Synthesis of Two Tetrasaccharides Related to the O-Antigen from Azospirillum brasilense S17 and Azospirillum lipoferum SR65

Synthesis of two isomeric tetrasaccharides, β-D-Glup-(1→2)-α-L-Rhap-(1→3)-α-L- Rhap-(1→2)-α-L-Rhap (I) and β-D-Glup-(1→3)-α-L-Rhap-(1→3)-α-L-Rhap-(1→3)-α-L-Rhap (II), the repeating units from the lipopolysaccharides of the nitrogen-fixing bacterium Azospirillum brasilense S17 and Azospirillum lipoferum SR65, was achieved via assembly of the building blocks 2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl trichloroacetimidate (2), p-methoxyphenyl 3,4-di-O-benzoyl-α-L-rhamnopyranoside (3), 3-O-allyloxycarbonyl-2,4-di-O-benzoyl-α-L-rhamnopyranosyl trichloroacetimidate (6), 2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl trichloroacetimidate (8), and p-methoxy phenyl 2,4-di-O-benzoyl-α-L-rhamnopyranoside (14). Condensation of 3 with 6 or 8 provided the disaccharides 9 or 11, respectively. Deallyloxycarbonylation of 11 gave the disaccharide aceptor 12, while removal of the p-methoxyphenyl group in 9 followed by trichloroacetimidation of the anomeric hydroxyl group afforded the disaccharide donor 10. Meanwhile, disaccharide donor 16 and acceptor 18 were prepared from 6, 8, and 14 similarly. Finally, condensation of 10 with 12 or 16 with 18, followed by deprotection, gave the target tetrasaccharides I or II, respectively.     

Structural features of an acidic polysaccharide with the potential of promoting osteoblast differentiation from Lycium ruthenicum Murr.

Abstract

The enhanced osteoblast differentiation is beneficial to the prevention of osteoporosis. In this study, a homogeneous polysaccharide (LRP-S2A) with the potential of promoting osteoblast differentiation was obtained from the fruits of Lycium ruthenicum, a traditional herb for treatment of postmenopausal metabolic disorders. Structural identification indicated that LRP-S2A, with a relative molecular weight of 2.65 × 10⁶ Da and an uronic acid content of 41.8%, contained Rha, Ara, Gal, Glc and GlcA in a molar ratio of 1.00 : 2.07 : 0.57 : 2.59 : 4.33 and was composed of a backbone consisting of 6-O-Me-α-(1→4)-D-GlcpA, 2-O-acetyl-α-(1→4)-D-Glcp, α-(1→2,4)-L-Rhap, β-(1→3)-D-Galp andα-(1→3,5)-L-Araf, and some branches consisting of 6-O-Me-α-(1→4)-D-GlcpA and terminal α-L-Araf. These results suggested that LRP-S2A with the potential of promoting osteoblast differentiation was a new acidic polysaccharide.     

1H NMR Studies of Hydroxy Protons of ASN- and Ser-Linked Disaccharides in Aqueous Solution

ABSTRACT

The hydroxy protons of β-D-GlcpNAc-(1→4)-β-D-GlcpNAc, β-D-GlcpNAc-(1→4)-β-D-GlcpNAc-N-Asn, β-D-Galp-(1→3)-α-D-GalpNAc-O-Me and of β-D-Galp-(1→3)-α-D-GalpNAc-O-Ser in aqueous solution have been investigated using ¹H NMR spectroscopy. The chemical shifts, coupling constants, temperature coefficients, exchange rates and NOEs have been measured. The O(3)H proton of β-D-GlcpNAc-(1→4)-β-D-GlcpNAc and β-D-GlcpNAc-(1→4)-β-D-GlcpNAc-N-Asn, and the O(2')H proton of β-D-Galp-(1→3)-α-D-GalpNAc and β-D-Galp-(1→3)-α-D-GalpNAc-O-Ser have values which differ significantly from the other hydroxy protons. Both these hydroxy protons are shielded when compared to those of the corresponding monosaccharide methyl glycosides. This shielding is attributed to the proximity of these protons to the O(5') oxygen and to the 2-acetamido group, respectively. In β-D-GlcpNAc-(1→4)-β-D-GlcpNAc and β-D-GlcpNAc-(1→4)-β-D-GlcpNAc-N-Asn, the O(3)H proton has restricted conformational freedom with a preferred orientation towards the O(5') oxygen, and is protected from exchange with the bulk water through a weak hydrogen bond interaction with O(5'). In β-D-Galp-(1→3)-α-D-GalpNAc-O-Me and β-D-Galp-(1→3)-α-D-GalpNAc-O-Ser, the O(2')H is protected from exchange with the bulk water by the 2-acetamido group. The conformations of the disaccharides are not affected by the amino acid, and no interaction in terms of hydrogen bonding between the sugars and the amino acid residue could be observed.     

Synthesis of Oligosaccharides Designed to form Micelles,Corresponding to Structures Found in Ovarian Cyst Fluid

ABSTRACT

The syntheses of α-D-GlcpNAc-(1→4)-β-D-Galp-(1→4)-β-D-GlcNAc-(1→O)-(CH₂)₁₅CH₃ (1) and fragments thereof, corresponding to structures found in human ovarian cyst fluid, are described. Silver triflate promoted coupling of 3,4,6-tri-O-acetyl-2-azido-2-deoxy-β-D-glucopyranosyl bromide (12) and galactose acceptor (11) gave a disaccharide donor (13), which was readily transformed into the corresponding bromo-derivative 18. For the synthesis of disaccharide β-D-Galp-(1→4)-D-GlcNAc, several differently protected glucosamine acceptors were prepared. It was found that cetyl alcohol needed to be introduced after the formation of the β-galactoside bond. Glycosylation of pent-4-enyl 3,6-di-O-benzyl-2-deoxy-2-tetrachlorophthalimido-β-D-glucopyranoside (30) with (3,4,6-tri-O-acetyl-2-azido-2-deoxy-α-D-glucopyranosyl)-(1→4)-2,3,6-tri-O-benzoyl-α-D-galactopyranosyl bromide (18) by use of silver triflate as promoter gave the desired trisaccharide 31. Finally 31 was transformed via coupling to the long alkyl chain aglycon and deprotection into the title compound 1.     

Studies on the Solution Conformation and Dynamics of the Trisaccharide Repeating Unit of the Kps from Sinorhizobium Fredii Svq293

ABSTRACT

The conformational behaviour of the major trisaccharide repeating unit (α-D-Galp-(1→2)-β-D-Ribf-(1→9)-α-5-O-Me-Kdnp-) of the polysaccharide from Sinorhizobium fredii SVQ293, a mutant derivative has been analysed by NMR spectroscopy and extensive molecular dynamics simulations. The results obtained indicate that the five-membered ring adopts an almost unique conformation as do the pyranose rings. The Ribf-(1→9)-α-5-O-Me-Kdnp linkage may adopt a variety of conformations while the α-D-Galp-(1→2)-β-D-Ribf- also populates an extended surface of the Φ/Ψ map. Two 10 ns MD simulations using the GB/SA continuum solvent model for water and the MM3* force field provides a population distribution of conformers which satisfactorily agrees with the experimental NMR data for both the glycosidic linkages and the hydroxymethyl groups.     

Conformational Analysis of Sulfated α-(1→3)-Linked D-Galactobioses Using the Mm3 Force-Field

Abstract

The conformational maps of eight derivatives of the disaccharide α-D-Galp-(1→3)-β-D-Galp sulfated in different positions were obtained using the MM3 force-field specially parameterized for sulfate ester groups. As occurred with MM2, the conformational flexibility of the glycosidic linkage is only slightly hindered by sulfation. A substantial effect of sulfation of the β-D-galactose unit on position 4 shifts the global minimum to positive Ψ_H (C1′-O3-C3-H3) angles, while sulfation at position 2 of the same unit deepens the well at negative Ψ^H angles. On the other hand, sulfation on the α-D-galactose unit has a lesser effect, which in any case tends to stabilize the minimum at negative Ψ_H angles.     

Triterpene glycosides of Hedera taurica

From the leaves of Crimean ivy we have isolated the previously known glycosides 3-O-α-L-Ara_p-28-O-[O-α-L-Rha_p-(1→4)-O-β-D-Glc_p-(1→6)-β-D-Glc_p]hederagenin, 3-O-[O-α-L-Rha_p-(1→2)-α-L-Ara_p]-28-O-[O-α-L-Rha_p-(1→4)-O-β-D-Glc_p-(1→6)-β-D-Glc_p]oleanic acid and -hederagenin, and 3-O-[O-α-L-Rha_p-(1→2)-α-L-Ara_p]-28-O-[O-β-D-Glc_p-(1→6)-β-D-Glc_p]hederagenin and a new one: tauroside H₁ — 3-O-[O-α-L-Rha_p-(1→2)-O-α-L-Ara_p]-28-O-[O-α-L-Rha_p-(1→4)-O-β-D-Glc_p-(1→6)-β-D-Glc_p]echinocystic acid.     

Isolation and structural characterization of a low-molecular-weight pectic polysaccharide SHPPB-1 isolated from sunflower heads

A novel low-molecular-weight pectic polysaccharide was isolated from sunflower heads that are a useless side product produced from sunflower oil processing. The low-molecular-weight pectic polysaccharide was purified by using an optimized four-step procedure and named as SHPPB-1. The molecular weight of SHPPB-1 is about 1.69× 10⁴ Da. Structure characterizations of SHPPB-1 by monosaccharide composition, methylation analysis, and Fourier transform infrared (FT-IR) spectroscopy showed that SHPPB-1 is consisted of 1,4-linked α-D-GalpA and 1,4-linked 2-OAc-5-COOMe-α-D-GalpA with rare α/β-D-Rhap, α/β-D-Manp, and α/β-D-GalpA. This was combined with NMR spectroscopic analysis to propose a structure of SHPPB-1 as: →4)-[α/β-D-monosaccharide-(1→3)]-α-D-GalpA-(1→4)-2-OAc-5-COOMe-α-D-GalpA-(1→ .     

Structure analysis and anti-fatigue activity of a polysaccharide from Lepidium meyenii Walp

A polysaccharide was obtained from Lepidium meyenii Walp by hot water extraction and purification by Millipore (100 kD) and Sephadex G-200. The content of polysaccharide was examined to be 89.9% with phenol-sulfuric acid method. Its average molecular weight was estimated to be 2.213 × 10⁶ Da by High Performance Gel Permeation Chromatography (HPGPC). Monosaccharide analysis showed that the polysaccharide was composed of arabinose, mannose, glucose and galactose with the molar ratio of 2.134: 1: 2.78: 2.82. After Smith degradation, methylation, infrared spectroscopy and NMR, the primary structure of the polysaccharide was identified. The backbone of the polysaccharide was composed of →4)-β-D-Galp-(1→ and →4)-α-D-Galp-(1→, while the branches were comprised of →6)-β-D-Glup-(1→, →5)- β-D-Araf-(1→, →3,6)-α-D-Manp-(1→, →3)-α-D-Galp-(1→, and α-D-Glup-(1→. The anti-fatigue effect of the polysaccharide was evaluated using exhaustive swimming test and biochemical indexes. The results indicated the polysaccharide has anti-fatigue effect.     

Conformational Analysis of Neocarrabiose and its Sulfated and/or Pyruvylated Derivatives using the MM3 Force-Field

ABSTRACT

The adiabatic conformational surfaces of neocarrabiose (3,6-An-α-D-Galp-(1→3)-β-D-Galp) and of nine sulfated and/or pyruvylated derivatives were obtained using the MM3 force-field. These maps indicate greater flexibility of the glycosidic linkage than found for similar compounds that are based on α-D-galactose instead of 3,6-anhydrogalactose units. Sulfation of the β-D-galactose unit on position 2 shifts the global minimum to negative Ψ_H (θ_C1'-03-C3-H3) angles, whereas sulfation at either position 4 of the same unit or at position 2 of the 3,6-anhydro-α-D-galactose unit has less effect. The results are consistent with the X-ray diffraction data on crystalline neocarrabiose and carrageenan fibers. Free energy calculations show that entropy is not uniformly distributed among confomers.     

Synthesis of Haptenic Trimers Corresponding to the Cell Wall Glycopeptidolipids of Mycobacterium Avium Serovar 12

ABSTRACT

The preparation of the spacer-containing trimers 2, 3-aminopropyl 3-O-[4-O-Me-3-O-(4-N-D,L-lactoyl-3-O-Me-β-D-Quip)-α-L-Rhap]-α-L-Rhap, derivatives of the antigenic determinant of the glycopeptidolipid from Mycobacterium avium serotype 12, are described. Thus, iodonium ion-mediated glycosylation of the spacer-containing acceptor 7 with ethyl 1-thio-rhamnopyranoside donor 10, followed by selective deprotection of the p-methoxybenzyl group of thus obtained 19 gave bis-rhamnopyranoside acceptor 20. Elongation of 20 with ethyl 4-azido-1-thio-β-D-quinovopyranoside 18 and subsequent reduction of the azido function in 21 led to trimer 22. The amino group in 22 was coupled with both D- and L-lactic acid to give, after removal of the protective groups, trimers 2.     

Synthesis of a Single Repeat Unit of Type VIII Group B Streptococcus Capsular Polysaccharide 1

Abstract

We have synthesized a single repeat unit of type VIII Group B Streptococcus capsular polysaccharide, the structure of which is {L-Rhap(β1→4)-D-Glcp(β1→4)[Neu5Ac(α2→3)]-D-Galp(β→4)}_n. The synthesis presented three significant synthetic challenges namely: the L-Rhap(β→4)-D-Glcp bond, the Neu5Ac(α2→3)-D-Galp bond and 3,4-D-Galp branching. The L-Rhap bond was constructed in 60% yield (α:β 1:1.2) using 4-O-acetyl-2,3-di-O-benzoyl-α-L-rhamnopyranosyl bromide 6 as donor, silver silicate as promotor and 6-O-benzyl-2,3-di-O-benzoyl-1-thio-β-D-glucopyranoside as acceptor to yield disaccharide 18. The Neu5Ac(α2→3) linkage was synthesized in 66% yield using methyl [phenyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-D-glycero-D-galacto-nonulopyranosid]onate as donor and triol 2-(trimethylsilyl) ethyl 6-O-benzyl-β-D-galactopyranoside as acceptor to give disaccharide 21. The 3,4-D-Galp branching was achieved by regioselective glycosylation of disaccharide diol 21 by disaccharide 18 in 28% yield to give protected tetrasaccharide 22. Tetrasaccharide 22 was deprotected to give as its 2-(trimethylsilyl)ethyl glycoside the title compound 1a. In addition the 2-(trimethylsilyl)ethyl group was cleaved and the tetrasaccharide coupled by glycosylation (via tetrasaccharide trichloroacetimidate) to a linker suitable for conjugation.

     

SYNTHESIS OF A XYLOSYLATED RHAMNOSE PENTASACCHARIDE—THE REPEATING UNIT OF THE O-SPECIFIC SIDE CHAIN OF LIPOPOLYSACCHARIDES FROM THE REFERENCE STRAINS FOR Stenotrophomonas maltophilia SEROGROUP O18

A xylosylated rhamnose pentasaccharide, α- l-Rha p-(1→3)-[β- l-Xyl p-(1→2)-] [β- l-Xyl p-(1→4)-]α- l-Rha p-(1→3)- l-Rha p, the repeating unit of the O-specific side chain of the lipopolysaccharides from the reference strains for Stenotrophomonas maltophilia serogroup O18, was synthesized by a highly regio- and stereoselective procedure. Thus coupling of methyl rhamnopyranoside (9) with 2,3,4-tri- O-acetyl-α- l-rhamnopyranosyl trichloroacetimidate (8) gave the (1→3)-linked disaccharide (10), and subsequent benzoylation and deacetylation afforded the disaccharide acceptor 12. Condensation of 12 with 8 yielded methyl 2,3,4-tri- O-acetyl-α- l-rhamnopyranosyl-(1→3)-α- l-rhamnopyranosyl-(1→3)-2,4-di- O-benzoyl-α- l-rhamnopyranoside (13). Coupling of 13 with 2,3,4-tri- O-benzoyl-α- l-xylopyranosyl trichloroacetimidate (4) followed by deprotection gave the target pentasaccharide (15).     

Structure Studies of the Extracellular Polysaccharide from <Emphasis Type="Italic">Trichoderma</Emphasis> sp. KK19L1 and Its Antitumor Effect via Cell Cycle Arrest and Apoptosis

An extracellular polysaccharide TP1A was purified from the fermented broth of Trichoderma sp. KK19L1 by combination of Q Sepharose fast flow and Sephacryl S-300 chromatography. TP1A was composed of Man, Gal, and Glc in a molar ratio of about 3.0:5.1:8.1. The molar mass of TP1A was about 40.0 kDa. Methylation and NMR analysis indicated that the probable structure of TP1A was [→4,6)-α-D-Glcp(1→6)-β-D-Galf(1→6)-β-D-Galf(1→2,6)-β-D-Galf(1→2,6)-β-D-Galf(1→2,6)-β-D-Galf(1→2,6)-α-D-Manp(1→2,6)-α-D-Manp(1→] with [α-D-Glcp(1→] and [α-D-Manp(1→6)-α-D-Glcp(1→6)-α-D-Glcp(1→] as branches. The antitumor study showed that TP1A was able to inhibit the cell viability of HeLa and MCF-7 cells. TP1A could arrest HeLa cells in G2/M phase and induce HeLa cell apoptosis. These findings suggest that fungal polysaccharides could be a potential source for antitumor agents.     

A FACILE SYNTHESIS OF MANNOSE TRI- AND TETRASACCHARIDE REPEATING UNITS OF FUNGAL CELL-WALL POLYSACCHARIDE FROM MICROSPORUM AND TRYCHOPHYTON SPECIES

ABSTRACT

A facile synthesis of the trisaccharide α-D-mannopyranosyl-(1→2)-α-D-mannopyranosyl-(1→6)-α-D-mannopyranose and the tetrasaccharide α-D-mannopyranosyl-(1→2)-α-D-mannopyranosyl-(1→6)-α-D-mannopyranosyl-(1→6)-D-mannopyranose, the repeating units of fungal cell-wall polysaccharide from Microsporum gypseum and Trychophyton, was achieved using α-(1→2)-linked disaccharide imidate as the donor. The disaccharide imidate was prepared from the self-condensation of 3,4,6-tri-O-benzoyl-1,2-O-allyloxyethylidene-β-D-mannopyranose.     

Synthesis of a Tri- and Tetrasaccharide Fragment Specific for the Shigella flexneri Serotype 5a O-Antigen. A Reinvestigation

ABSTRACT

Stereocontrolled, stepwise synthesis of methyl α-L-rhamnopyranosyl-(1→2)-[α-D-glucopyranosyl-(1→3)]-α-L-rhamnopyranoside (A(E)B, 1) and methyl 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→2)-α-L-rhamnopyranosyl-(1→2)-[α-D-glucopyranosyl-(1→3)]-α-L-rhamnopyranoside (DA(E)B, 2) is described; these constitute the methyl glycosides of fragments of the O-specific polysaccharide of Shigella flexneri serotype 5a. Two routes to trisaccharide 1 were considered. Route 1 involved the coupling of a precursor to residue A and a disaccharide EB, whereas route 2 was based on the condensation of a precursor to residue E and a disaccharide AB. Rather surprisingly, the latter afforded the β-anomer of 1, namely methyl α-L-rhamnopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)]-α-L-rhamnopyranoside as the major product. Route 1 was preferred. Overall, several observations made during this study suggested that, for the construction of higher fragments, a suitable precursor to rhamnose A would require protecting groups of low bulkiness at position 3 and 4. Therefore, the 2-O-acetyl-3,4-di-O-allyl-α-L-rhamnopyranosyl trichloroacetimidate (35) was the precursor of choice to residue A in the synthesis of the tetrasaccharide 2. The condensation product of 35 and methyl 2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl-4-O-benzyl-α-L-rhamnopyranoside was selectively deacylated and condensed to 2-trichloroacetamido-3,4,6-tri-O-acetyl-2-deoxy-α-D-glucopyranosyl trichloroacetimidate to afford the corresponding fully protected tetrasaccharide 45. Controlled stepwise deprotection of the latter proceeded smoothly to afford the target 2. It should be emphasised that the preparation of 45 was not straightforward, several donors and coupling conditions that were tested resulted only in the complete recovery of the acceptor. Distortion of several signals in the ¹³C NMR spectra of the fully or partially protected tetrasaccharide intermediates suggested that steric hindrance, added to the known low reactivity of HO-2 of rhamnosyl acceptors, probably played a major role in the outcome of the glycosidation attempts.     

Synthesis of Rhamnogalacturonan I Fragments

ABSTRACT

The partially deprotected trisaccharide 17 has been synthesized as an analogue of the repeating unit of the backbone of rhamnogalacturonan I. The trisaccharide 17 was obtained after prior selective derivatization of HO-3 and HO-4 of a rhamnopyranose cyanoethylidene glycosyl donor, followed by coupling with a tritylated galactopyranosyluronic acceptor (11), selective removal of the acetyl group at the O-2' position of the formed disaccharide 12, and glycosylation of the HO-2' position with methyl (ethyl 2,3-di-O-benzyl-4-O-p-methoxybenzyl-1-thio-β-D-galactopyranosid)uronate (14) providing methyl (methyl 2,3-di-O-benzyl-4-O-p-methoxybenzyl-α-D-galactopyranosyluronate)-(1→2)-(4-O-benzoyl-3-O-benzyl-α-L-rhamnopyranosyl)-(1→4)-(allyl 2,3-di-O-benzyl-β-D-galactopyranosid)uronate (15). Finally, palladium chloride catalyzed deallylation (16) and hydrogenolysis over Pd-C resulted in methyl (methyl α-D-galactopyranosyluronate)-(1→2)-(4-O-benzoyl-α-L-rhamnopyranosyl)-(1→4)-α/β D-galactopyranuronate (17).     

The Synthesis of Four Dideoxygenated Analogues of β-Maltosyl-(1→4)-Trehalose

Abstract

Four derivatives of β-maltosyl-(1→4)-trehalose were prepared, each with two deoxy functions in one of the constitutive disaccharide building blocks. 2,3-Di-O-acetyl-4,6-dideoxy-4,6-diiodo-α-D-galactopyranosyl- (1→4) ?1,2,3,6-tetra-O-acetyl-D-glucopyranose (3) was employed as a precursor for the 4?,6?-dideoxygenated tetrasaccharide 9: coupling of 3 with 2,3,6-tri-O-benzyl-α-D-glucopyranosyl 2,3,6-tri-O-benzylidene-α-D-glucopyranoside (4) furnished the tetrasaccharide 5 which was deiodinated and deprotected to yield the target tetrasaccharide 9. Secondly, the dideoxygenated maltose derivative 3-deoxy-4,6-O-isopropylidene-2-O-pivaloyl-β-D-glucopyranosyl- (1→4) ?1,6-anhydro-3-deoxy-2-O-pivaloyl-β-D-glucopyranose (10) was ring-opened to the anomeric acetate 11. A [2+2] block synthesis with 4 in TMS triflate mediated glycosylation gave a tetrasaccharide which was deprotected to the 3″,3?-dideoxygenated analogue of β-maltosyl-(1→4)-trehalose. For the third tetrasaccharide, 2,3,2″,3′-tetra-O-benzyl-α,α-trehalose was iodinated at the primary positions and deiodinated in the presence of palladium-on-carbon, then this acceptor was selectively glycosylated with hepta-O-acetyl-maltosyl bromide (20). Removal of protective groups furnished the maltosyl trehalose tetrasaccharide deoxygenated at positions C-6 and C-6′. to prepare a 3,3′-dideoxygenated trehalose, the free hydroxyl groups of 2-O-benzyl-4,6-O-(R)-benzylidene-α-D-glucopyranosyl 2-O-benzyl-4,6-O-(R)-benzylidene-α-D-glucopyranoside (25) were reduced by Barton-McCombie deoxygenation. One of the benzylidene groups was opened reductively with sodium cyanoborohydride. The resulting free hydroxyl group at the 4′-position was glycosylated in a Koenigs-Knorr reaction with 20 to yield the 3,3′-dideoxygenated tetrasaccharide 32, the fourth target oligosaccharide, after deprotection.     

Structural elucidation and antioxidant activity of a novel heteroglycan from Tricholoma Lobayense

A novel heteroglycan TLH-G was successfully isolated from Tricholoma lobayense cultivated in south China. TLH-G was composed of five monosaccharides, namely, mannose, rhamnose, glucuronic acid, glucose and galactose, and consisted of →1)-β-D-Glcp-(6→ and →1)-α-D-Galp-(3→ motifs. TLH-G had excellent scavenging activities towards 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) radicals. Structure-activity relationship analysis demonstrated that glycosidic bond types, monosaccharide composition, and branching degree were more important for the antioxidant activity of polysaccharides from Tricholoma lobayense than other structural factors. All the above results showed that the growth environment played a crucial role in the structure and antioxidant activity of polysaccharides from Tricholoma lobayense.     

Synthesis of the Methyl Glycosides of a Di- and Two Trisaccharide Fragments Specific for the Shigella flexneri Serotype 2a O-Antigen

ABSTRACT

The stereocontrolled synthesis of methyl α-D-glucopyranosyl-(1→4)-α-L-rhamnopyranoside (EC, 1), methyl α-L-rhamnopyranosyl-(1→3)-[α-D-glucopyranosyl-(1→4)]-α-L-rhamnopyranoside (B(E)C, 3) and methyl α-D-glucopyranosyl-(1→4)-α-L-rhamnopyranosyl-(1→3)-2-acetamido-2-deoxy-β-D-glucopyranoside (ECD, 4) is described; these constitute the methyl glycosides of branched and linear fragments of the O-specific polysaccharide of Shigella flexneri serotype 2a. Emphasis was put on the construction of the 1,2-cis EC glycosidic linkage resulting in the selection of 2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl fluoride (8) as the donor. Condensation of methyl 2,3-O-isopropylidene-4-O-trimethylsilyl-α-L-rhamnopyranoside (11) and 8 afforded the fully protected αE-disaccharide 20, as a common intermediate in the synthesis of 1 and 3, together with the corresponding βE-anomer 21. Deacetalation and regioselective benzoylation of 20, followed by glycosylation with 2,3,4-tri-O-benzoyl-α-L-rhamnopyranosyl trichloroacetimidate (15) afforded the branched trisaccharide 25. Full deprotection of 20 and 25 afforded the targets 1 and 3, respectively. The corresponding βE-disaccharide, namely, methyl β-D-glucopyranosyl-(1→4)-α-L-rhamnopyranoside (βEC, 2) was prepared analogously from 21. Two routes to trisaccharide 4 were considered. Route 1 involved the coupling of a precursor to residue E and a disaccharide CD. Route 2 was based on the condensation of an appropriate EC donor and a precursor to residue D. The former route afforded a 1:2 mixture of the αE and βE condensation products which could not be separated, neither at this stage, nor after deacetalation. In route 2, the required αE-anomer was isolated at the disaccharide stage and transformed into 2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl-(1→4)-2,3-di-O-benzoyl-α-L-rhamnopyranosyl trichloroacetimidate (48) as the EC donor. Methyl 2-acetamido-2-deoxy-4,6-O-isopropylidene-β-D-glucopyran-oside (19) was preferred to its benzylidene analogue as the precursor to residue D. Condensation of 19 and 48 and stepwise deprotection of the glycosylation product afforded the target 4.