RESUMO
Herein, we report a mild and general protocol for chemoselective deacetylation of mixed acetyl- and benzoyl-protected carbohydrates under mild acidic conditions. The protocol allows quick access to partially protected carbohydrates, which serve as versatile synthetic intermediates during the total synthesis of various mono- and oligosaccharide targets. The applicability of the developed protocol was successfully demonstrated on a range of carbohydrate substrates of various configurations and substitution patterns featuring functionalized aliphatic and aromatic aglycones. The protocol has shown excellent compatibility with the widely used O-anomeric protecting groups, prespacer aglycones, and thioglycoside glycosyl donors.
RESUMO
Optical rotation of aqueous solutions of D-levoglucosan was studied experimentally in the 0.03-4.0 mol L(-1) concentration range and a nonlinear concentration dependence of specific optical rotation (SR) was revealed. Discontinuities observed in the concentration plot of SR (at 0.1, 0.3, 0.5, 1.0, and 2.0 mol L(-1)) are well correlated with those found by static and dynamic light scattering and identify concentration ranges in which different solution domains (supramers) may exist. The average SR experimental value for a D-levoglucosan aqueous solution ([α]D(28) -58.5±8.7 deg dm(-1) cm(-3) g(-1)) was found to be in good agreement with values obtained by theoretical calculation (TD-DFT/GIAO) of SR for 15 different conformers revealed by conformational sampling at the PCM/B3LYP/6-311++G(2d,2p)//B3LYP/6-31+G(d,p) level, which were shown to be strongly affected by the solvation microenvironment (0, 1, 2, and 3 explicit solvent molecules considered) due to local geometrical changes induced in the solute molecule. This exceptionally high sensitivity of SR makes polarimetry a unique method capable of sensing changes in the structure of supramers detected in this study.
RESUMO
A synthesis of 2-(2,2,2-trichloroethoxy)-(3,4,6-tri-O-acetyl-1,2-dideoxy-α-d-galactopyrano)-[2,1-d]-2-oxazoline - a previously unknown 2-alkoxy glyco-[2,1-d]-2-oxazoline derivative with d-galacto configuration was carried out. Glycosylating activity of the obtained galactooxazoline has been studied and it has been shown that in the presence of a weak protic acid, such as sym-collidinium triflate, this substance exhibits properties of a reactive and 1,2-trans-stereoselective glycosyl donor. The homopolymerization reaction of oxazoline derivatives of sugars has been found to proceed under the same conditions, leading to the formation of pseudo-oligosaccharide products. It has been found that this undesirable side reaction could be suppressed by changing the acid catalyst concentration, resulting in the development of efficient methods for the synthesis of glycoside and oligosaccharide derivatives of ß-d-galactosamine using the synthesized 2-(2,2,2-trichloroethoxy)-2-oxazoline glycosyl donor under very mild conditions.
Assuntos
Glicosídeos , Oligossacarídeos , Catálise , Configuração de CarboidratosRESUMO
Glycosyl halides are historically one of the first glycosyl donors used in glycosylation reactions, and interest in glycosylation reactions involving this class of glycosyl donors is currently increasing. New methods for their activation have been proposed and effective syntheses of oligosaccharides with their participation have been developed. At the same time, the possibilities of using these approaches to the synthesis of sialosides are restricted by the limited diversity of known sialyl halides (previously, mainly sialyl chlorides, less often sialyl bromides and sialyl fluorides, with acetyl (Ac) groups at the oxygen atoms and AcNH, Ac2N and N3 groups at C-5 were used). This work describes the synthesis of six new N-acetyl- and N-trifluoroacetyl-sialyl chlorides and bromides with O-chloroacetyl and O-trifluoroacetyl protective groups. Preparation of N,O-trifluoroacetyl protected derivatives was made possible due to development of the synthesis of sialic acid methyl ester pentaol with N-trifluoroacetyl group.
Assuntos
Brometos , Ácido N-Acetilneuramínico , Glicosilação , Oligossacarídeos , ÉsteresRESUMO
Stereocontrolled 1,2-trans-α-arabinofuranosylation using polysilylated mono- and disaccharide glycosyl donors was investigated. A complete α-stereoselectivity of 1,2-trans-arabinofuranosylation was found for Ara-ß-(1 â 2)-Ara disaccharide glycosyl donors containing five triisopropylsilyl (TIPS) groups with arylthiol (1) (as shown in our previous publications) or N-phenyltrifluoroacetimidoyl (2) (this work) leaving groups. Conversely, in case of monosaccharide thioglycosides polysilylated with acyclic silyl groups (TIPS, TBDPS), stereoselectivity of glycosylation was lower (α:ß = 7-8:1), although the desired α-isomer still dominated. Disaccharide glycosyl donor 2 was successfully used in the synthesis of linear α-(1 â 5)-, ß-(1 â 2)-linked hexaarabinofuranoside useful for further preparation of conjugates thereof as antigens valuable for the diagnosis of mycobacterioses.
RESUMO
We discovered an unusual triflic acid-promoted oligomerization of arabinofuranosides during glycosylation of the primary hydroxy group of α-(1 â 5)-linked tetraarabinofuranoside bearing 4-(2-chloroethoxy)phenyl aglycone with α-(1 â 5), ß-(1 â 2)-linked tetraarabinofuranoside containing N-phenyltrifluoroacetimidoyl leaving group, which led to octa-, dodeca- and hexadecaarabinofuranosides. The possible mechanism of triflic acid-promoted oligomerization was proposed. The choice of promoter was found to be a critical factor for the discovered oligomerization of arabinofuranosides. The obtained octa-, dodeca- and hexadecaarabinofuranosides may serve as useful blocks in the synthesis of oligosaccharide fragments of polysaccharides of Mycobacterium tuberculosis.
Assuntos
Arabinose , Mesilatos , Glicosilação , Arabinose/química , Mesilatos/química , Mycobacterium tuberculosis/efeitos dos fármacos , Mycobacterium tuberculosis/química , Configuração de CarboidratosRESUMO
Base-promoted (MeONa in MeOH or imidazole in DMF) isomerization of a series of 3,4,6-tri-O-benzyl-d-gluco- and d-mannopyranose derivatives with triisopropylsilyl (TIPS) substituents was studied. The presence of a bulky TIPS group at O-1 or O-2 was shown to be favorable for the isomerization of benzyl protected d-gluco- and d-mannopyranose derivatives to d-fructofuranose derivatives, in which the bulky silyl group occupies less sterically hindered primary position. The highest yield (33%) of the fructofuranose derivative was achieved when 3,4,6-tri-O-benzyl-2-O-triisopropylsilyl-d-mannopyranose was treated with MeONa in MeON at 50 °C.
Assuntos
Manose , IsomerismoRESUMO
New methods for the synthesis of the title oxazoline 2 from the corresponding 2-deoxy-2-(2,2,2- trichloroethoxycarbonylamino)glucosyl bromide were developed. The target 2-(2,2,2-trichloroethoxy) gluco-[2,1-d]-2-oxazoline 2 can be synthesized under conditions of halide ion catalysis, using triethylamine as a base. The synthesized 2-(2,2,2-trichloroethoxy)-2-oxazoline glycosyl donor was used for stereo-, regio-, and chemoselective glycosylation reactions under extremely mild conditions. The undesirable side reaction of intermolecular aglycone transfer between an ethyl thioglycoside glycosyl acceptor and the 2-(2,2,2-trichloroethoxy)-2-oxazoline glycosyl donor occurred to a relatively small extent. Regio-, and chemoselectivity of the disaccharide synthesis with the oxazoline glycosyl donor depended on the reaction conditions.
Assuntos
Oligossacarídeos , Glicosilação , Espectroscopia de Ressonância MagnéticaRESUMO
The formation of macrocyclic pseudo-tetrasaccharide derivative of d-glucosamine as a result of the acid-catalyzed reaction between 2-methyl- and 2-(2,2,2-trichloroethoxy)-substituted oxazoline derivatives of sugars was discovered. The structure of the obtained product was determined using NMR spectroscopy and mass spectrometry. An explanation of the obtained results based on the mechanism of the reaction of electrophilic polymerization of 2-substituted glyco-[2,1-d]-2-oxazolines and the principle of hard and soft acids and bases (HSAB) was proposed.
Assuntos
Glucosamina/química , Compostos Macrocíclicos/síntese química , Oligossacarídeos/síntese química , Oxazóis/química , Ácidos Sulfônicos/química , Configuração de Carboidratos , Catálise , Glucosamina/análogos & derivados , Compostos Macrocíclicos/química , Oligossacarídeos/químicaRESUMO
Highly regioselective acetylation of primary hydroxy groups in thioglycoside derivatives with gluco- and galacto-configurations was achieved by treatment with aqueous or anhydrous acetic acid (60-100% AcOH) at elevated temperatures (80-118 °C), avoiding complex, costly and time-consuming manipulations with protective groups. Acetylation of both 4,6-O-benzylidene acetals and the corresponding diols as well as the unprotected tetraol with AcOH was shown to lead selectively to formation of 6-O-acetyl derivatives. For example, the treatment of phenyl 1-thio-ß-d-glucopyranoside with anhydrous AcOH at 80 °C for 24 h gave the corresponding 6-O-acetylated derivative in 47% yield (71% based on the reacted starting material) and unreacted starting tetraol in 34% yield, which can easily be recovered by silica gel chromatography and reused in further acetylation.
RESUMO
Efficient procedures for the preparative synthesis of per-O-acyl derivatives of 4-(3-chloropropoxy)phenyl (CPP) glycosides of a series of common mono- and disaccharides (d-glucose, d-galactose, d-mannose, l-rhamnose, d-arabinofuranose, d-glucosamine, lactose) are described. The CPP glycosides obtained were transformed in almost quantitative yields to the corresponding unprotected 4-(3-azidopropoxy)phenyl (APP) glycosides, which could become next-generation Janus glycosides with cleavable spacer aglycon, ready for conjugation or further transformation.
Assuntos
Dissacarídeos/química , Glicosídeos/síntese química , Monossacarídeos/química , Técnicas de Química Sintética , Glicosídeos/química , Glicosilação , Estrutura MolecularRESUMO
A new glycosyl acceptor to be used in sialylation was designed as a 3-hydroxy derivative of 4-methoxyphenyl ß-d-galactopyranoside with 2-O-acetyl group and O-4 and O-6 protected as benzylidene acetal. Two alternative syntheses of this compound were compared. Sialylation of 3-OH group of the glycosyl acceptor with O-chloroacetylated N-trifluoroacetylneuraminic acid phenyl thioglycoside (NIS, TfOH, MeCN, MS 3â¯Å, -40⯰C) was studied in a wide concentration range (5-150â¯mmolâ¯L-1). The outcome of sialylation generally followed the predictions of supramer analysis of solutions of sialyl donor in MeCN, which was performed by polarimetry and static light scattering and revealed two concentration ranges differing in solution structure and the structures of supramers of glycosyl donor. The optimized conditions of sialylation (Câ¯=â¯50â¯mmolâ¯L-1) were used to synthesize protected Neu-α(2-3)-Gal disaccharide (78%, α:ßâ¯=â¯13:1), which was then converted to sialyl-α(2-3)-galactose imidate building block useful for the synthesis of complex sialo-oligosaccharides.
Assuntos
Galactose/síntese química , Configuração de Carboidratos , Galactose/químicaRESUMO
A pyranose ring contraction of ethyl 1-thio-ß-d-galactopyranosides has been discovered that proceeds with retention of aglycon under mildly acidic conditions (aq TFA in CH2Cl2). Key factors for success of this rearrangement are the presence of bulky silyl (TIPS or TBDPS) substituents at both O-2 and O-3 and a free hydroxy group at C-4 (derivatives with acid-labile protective groups at O-4 will also engage in this reaction). The rearrangement cleanly proceeds for 2,3-di- O-TIPS derivatives with two hydroxy groups at C-4 and C-6, acid-labile TES groups at O-4 and O-6, or one acyl substituent (Bz, ClAc) at O-6. A possibility to switch the direction of the debenzylidenation reaction in 4,6- O-benzylidene-2,3-di- O-TIPS/TBDPS derivatives by the choice of an acid (TFA, which cleanly gives furanose, versus AcOH, which cleaves benzylidene acetal only) may present an advantage in the divergent synthesis of selectively protected glycosyl donors (either in furanose or pyranose form) useful for the synthesis of biologically important oligosaccharides.
RESUMO
Both protective and pre-spacer features of 4-(2-chloroethoxy)phenyl (CEP) aglycon, which belong to the class of Janus aglycons, were engaged in a benzyl-free synthesis of oligosaccharide fragments of polysaccharides from rhizobacterium Azospirillum brasilense sp7. Introduction of α-1,4-linked L-fucose residue was performed using 3,4-di-O-benzoyl-2-O-triisopropylsilyl-α-L-fucopyranosyl N-phenyltrifluoroacetimidate in excellent stereoselectivity and high yields. The obtained deprotected di-, tri- and tetrasaccharides contain 4-(2-azidoethoxy)phenyl (AEP) spacer aglycon, which allows straightforward preparation of neoglycoconjugates that will be used for the study of the role of lipopolysaccharide of rhizobacterium A. brasilense sp7 in plant-microbe symbiosis. The intermediate protected oligosaccharide building blocks with cleavable CEP/AEP aglycons have a strong potential for further application in the synthesis of more complex oligosaccharides.
Assuntos
Azospirillum brasilense/química , Oligossacarídeos/química , Oligossacarídeos/síntese química , Polissacarídeos Bacterianos/química , Técnicas de Química Sintética , GlicosilaçãoRESUMO
Methanesulfonic acid was shown to be an efficient and convenient substitute for ethereal HCl in reductive 4,6-O-benzylidene acetal ring-opening reaction with sodium cyanoborohydride in THF. Normal regioselectivity was observed, the 6-O-benzyl ethers with free 4-OH group being the major products of the reaction.
Assuntos
Acetais/química , Compostos de Benzilideno/química , Mesilatos/química , Boroidretos/química , OxirreduçãoRESUMO
Under gradual acidification of growth medium resulting in the formation of dormant Mycobacterium smegmatis, a significant accumulation of free trehalose in dormant cells was observed. According to 1H- and 13C-NMR spectroscopy up to 64% of total organic substances in the dormant cell extract was represented by trehalose whilst the trehalose content in an extract of active cells taken from early stationary phase was not more than 15%. Trehalose biosynthesis during transition to the dormant state is provided by activation of genes involved in the OtsA-OtsB and TreY-TreZ pathways (according to RT-PCR). Varying the concentration of free trehalose in dormant cells by expression of MSMEG_4535 coding for trehalase we found that cell viability depends on trehalose level: cells with a high amount of trehalose survive much better than cells with a low amount. Upon resuscitation of dormant M. smegmatis, a decrease of free trehalose and an increase in glucose concentration occurred in the early period of resuscitation (after 2 h). Evidently, breakdown of trehalose by trehalase takes place at this time as a transient increase in trehalase activity was observed between 1 and 3 h of resuscitation. Activation of trehalase was not due to de novo biosynthesis but because of self-activation of the enzyme from the inactive state in dormant cells. Because, even a low concentration of ATP (2 mM) prevents self-activation of trehalase in vitro and after activation the enzyme is still sensitive to ATP we suggest that the transient character of trehalase activation in cells is due to variation in intracellular ATP concentration found in the early resuscitation period. The negative influence of the trehalase inhibitor validamycin A on the resuscitation of dormant cells proves the importance of trehalase for resuscitation. These experiments demonstrate the significance of free trehalose accumulation for the maintenance of dormant mycobacterial viability and the involvement of trehalose breakdown in early events leading to cell reactivation similar to yeast and fungal spores.
RESUMO
A series of novel sialyl donors containing O-trifluoroacetyl (TFA) groups at various positions was synthesized. The choice of protecting groups in sialyl donors was based on hypothesis that variations in ability of different acyl groups to act as hydrogen bond acceptors would influence the supramolecular structure of reaction mixture (solution structure), hence the outcome of sialylation. These glycosyl donors were examined in the model glycosylation of the primary hydroxyl group of 1,2:3,4-di-O-isopropylidene-α-D-galactopyranose in comparison with sialyl donors without O-TFA groups. The presence of O-TFA groups in a sialyl donor strongly affected the outcome of sialylation. Several sialyl donors studied showed promising results: yields of disaccharides can be as high as 86% as can be the stereoselectivities (α/ß up to 15:1). The results obtained suggest that varying acyl O-protecting groups in sialyl donor may result in dramatic changes in the outcome of sialylation although further studies are required to dissect the influence of intermolecular hydrogen bonding and intramolecular substituent effects related to variations of electron-withdrawing properties of different acyl groups.
Assuntos
Burkholderia cenocepacia/química , Lipopolissacarídeos/química , Glicosilação , Ligação de Hidrogênio , EstereoisomerismoRESUMO
Rapid and simple synthesis of oligosaccharides related to one of the terminal motifs of mycobacterial lipoarabinomannan is described. An array of homologous linear α(1 â 5)-linked oligoarabinofuranosides with 4-(2-chloroethoxy)phenyl aglycon and selectively unprotected 5-OH group at the non-reducing end was obtained by oligomerization of 3-O-benzoyl ß-D-arabinofuranose 1,2,5-orthobenzoate. Subsequent introduction of ß(1 â 2)-linked arabinofuranose disaccharide moiety by step-wise glycosylation furnished the target oligosaccharides which were conjugated with bovine serum albumin.
Assuntos
Lipopolissacarídeos/química , Oligossacarídeos/síntese química , Glicoproteínas , Estrutura Molecular , Oligossacarídeos/química , PolimerizaçãoRESUMO
1-O-Acetyl-beta-D-galactopyranose (AcGal), a new substrate for beta-galactosidase, was synthesized in a stereoselective manner by the trichloroacetimidate procedure. Kinetic parameters (K(M) and k(cat)) for the hydrolysis of 1-O-acetyl-beta-D-galactopyranose catalyzed by the beta-D-galactosidase from Penicillium sp. were compared with similar characteristics for a number of natural and synthetic substrates. The value for k(cat) in the hydrolysis of AcGal was three orders of magnitude greater than for other known substrates. The beta-galactosidase hydrolyzes AcGal with retention of anomeric configuration. The transglycosylation activity of the beta-D-galactosidase in the reaction of AcGal and methyl beta-D-galactopyranoside (1) as substrates was investigated by 1H NMR spectroscopy and HPLC techniques. The transglycosylation product using AcGal as a substrate was beta-D-galactopyranosyl-(1-->6)-1-O-acetyl-beta-D-galactopyranose (with a yield of approximately 70%). In the case of 1 as a substrate, the main transglycosylation product was methyl beta-D-galactopyranosyl-(1-->6)-beta-D-galactopyranoside. Methyl beta-D-galactopyranosyl-(1-->3)-beta-D-galactopyranoside was found to be minor product in the latter reaction.
Assuntos
Galactosídeos/metabolismo , Penicillium/enzimologia , beta-Galactosidase/metabolismo , Galactosídeos/síntese química , Galactosídeos/química , Cinética , Lactose/metabolismo , Metilgalactosídeos/metabolismo , Nitrofenilgalactosídeos/metabolismo , Especificidade por SubstratoRESUMO
The influence of O-trifluoroacetyl (TFA) groups at different positions of thioglycoside glycosyl donors on stereoselectivity of α-arabinofuranosylation leading to corresponding disaccharides was studied. It was shown that TFA group in thioglycoside glycosyl donors, when combined with 2-O-(triisopropylsilyl) (TIPS) non-participating group, may be regarded as an electron-withdrawing protecting group that may enhance 1,2-cis-selectivity in arabinofuranosylation, the results strongly depending on the nature of glycosyl acceptor. The reactivities of the glycosyl donors were compared with those of a similar thioglycoside with O-pentafluoropropionyl groups and the known phenyl 3,5-O-(di-tert-butylsilylene)-1-thio-α-d-arabinofuranosides with 2-O-TIPS and 2-O-benzyl groups. The 'matching' in the donor-acceptor combination was found to be critical for achieving both high reactivity of glycosyl donor and ß-stereoselectivity of arabinofuranosylation. The use of glycosyl donors with TFA and silyl protection may be useful in the realization of the benzyl-free approach to oligoarabinofuranosides with azido group in aglycon-convenient building blocks for the preparation of neoglycoconjugates.