Ac(α2,3)Gal(β1,3)[Neu5Ac(α2,6)]GlcNAc ( 1 ), which is found at the termini of some N‐glycans, has been synthesized. Compound 1 was obtained through an α‐sialylation reaction between a sialic acid donor and a trisaccharide that was synthesized from the glycosylation of a sialylated disaccharide with a glucosaminyl donor. This synthetic route enabled the synthesis of the as‐described disialylated structure. A more‐convergent route based on the glycosylation of two sialylated disaccharides was also established to scale up the synthesis. Protection of the amide groups in the sialic acid residues significantly increased the yield of the glycosylation reaction between the two sialylated disaccharides, thus suggesting that the presence of hydrogen bonds on the sialic acid residues diminished their reactivity.     

One-pot synthesis of β-2,6-dideoxyglycosides via glycosyl iodide intermediates

An efficient and convenient approach for stereoselective synthesis of β-linked 2,6-dideoxyglycosides has been developed through one-pot glycosylation strategy using glycosyl iodide intermediates. The glycosidation reaction, promoted by easily available silver nitrate, provided the corresponding 2,6-dideoxyglycosides and oligosaccharides with preponderant β-configuration (β/α = 2.3:1 to 11:1) and good to excellent yields (52–84%).     

Chemoenzymatic synthesis of glycosyl-deoxyinositol derivatives. First example of a fagopyritol β-analogue containing an aminoinositol unit

The first synthesis of two fagopyritol β-analogues (β-d-galactopyranosyl-(1′→1)-conduramine F-4 and β-d-galactopyranosyl-(1′→3)-4-aminodeoxy-l-chiro-inositol) has been accomplished by a chemoenzymatic route in satisfactory yields. The key step of the synthesis is the TMSOTf-promoted glycosylation reaction of a deoxyconduritol derivative. The methodology is amenable to scale-up and expandable to the preparation of other pseudofagopyritols.     

First synthesis of a pentasaccharide repeating unit of the O-antigenic polysaccharide from enterohaemorrhagic Escherichia coli O48:H21

A concise synthesis of a pentasaccharide as its 4-methoxyphenyl glycoside, found in the O-antigenic polysaccharide of enterohaemorrhagic Escherichia coli O48:H21 has been achieved for the first time in excellent yield. Most of the intermediate steps are high yielding and the stereooutcome of each glycosylation step was excellent. Stereoselective glycosylation and removal of the 4-methoxybenzyl group were achieved in one-pot by tuning the reaction conditions. A late-stage TEMPO-mediated oxidation strategy has been adopted for the oxidation of a primary hydroxyl group to carboxylic acid.     

Asymmetric synthesis of novel thioiso dideoxynucleosides with exocyclic methylene as potential antiviral agents

Novel thioiso pyrimidine and purine nucleosides substituted with exocyclic methylene have been synthesized, starting from D-xylose. The glycosyl donor 14 was synthesized from D-xylose, using cyclization of dimesylate 10 with sodium sulfide as a key step. Cyclization proceeded in pure S(N)2 reaction without going through S(N)1 reaction in the presence of an allylic functional group at low reaction temperature (0 degrees C) in polar solvent (DMF), affording compound 12 as a major product. At higher temperatures, S(N)2' product 11 was almost exclusively obtained as a major product. On the other hand, glycosylation of 14 with 6-chloropurine under Mitsunobu conditions afforded the desired S(N)2 product 26, while palladium-catalyzed glycosylation resulted in the sole formation of S(N)2' product 34.