Synthetic Studies on the Incorporation of N-Acetylallosamine in Hyaluronic Acid-Inspired Thiodisaccharides.

Two approaches for the synthesis of the thiodisaccharide ß-S-GlcA(1→3)ß-S-AllNAc are described here. The target disaccharide was a C-3 epimer and thio-analogue of the hyaluronic acid repetitive unit, tuned with a thiopropargyl anomeric group for further click conjugation. Thus, we analysed and tested two convenient sequences, combining the two key steps required to introduce the thioglycosidic bonds and consequently reach the target molecule: the SN2 substitution of a good leaving group (triflate) present at C-3 of a GlcNAc derivative and the introduction of the anomeric thiopropargyl substituent. The use of a 2-azido precursor showed to be a convenient substrate for the SN2 step. Nevertheless, further protecting group manipulation and the introduction of the thiopropargyl anomeric residue were then required. This approach showed to provide access to a variety of thiodisaccharide derivatives as interesting building blocks for the construction of neoglycoconjugates.

Design and Synthesis of 2-Acetamido-2,3-dideoxythiodisaccharides via Diastereoselective Conjugate Addition to Sugar Enone O-Acetyl Oximes. Galactosidase Inhibition Studies.

The key step in a new synthesis of 2-acetamido-2,3-dideoxy-(1→4)-thiodisaccharides was the conjugate addition of a 1-thiogalactose derivative to E and Z acetyl oximes derived from sugar enones. This reaction was shown to be completely diastereoselective for both the formation of the thioglycosidic linkage and the configuration of acetyl oxime. The thiodisaccharides have been designed as inhibitors of the ß-galactosidase from E. coli, and they have been shown to successfully meet such requirements.

Characterization of a ß-galactosidase from Bacillus subtilis with transgalactosylation activity.

Microbial ß-galactosidases (EC 3.1.2.23) have applications in the production of galacto-oligosaccharides, which are established prebiotic food ingredients. The ß-galactosidase from Bacillus subtilis (YesZ) was expressed as a heterologous protein in Escherichia coli, and presented an optimum activity at pH 6.5 and 40 °C. The catalytic constants Km and Vmax of the enzyme were 8.26 mM and 1.42 µmol·min-1·mg-1 against pNP-ß-d-galactopyranoside, respectively. Structural characterization revealed that YesZ is a homotrimer in solution, and homology modeling suggested that the YesZ conserves a Cys cluster zinc binding site. Flame photometry experiments confirmed the presence of bound zinc in the recombinant enzyme, and YesZ activity was inhibited by 1 mM zinc, copper and silver ions. Transgalactosylation activity of YesZ was observed with the synthetic substrate p-NP-ßGal in the presence of a d-xylose acceptor, producing a ß-d-galactopyranosyl-(1 → 4)-d-xylopyranose disaccharide. Analysis of this disaccharide by MALDI-ToF-MS/MS suggested a ß-1,4 glycosidic linkage between a non-reducing galactose residue and the xylose. The ß-galactosidase YesZ from B. subtilis is a candidate for enzymatic synthesis showing favorable thermostability (with residual activity of 50% after incubation at 30 °C for 25 h) and transgalactosylation activity.

Synthesis of neamine-based pseudodisaccharides as potential vestibulotoxic agents to treat vertigo in Ménière's disease.

Ménière's disease (MD) is a progressive disease of the inner ear characterized by recurring attacks of disabling vertigo, hearing loss and tinnitus. Patients who do not respond to vestibular sedatives or steroids may require an intratympanic application of aminoglycoside antibiotics, which destroys the vestibular function of the affected ear in order to avoid the debilitating vertigo attacks. Although effective, this procedure causes hearing loss in almost one third of the patients due to the aminoglycosides cochlear toxicity. Here we describe the synthesis of two pseudodisaccharides structurally related to neamime aiming to mimic the aminoglycosides pharmacophore core by replacing their toxic amine by azide and hydroxyl groups. Products 1 and 2 selectively promoted 'in vivo' damage to vestibular tissues without causing hearing loss or cochlear toxicity. Therefore, these pseudodisaccharides stand as promising lead compounds for the development of a safer and more effective therapeutic procedure to manage the symptoms of MD severe dizziness.

Synthesis of the (1→6)-linked thiodisaccharide of galactofuranose: inhibitory activity against a ß-galactofuranosidase.

A new (1→6)-linked thiodisaccharide formed by two galactofuranosyl units has been synthesized. Methyl (methyl α,ß-D-galactofuranosid)uronate was employed as the starting compound, which was per-O-silylated with TBSCl and reduced with LiAlH4 to afford methyl 2,3,5-tri-O-tert-butyldimethylsilyl-ß-D-galactofuranoside (2ß) as a key precursor for the preparation of methyl per-O-tert-butyldimethylsilyl-6-thio-ß-D-galactofuranoside (12). The free thiol group of 12 was glycosylated and the product O-deprotected to afford the target ß-D-Galf-S-(1→6)-ß-d-Galf-OMe (14). The conformations of this thiodisaccharide were preliminarily studied using combined theoretical calculations and NMR data. Furthermore, the glycomimetic 14 showed to be a competitive inhibitor of the ß-galactofuranosidase from Penicillum fellutanum (K(i)=3.62 mM).

Transglycosylation specificity of Acremonium sp. α-rhamnosyl-ß-glucosidase and its application to the synthesis of the new fluorogenic substrate 4-methylumbelliferyl-rutinoside.

Transglycosylation potential of the fungal diglycosidase α-rhamnosyl-ß-glucosidase was explored. The biocatalyst was shown to have broad acceptor specificity toward aliphatic and aromatic alcohols. This feature allowed the synthesis of the diglycoconjugated fluorogenic substrate 4-methylumbelliferyl-rutinoside. The synthesis was performed in one step from the corresponding aglycone, 4-methylumbelliferone, and hesperidin as rutinose donor. 4-Methylumbelliferyl-rutinoside was produced in an agitated reactor using the immobilized biocatalyst with a 16% yield regarding the sugar acceptor. The compound was purified by solvent extraction and silica gel chromatography. MALDI-TOF/TOF data recorded for the [M+Na](+) ions correlated with the theoretical monoisotopic mass (calcd [M+Na](+): 507.44 m/z; obs. [M+Na](+): 507.465 m/z). 4-Methylumbelliferyl-rutinoside differs from 4-methylumbelliferyl-glucoside in the rhamnosyl substitution at the C-6 of glucose, and this property brings about the possibility to explore in nature the occurrence of endo-ß-glucosidases by zymographic analysis.

Synthesis of branched dithiotrisaccharides via ring-opening reaction of sugar thiiranes.

Satisfactory procedures are described for the synthesis of 5,6- and 3,4-thiirane derivatives from the respective hexofuranose or hexopyranose epoxide precursors. The controlled ring-opening reaction of thiiranes by 1-thioaldoses was successfully accomplished to afford, regio- and stereoselectively, ß-S-(1→4)-3,4-dithiodisaccharides. For instance, the regioselective attack of per-O-acetyl-1-thioglucose (16) to C-4 of 2-propyl 2,6-di-O-acetyl-3,4-epithio-α-D-galactopyranoside (14) gave the derivative of Glcp-ß-S-(1→4)-3,4-dithioGlcp-O-iPr (17). This thiodisaccharide was accompanied by the (1→3)-disulfide 18, formed between 16 and 17, and the symmetric (3→3)-disulfide 19, which resulted from the oxidative dimerization of 17. However, the S-acetyl derivative of 17 could be obtained in good yield (62%) by LiAlH(4) reduction of the crude mixture 17-19, followed by acetylation. The same sequence of reactions starting from 14 and the 1-thiolate of Galp afforded the per-O,S-acetyl derivative of Galp-ß-S-(1→4)-3,4-dithio-α-D-Glcp-O-iPr (23), which was selectively S-deacetylated to give 25. The dithiosaccharides 17 and 25 are 3,4-di-S-analogues of derivatives of the natural disaccharides cellobiose and lactose, respectively. The ring-opening reaction of 5,6-epithiohexofuranoses of D-galacto (8) or L-altro (11) configuration with 1-thioaldoses was also regio- and stereoselective to give the respective ß-S-(1→6)-linked 5,6-dithiodisaccharides 26 or 29 in excellent yields. Glycosylation of the free thiol group of 17, 25, or 26, using trichloroacetimidates as glycosyl donors, led to the corresponding branched dithiotrisaccharides. Some of them are sulfur analogues of derivatives of branched trisaccharides found in natural polysaccharides.

Synthesis of pentopyranosyl-containing thiodisaccharides. Inhibitory activity against beta-glycosidases.

Beta-(1-->4)-thiodisaccharides formed by a pentopyranose unit as reducing or non reducing end have been synthesized using a sugar enone derived from a hexose or pentose as Michael acceptor of a 1-thiopentopyranose or 1-thiohexopyranose derivatives. Thus, 2-propyl per-O-acetyl-3-deoxy-4-S-(beta-D-Xylp)-4-thiohexopyranosid-2-ulose (3) and benzyl per-O-acetyl-3-deoxy-4-S-(beta-D-Galp)-4-thiopentopyranosid-2-ulose (11) were obtained in almost quantitative yields. The carbonyl function of these uloses was reduced with NaBH(4) or K-Selectride, and the stereochemical course of the reduction was highly dependent on the reaction temperature, reducing agent and solvent. Unexpectedly, reduction of 3 with NaBH(4)-THF at 0 degrees C gave a 3-deoxy-4-S-(beta-D-Xylp)-4-thio-alpha-D-ribo-hexopyranoside derivative (6) as major product (74% yield), with isomerization of the sulfur-substituted C-4 stereocenter of the pyranone. Reduction of 11 gave always as major product the benzyl 3-deoxy-4-S-(Galp)-4-thio-beta-D-threo-pentopyranoside derivative 14, which was the only product isolated (80% yield) in the reduction with K-Selectride in THF at -78 degrees C. Deprotection of 14 and its epimer at C-2 (13) afforded, respectively the free thiodisaccharides 19 and 18. They displayed strong inhibitory activity against the beta-galactosidase from Escherichia coli. Thus, compound 18 proved to be a non-competitive inhibitor of the enzyme (K(i)=0.80 mM), whereas 19 was a mixed-type inhibitor (K(i)=32 microM).

Thiodisaccharides with galactofuranose or arabinofuranose as terminal units: synthesis and inhibitory activity of an exo beta-D-galactofuranosidase.

Thiodisaccharides having beta-D-Galf or alpha-L-Araf units as non-reducing end have been synthesized by the SnCl(4)- or MoO(2)Cl(2)-promoted thioglycosylation of per-O-benzoyl-D-galactofuranose (1), its 1-O-acetyl analogue 4, or per-O-acetyl-alpha-L-arabinofuranose (16) with 6-thioglucose or 6-thiogalactose derivatives. After convenient removal of the protecting groups, the free thiodisaccharides having the basic structure beta-D-Galf(1-->6)-6-thio-alpha-D-Glcp-OMe (5) or beta-D-Galf(1-->6)-6-thio-alpha-D-Galp-OMe (15) were obtained. The respective alpha-L-Araf analogues 18 and 20 were prepared similarly from 16. Alternatively, beta-D-Galf(1-->4)-4-thio-3-deoxy-alpha-L-Xylp-OiPr was synthesized by Michael addition to a sugar enone of 1-thio-beta-d-Galf derivative, generated in situ from the glycosyl isothiourea derivative of 1. The free S-linked disaccharides were evaluated as inhibitors of the beta-galactofuranosidase from Penicillium fellutanum, being 15 and 20 the more active inhibitors against this enzyme.

Straightforward synthesis of thiodisaccharides by ring-opening of sugar epoxides.

3,4-Anhydro hexopyranosides have been prepared by diastereoselective epoxidation of derivatives of 2-propyl 3,4-dideoxy-alpha-D-erythro-hex-3-enopyranoside (5), selectively protected at HO-2 and HO-6. The allylic group at C-2, in 5 and derivatives, plays a critical role in the facial selectivity of the epoxidation reaction. Thus, the free HO-2 in 3 (the 6-O-acetyl derivative of 5) directs the attack of m-chloroperbenzoic acid from the more hindered alpha face of the molecule to give 2-propyl 6-O-acetyl-3,4-anhydro-alpha-D-allopyranoside (7) accompanied by the beta epoxide 6 as a very minor product. Reverse diastereoselectivity has been obtained when the HO-2 in 3 was substituted by a bulky tert-butyldimethylsilyl (TBS) group. In this case, the major isomer was the 2-O-TBS derivative of 6 (alpha-D-galacto configuration). The ring-opening of sugar epoxides by nucleophilic per-O-acetyl-1-thio-beta-D-glucopyranose (11) was employed as a convenient approach to the synthesis of (1-->3)- and (1-->4)-thiodisaccharides. For example, ring-opening of the oxirane 7 by 11 led to the expected regioisomeric per-O-acetyl thiodisaccharides beta-D-Glc-S-(1-->3)-4-thio-alpha-D-Glc-O-iPr (12) and beta-D-Glc-S-(1-->4)-4-thio-alpha-D-Gul-O-iPr (13). Regioselectivity in the construction of the (1-->4)-thioglycosidic linkage could be achieved by hindering C-3 of the 3,4-anhydro sugar with a bulky silyloxy group at the vicinal C-2. For instance, coupling of the 2-O-TBS derivative of 7 with 11 led regioselectively to the protected thiodisaccharide beta-D-Glc-S-(1-->4)-4-thio-alpha-D-Glc-O-iPr (27). The utility of the approach was demonstrated through the synthesis of sulfur-linked analogues of naturally occurring (laminarabiose and cellobiose) and non-natural disaccharides (i.e., beta-D-Glc-(1-->4)-alpha-D-Gul).

Synthesis of non-glycosidic 4,6'-thioether-linked disaccharides as hydrolytically stable glycomimetics.

Michael addition of 1,2:3,4-di-O-isopropylidene-6-thio-alpha-D-galactose (2) to 2-propyl 6-O-acetyl-3,4-dideoxy-alpha-D-glycero-hex-3-enopyranosid-2-ulose (1) afforded, as the major diastereoisomer, 2-propyl 6-O-acetyl-3-deoxy-4-S-(6-deoxy-1,2:3,4-di-O-isopropylidene-alpha-D-galactopyranos-6-yl)-4-thio-alpha-D-threo-hexopyranosid-2-ulose (3, 91% yield). Reduction of the carbonyl group of 3, followed by O-deacetylation gave the two epimers 7 (alpha-D-lyxo) and 8 (alpha-D-xylo) in a 1:2 ratio. On removal of the protecting groups of 8 by acid hydrolysis, formation of an 1,6-anhydro bridge was observed in the 3-deoxy-4-thiohexopyranose unit (10). The free non-glycosidic thioether-linked disaccharide 3-deoxy-4-S-(6-deoxy-alpha,beta-D-galactopyranos-6-yl)-4-thio-alpha,beta-D-xylo-hexopyranose (11) was obtained by acetolysis of 10 followed by O-deacetylation. A similar sequence starting from the enone 1 and methyl 2,3,4-tri-O-benzoyl-6-thio-alpha-D-glucopyranoside (12) led successfully to 2-propyl 3-deoxy-4-S-(methyl 6-deoxy-alpha-D-glucopyranos-6-yl)-4-thio-alpha-D-lyxo-hexopyranoside (17) and its alpha-D-xylo analog (19, major product). In this synthetic route, orthogonal sets of protecting groups were employed to preserve the configuration of both reducing ends and to avoid the formation of the 1,6-anhydro ring.

Microwave-assisted efficient preparation of novel carbohydrate tetrazole derivatives.

A series of novel N-1, N-2 and S-5 saccharide substituted tetrazole derivatives linked at anomeric and nonanomeric positions were obtained from commercial tetrazoles under microwave irradiation. Yields are compared with conventional methodologies.

Depolymerization of beta-chitin to mono- and disaccharides by the serum fraction from the para rubber tree, Hevea brasiliensis.

The serum fraction of latex from Hevea brasiliensis, the para rubber tree, is known to contain an endo-chitinolytic enzyme, hevamine. Herein the activity of the rubber serum towards beta-chitin is investigated. The serum contained 6 mg/mL of protein and a chitinolytic activity of 18 mU permg of protein. The optimum ratio of enzyme to chitin was 0.22 mU/mg, and the optimum substrate concentration was 60 mg/mL. The optimum pH range was pH2-4, and the optimum temperature was 45 degrees C. At these conditions both (GlcNAc)2 and GlcNAc were produced in a molar ratio of approximately 2:1. The hydrolysis of 300 mg of chitin with 64 mU of the rubber serum for 8 days under the optimum conditions gave 39 mg of GlcNAc and 108 mg of (GlcNAc)2 as determined by HPLC. Mixing the rubber serum preparation with an Aspergillus niger pectinase preparation containing beta-N-acetylhexosaminidase can be used to produce almost exclusively the GlcNAc monomer in about 50% yield.

Synthesis of 4-nitrophenyl beta-D-fucofuranoside and beta-D-fucofuranosyl-(1-->3)-D-mannopyranose: modified substrates for studies on catalytic requirements of beta-D-galactofuranosidase.

Syntheses of 4-nitrophenyl beta-D-fucofuranoside (6) and beta-D-fucofuranosyl-(1-->3)-D-mannopyranose (10) are reported. These compounds, as analogues of galactofuranosides, were used for studying the influence of the hydroxyl group at C-6 in the interaction of the substrate with beta-D-galactofuranosidase. For the synthesis of the fucofuranosides, 2,3,5-tri-O-benzoyl-6-bromo-6-deoxy-D-galactono-1,4-lactone (1) was the key intermediate, which upon reduction of the lactone group with diisoamylborane, acetylation of the anomeric hydroxyl group, and catalytic hydrogenolysis of the bromine at C-6, led to 1-O-acetyl-2,3,5-tri-O-benzoyl-alpha,beta-D-fucofuranose (4), a convenient derivative for the preparation of fucofuranosides. Compound 4 was glycosylated in the presence of SnCl4, either with 4-nitrophenol for the preparation of 6, or with 2,5,6-tri-O-benzoyl-D-mannono-1,4-lactone (7), for the synthesis of disaccharide 10, via the glycosyl-aldonolactone approach. The synthetic route developed for the beta-D-fucofuranosides is simple and efficient. Compound 6 was not hydrolyzed by incubation with the exo beta-D-galactofuranosidase from Penicillium fellutanum, showing that HO-6 is essential for interaction of the substrate with the enzyme.

Synthesis of beta-D-Galf-(1-3)-D-GlcNAc by the trichloroacetamidate method and of beta-D-Galf-(1-6)-D-GlcNAc by SnCl4-promoted glycosylation.

In a continuation of our studies on the characterization of the glycoproteins of T. cruzi new galactofuranosyl disaccharides were synthesized. Beta-D-Galf-(1-3)-D-GlcNAc was prepared by employing the trichloroacetamidate procedure for the glycosylation step. The mild conditions of this reaction are appropriate for condensation of 2,3,5,6-tetra-O-benzoyl-beta-D-galactofuranosyl trichloroacetamidate with acid-labile benzyl 2-acetamido-4,6-O-benzylidene-2-deoxy-alpha-D-glucopyranoside. On the other hand, tin(IV) chloride promoted condensation of benzyl 2-acetamido-3-O-benzoyl-2-deoxy-alpha-D-glucopyranoside with penta-O-benzoyl-alpha-beta-D-galactofuranose gave the derivative of beta-D-Galf-(1-6)-D-GlcNAc in 78% yield.

The glycosyl-aldonolactone approach for the synthesis of beta-D-Galf-(1-->3)-D-Manp and 3-deoxy-beta-D-xylo-hexofuranosyl-(1-->3)-D-Manp.

A convenient synthesis of free beta-D-Galf-(1-->3)-D-Manp (8a) is reported. The disaccharide is present as external unit in the lipopeptidophosphoglycan (LPPG) of Trypanosoma cruzi and internally in the lipophosphoglycan (LPG) of Leishmania. Condensation of 2,5,6-tri-O-benzoyl-D-mannono-1,4-lactone (1) with 1,2,3,5,6-penta-O-benzoyl-D-galactofuranose, promoted by SnCl4, led to the beta-glycosyl-lactone, a key intermediate for disaccharide 8a, readily obtained by successive reduction of the lactone with diisoamylborane and debenzoylation. As in the LPG of Leishmania the HO-3 group of the galactofuranose is glycosylated by alpha-D-Galp, we also synthesized 3-deoxy-beta-D-xylo-hexofuranosyl-(1-->3)-D-Manp (8b) and p-nitrophenyl 3-deoxy-beta-D-xylo-hexofuranoside for studying the influence of HO-3 in the interaction with specific glycosidases. The disaccharide 8a, and its corresponding alditol, were good substrates for the beta-D-galactofuranosidase from Penicillium fellutanum, whereas the 3-deoxyglycosides were not hydrolyzed by the enzyme.

Synthesis of terminal disaccharide elements corresponding to the Ogawa and Inaba antigenic determinant from Vibrio cholerae O1.

Vibrio cholerae O1 LPS terminal mono- and disaccharide elements were synthesized by reduction of the azido group in several 4-amino-4,6-dideoxy-D-mannose mono- and disaccharide derivatives, followed by coupling with 2, 4-di-O-acetyl-3-deoxy-L-glycero-tetronic acid in the presence of 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline. This compound represents a useful model in order to elucidate the size of the epitopes which define Ogawa and Inaba serotypes from Vibrio cholerae O1.

Convenient syntheses of 5-O- and 3,5-di-O-(beta-D-galactofuranosyl)-D-galactofuranose.

Benzoylation of D-galactono-1,4-lactone with 2.2 mol of benzoyl chloride, at -23 degrees, gave the 2,6-dibenzoate (2, 62%). Tin(IV) chloride-catalyzed glycosylation of 2 with 1,2,3,5,6-penta-O-benzoyl-alpha,beta-D-galactofuranose (1) afforded 2,6-di-O-benzoyl-5-O-(2,3,5,6-tetra-O-benzoyl-beta-D-galactofuranosyl)-D - galactono-1,4-lactone (4, 70%), HO-3 of which was benzoylated to give 5. The structure of 4 was confirmed by its conversion into crystalline beta-D-Galf-(1----5)-D-Gal-ol (8). Reduction of the lactone function of 5 with di-isoamylborane followed by debenzoylation gave beta-D-Galf-(1----5)-D-Galf (7). A by-product of the condensation of 1 with 2 was characterized as 2,6-di-O-benzoyl-3,5-di-O-(2,3,5,6-tetra-O-benzoyl-beta-D-galactofuranos yl)-D- galactono-1,4-lactone (9), which was converted, as for 5, into beta-D-Galf-(1----3)[beta-D-Galf-(1----5)]-D-Galf (13).

Synthesis of galactofuranose disaccharides of biological significance.

Methyl beta-D-galactofuranoside was readily obtained by tin(IV) chloride-catalyzed glycosylation of penta-O-benzoyl-alpha,beta-D-galactofuranose, followed by debenzoylation with sodium methoxide. Glycosylation of 1 with 2,3,5-tri-O-benzoyl-D-galactono-1,4-lactone or with the 6-O-trityl-lactone derivative 5 gave the benzoylated beta-D-galactofuranosyl-(1----6)-D-galactono-1,4-lactone 6 in excellent yield. The structure of disaccharide 6 was confirmed by borohydride reduction to the glycosyl-alditol 7. A byproduct of the condensation reaction of 1 with 4 or 5 was identified as the benzoylated (1----1)-beta,beta'-D-galactofuranosyl disaccharide 8. Compound 8 was readily prepared (88% yield) by controlled addition of water to 1, in the presence of stannic chloride. O-Debenzoylation of 8 afforded crystalline beta'-D-galactofuranosyl-(1----1)-beta-D-galactofuranoside. The glycosyl-lactone 6 constitutes a key intermediate for the synthesis of a disaccharide derivative having both units in the furanoid form. Thus, diisoamylborane reduction of the lactone function of 6 led to the disaccharide derivative 10, from which the methyl glycoside 12 was prepared. O-Debenzoylation of 12 gave the corresponding methyl beta-D-galactofuranosyl-(1----6)-beta-D-galactofuranoside. The free disaccharide beta-D-Galf-(1----6)-D-Galp and its acetylated derivative were also synthesized from 10.