Discrimination of sulfated isomers of chondroitin sulfate disaccharides by HILIC-MS.

Chondroitin sulfate (CS) glycosaminoglycans are biologically active sulfated polysaccharides that pose an analytical challenge for their structural analysis and functional evaluation. In this study, we developed a hydrophilic interaction liquid chromatography separation method and its on-line coupling to mass spectrometry (MS) allowing efficient differentiation and sensitive detection of mono-, di-, and trisulfated CS disaccharides and their positional isomers, without requiring prior derivatization. The composition of the mobile phase in terms of pH and concentration showed great influence on the chromatographic separation and was varied to allow the distinction of each CS without signal overlap for a total analysis time of 25 min. This methodology was applied to determine the disaccharide composition of biological reaction media resulting from various enzymatic transformations of CS, such as enzymatic desulfation of CS disaccharides by a CS 4-O-endosulfatase, and depolymerization of the CS endocan by chondroitinase lyase ABC.

Isomer separation and effect of the degree of polymerization on the gas-phase structure of chondroitin sulfate oligosaccharides analyzed by ion mobility and tandem mass spectrometry.

RATIONALE: Chondroitin sulfate (CS) glycosaminoglycans are bioactive sulfated polysaccharides comprising repeating units of uronic acid and N-acetyl galactose sulfated at various positions. The optimal length and sulfation pattern of the CS bioactive sequences remain elusive so that structure-activity relationships cannot be easily established. Development of efficient analytical methods allowing the differentiation of the various sulfation patterns of CS sequences is therefore of particular importance to correlate their biological functions to the sulfation pattern. METHODS: Discrimination of different oligomers (dp2 to dp6) of synthetic chondroitin sulfate isomers was evaluated by electrospray ionization tandem mass spectrometry (ESI-MS/MS) in the negative-ion mode from deprotonated and alkali adduct species. In addition, ion mobility mass spectrometry (IMS-MS) was used to study the influence of both the degree of polymerization and sulfate group location on the gas-phase conformation of CS oligomers. RESULTS: ESI-MS/MS spectra of chondroitin sulfate isomers show characteristic product ions exclusively from alkali adduct species (Li, Na, K and Cs). Whatever the alkali adducts studied, MS/MS of chondroitin oligosaccharides sulfated at position 6 yields a specific product ion at m/z 139 while CS oligosaccharides sulfated at position 4 show a specific product ion at m/z 154. Being observed for the different CS oligomers di-, tetra- and hexasaccharides, these fragment ions are considered as diagnostic ions for chondroitin 6-O-sulfate and chondroitin 4-O-sulfate, respectively. IMS-MS experiments reveal that collision cross-sections (CCS) of CS oligomers with low charge states evolved linearly with degrees of polymerization indicating a similar gas-phase conformation. CONCLUSIONS: This study allows the fast and unambiguous differentiation of CS isomers sulfated at position 6 or 4 for both saturated and unsaturated analogues from MS/MS experiments. In addition, the CCS linear evolution of CS oligomers in function of the degree of polymerization indicates that no folding occurs even for hexasaccharides.

Synthesis of a library of variously modified 4-methylumbelliferyl xylosides and a structure-activity study of human ß4GalT7.

Proteoglycans (PGs) are complex macromolecules that are composed of glycosaminoglycan (GAG) chains covalently attached to a core protein through a tetrasaccharide linker. The biosynthesis of PGs is complex and involves a large number of glycosyltranferases. Here we present a structure-activity study of human ß4GalT7, which transfers the first Gal residue onto a xyloside moiety of the linkage region. An efficient and regiocontrolled synthesis of a library of modified analogs of 4-methylumbelliferyl xyloside (XylMU) is reported herein. Hydroxyl groups at the position C-2, C-3 or C-4 have been epimerized and/or replaced by a hydrogen or a fluorine, while the anomeric oxygen was replaced by either a sulfur or a sulfone. The effect of these compounds on human ß4GalT7 activity in vitro and on GAG biosynthesis in cellulo was then evaluated.

Probing the acceptor active site organization of the human recombinant ß1,4-galactosyltransferase 7 and design of xyloside-based inhibitors.

Among glycosaminoglycan (GAG) biosynthetic enzymes, the human ß1,4-galactosyltransferase 7 (hß4GalT7) is characterized by its unique capacity to take over xyloside derivatives linked to a hydrophobic aglycone as substrates and/or inhibitors. This glycosyltransferase is thus a prime target for the development of regulators of GAG synthesis in therapeutics. Here, we report the structure-guided design of hß4GalT7 inhibitors. By combining molecular modeling, in vitro mutagenesis, and kinetic measurements, and in cellulo analysis of GAG anabolism and decorin glycosylation, we mapped the organization of the acceptor binding pocket, in complex with 4-methylumbelliferone-xylopyranoside as prototype substrate. We show that its organization is governed, on one side, by three tyrosine residues, Tyr(194), Tyr(196), and Tyr(199), which create a hydrophobic environment and provide stacking interactions with both xylopyranoside and aglycone rings. On the opposite side, a hydrogen-bond network is established between the charged amino acids Asp(228), Asp(229), and Arg(226), and the hydroxyl groups of xylose. We identified two key structural features, i.e. the strategic position of Tyr(194) forming stacking interactions with the aglycone, and the hydrogen bond between the His(195) nitrogen backbone and the carbonyl group of the coumarinyl molecule to develop a tight binder of hß4GalT7. This led to the synthesis of 4-deoxy-4-fluoroxylose linked to 4-methylumbelliferone that inhibited hß4GalT7 activity in vitro with a Ki 10 times lower than the Km value and efficiently impaired GAG synthesis in a cell assay. This study provides a valuable probe for the investigation of GAG biology and opens avenues toward the development of bioactive compounds to correct GAG synthesis disorders implicated in different types of malignancies.

Efficient and stereocontrolled synthesis of chondroitin mono- and disaccharide linked to variously sulfated biotinylated trisaccharides of the linkage region of proteoglycans.

Efficient and stereocontrolled preparation of a library of variously sulfated biotinylated tetra- and pentasaccharides possessing the backbone of the partial linkage region plus the first chondroitin sulfate mono- or disaccharide unit (d-GlcA)n-ß-d-(1,3)-GalNAc-ß-d-(1,4)-GlcA-ß-d-(1,3)-Gal-ß-d-(1,3)-Gal (n = 0 or 1) is reported herein for the first time. The synthesis of these compounds was achieved using common key intermediates and a disaccharide building block obtained by semisynthesis. Stereoselective glycosylation, selective protection/deprotection steps, efficient reduction of the N-trichloroacetyl group into the corresponding N-acetyl group, efficient sulfation strategy, deprotection and biotinylation afforded target oligomers in good yield with high purity.

A cationic motif upstream Engrailed2 homeodomain controls cell internalization through selective interaction with heparan sulfates.

Engrailed2 (En2) is a transcription factor that transfers from cell to cell through unconventional pathways. The poorly understood internalization mechanism of this cationic protein is proposed to require an initial interaction with cell-surface glycosaminoglycans (GAGs). To decipher the role of GAGs in En2 internalization, we have quantified the entry of its homeodomain region in model cells that differ in their content in cell-surface GAGs. The binding specificity to GAGs and the influence of this interaction on the structure and dynamics of En2 was also investigated at the amino acid level. Our results show that a high-affinity GAG-binding sequence (RKPKKKNPNKEDKRPR), upstream of the homeodomain, controls En2 internalization through selective interactions with highly-sulfated heparan sulfate GAGs. Our data underline the functional importance of the intrinsically disordered basic region upstream of En2 internalization domain, and demonstrate the critical role of GAGs as an entry gate, finely tuning homeoprotein capacity to internalize into cells.

Chondroitin sulfate N-acetylgalactosaminyltransferase-1 (CSGalNAcT-1) involved in chondroitin sulfate initiation: Impact of sulfation on activity and specificity.

Glycosaminoglycan (GAG) assembly initiates through the formation of a linkage tetrasaccharide region serving as a primer for both chondroitin sulfate (CS) and heparan sulfate (HS) chain polymerization. A possible role for sulfation of the linkage structure and of the constitutive disaccharide unit of CS chains in the regulation of CS-GAG chain synthesis has been suggested. To investigate this, we determined whether sulfate substitution of galactose (Gal) residues of the linkage region or of N-acetylgalactosamine (GalNAc) of the disaccharide unit influences activity and specificity of chondroitin sulfate N-acetylgalactosaminyltransferase-1 (CSGalNAcT-1), a key glycosyltransferase of CS biosynthesis. We synthesized a series of sulfated and unsulfated analogs of the linkage oligosaccharide and of the constitutive unit of CS and tested these molecules as potential acceptor substrates for the recombinant human CSGalNAcT-1. We show here that sulfation at C4 or C6 of the Gal residues markedly influences CSGalNAcT-1 initiation activity and catalytic efficiency. Kinetic analysis indicates that CSGalNAcT-1 exhibited 3.6-, 1.6-, and 2.2-fold higher enzymatic efficiency due to lower K(m) values toward monosulfated trisaccharides substituted at C4 or C6 position of Gal1, and at C6 of Gal2, respectively, compared with the unsulfated oligosaccharide. This highlights the critical influence of Gal substitution on both CSGalNAcT-1 activity and specifity. No GalNAcT activity was detected toward sulfated and unsulfated analogs of the CS constitutive disaccharide (GlcA-ß1,3-GalNAc), indicating that CSGalNAcT-1 was involved in initiation but not in elongation of CS chains. Our results strongly suggest that sulfation of the linkage region acts as a regulatory signal in CS chain initiation.

Development of a mouse monoclonal antibody against the chondroitin sulfate-protein linkage region derived from shark cartilage.

Glycosaminoglycans (GAGs) like chondroitin sulfate (CS) and heparan sulfate (HS) are synthesized on the tetrasaccharide linkage region, GlcAbeta1-3Galbeta1-3Galbeta1-4Xylbeta1-O-Ser, of proteoglycans. The Xyl can be modified by 2-O-phosphate in both CS and HS, whereas the Gal residues can be sulfated at C-4 and/or C-6 in CS but not in HS. To study the roles of these modifications, monoclonal antibodies were developed against linkage glycopeptides of shark cartilage CS proteoglycans, and one was characterized in detail. This antibody bound hexa- and pentasaccharide-peptides more strongly than unsaturated tetrasaccharide-peptides with the unnatural fourth sugar residue (unsaturated hexuronic acid), suggesting the importance of the fifth and/or fourth saccharide residue GalNAc-5 and/or GlcA-4. Its reactivity was not affected by treatment with chondro-4-sulfatase or alkaline phosphatase, suggesting that 4-O-sulfate on the Gal residues and 2-O-phosphate on the Xyl residue were not recognized. Treatment with weak alkali to cleave the Xyl-Ser linkage completely abolished the binding activity, suggesting the importance of the peptide moiety of the hexasaccharide-peptide for the binding. Based on the amino acid composition and matrix-assisted laser desorption ionization time-of-flight mass spectrometry analyses, it was revealed that the peptide moiety is composed of four amino acids, Ser, Pro, Gly, and Glu. Furthermore, the antibody stained wild-type CHO cells significantly, but much weakly mutant cells deficient in xylosyl- or galactosyltransferase-I required for the biosynthesis of the linkage region. These results suggest that the antibody recognizes the structure GalNAc(+/-6-O-sulfate)-GlcA-Gal-Gal-Xyl-Ser-(Pro, Gly, Glu). The antibody will be a useful tool for investigating the significance of the linkage region in the biosynthesis and/or intracellular transport of different GAG chains especially since such tools to study the linkage region are lacking.

A versatile strategy to synthesize N-methyl-anthranilic acid-labelled glycoprobes for fluorescence-based screening assays.

Here we propose a general strategy to label carbohydrates with N-methyl-anthranilic acid at the anomeric position. Through two examples, we demonstrate that the generated glycoprobes are suitable for fluorescence-based binding/competition assays. Our approach is expected to readily generate series of glycoprobes dedicated to screening assays for the discovery of drugs targeting carbohydrate-protein interactions.

Matrix-assisted laser desorption/ionization mass spectrometric analysis of polysulfated-derived oligosaccharides using pyrenemethylguanidine.

A better understanding of the biological roles of carbohydrates requires the use of tools able to provide efficient and rapid structural information. Unfortunately, highly acidic oligomers-such as polysulfated oligosaccharides-are very challenging to characterize because of their high polarity, structural diversity, and sulfate lability. These features pose special problems for matrix-assisted laser desorption/ionization mass spectrometric (MALDI-MS) analysis because polysulfated carbohydrates exhibit poor ionization efficiency and usually do not produce any signal. The present report demonstrates how MALDI-MS can be used to derive structural and compositional information from pure and mixed fractions of polysulfated oligosaccharides. Indeed, pyrenemethylguanidine (pmg, a derivatizing agent and ionization efficiency enhancer) was used for the analysis of di- to decasaccharides, carrying from two to nine sulfate groups. The method is applied to various highly sulfated chondroitin and carrageenan oligosaccharides as well as to the analysis of mixtures of compounds. In the mass spectra, the observation of a unique pmg-complexed ladder of peaks in both ionization modes allows an easy and rapid determination of both the number of sulfate groups carried by the analyte and its molecular weight. Moreover, we have developed a software tool for the rapid and automatic structural elucidation of carrageenans based on the mass spectra obtained.

From polymer to size-defined oligomers: a step economy process for the efficient and stereocontrolled construction of chondroitin oligosaccharides and biotinylated conjugates thereof: part 1.

Controlled acid hydrolysis of polymeric chondroitin sulfate of bovine origin afforded in good yield a basic disaccharide fragment that was used for the first time as a starting material for the expeditious preparation of a set of building blocks that in turn act as versatile synthons for the efficient and stereocontrolled construction of a collection of size-defined chondroitin oligomers (from di- to octasaccharides). This step economy process allows their preparation as reducing species, fitted with a fluorophore, or as biotinylated conjugates; all useful tools for the preparation of microarrays, or as probes for the study of the biosynthesis of chondroitin sulfate.

From polymer to size-defined oligomers: a highly divergent and stereocontrolled construction of chondroitin sulfate A, C, D, E, K, L, and M oligomers from a single precursor: part 2.

An efficient, stereocontrolled, and highly divergent approach for the preparation of oligomers of chondroitin sulfate (CS) A, C, D, E, K, L, and M variants, starting from a single precursor easily obtained by semisynthesis from abundant natural polymer is reported for the first time. Common intermediates were designed that allowed the straightforward construction of O-sulfonated species either on the D-galactosamine unit (CS-A, -C, and -E) or on both D-glucuronic acid and D-galactosamine units (CS-D and CS-K, -L, and -M). This strategy represents a successful improvement and brings a definitive answer toward the synthesis of such complex molecules with numerous relevant biological functions.

Hyaluronidase reaction kinetics evaluated by capillary electrophoresis with UV and high-resolution mass spectrometry (HRMS) detection.

The biology of hyaluronidase activity on age related turnover of the hyaluronic acid (HA) in skin dermis and epidermis has not been established. Elucidation of this phenomenon enables discovery of novel compounds for skin health. As a simple and green technique, capillary electrophoresis (CE) was used for the first time for the determination of the kinetic constants (Km, Vmax and IC50) of the enzymatic degradation of HA. Reaction products were identified using CE/high-resolution mass spectrometry (HRMS) after appropriate optimization. Best results in terms of signal sensitivity were obtained using 10 mM ammonium acetate (pH 9.0) BGE, a sheath liquid composed of methanol-water (80:20, v/v) with 0.02% (v/v) formic acid at 10 µL min-1 and an ESI voltage at -4 kV. Km and Vmax were determined (n = 3) using CE/UV at 200 nm as 0.24 ± 0.02 mg mL-1 and 150.4 ± 0.1 nM s-1, respectively. They were also successfully obtained by CE/HRMS (n = 3) with Km of 0.49 ± 0.02 mg mL-1 and Vmax of 155.7 ± 0.2 nM s-1. IC50 of a standard natural inhibitor, epigallocatechin gallate, was also determined by CE-UV/HRMS. Kinetic constant values obtained by CE compared well with literature which validated the developed CE-based assay. In addition, the activity of homemade tetrasaccharides of biotinylated chondroitin sulfate CS-A or CS-C (4- or 6- sulfated in a homogeneous or heterogeneous way) on the hydrolysis reaction of hyaluronidase was evaluated. Hyaluronidase was mostly dose-dependently inhibited by CS-A tetrasaccharides sulfated in a homogeneous way. Two trisaccharides from truncated linkage region of proteoglycans were also tested as inhibitors or activators. CE-based assay showed that even a small modification of one hydroxyl group changes the influence on hyaluronidase activity. CE-based assay can be used for the screening of natural and synthetic inhibitors of hyaluronidase activity for cosmetic and therapeutic applications.

Tumor hypoxia modulates podoplanin/CCL21 interactions in CCR7+ NK cell recruitment and CCR7+ tumor cell mobilization.

Podoplanin (PDPN), an O-glycosylated, transmembrane, mucin-type glycoprotein, is expressed by cancer associated fibroblasts (CAFs). In malignant transformation, PDPN is subjected to changes and its role is yet to be established. Here we show that it is involved in modulating the activity of the CCL21/CCR7 chemokine/receptor axis in a hypoxia-dependent manner. In the present model, breast cancer MDA-MB-231 cells and NKL3 cells express the surface CCR7 receptor for CCL21 chemokine which is a potent chemoattractant able to bind to PDPN. The impact of the CCL21/CCR7 axis in the molecular mechanism of the adhesion of NKL3 cells and of MDA-MB-231 breast cancer cells was reduced in a hypoxic tumor environment. In addition to its known effect on migration, CCL21/CCR7 interaction was shown to allow NK cell adhesion to endothelial cells (ECs) and its reduction by hypoxia. A PDPN expressing model of CAFs made it possible to demonstrate the same CCL21/CCR7 axis involvement in the tumor cells to CAFs recognition mechanism through PDPN binding of CCL21. PDPN was induced by hypoxia and its overexpression undergoes a reduction of adhesion, making it an anti-adhesion molecule in the absence of CCL21, in the tumor. CCL21/CCR7 modulated NK cells/ECs and MDA-MB-231 cells/CAF PDPN-dependent interactions were further shown to be linked to hypoxia-dependent microRNAs as miRs: miR-210 and specifically miR-21, miR-29b which influence PDPN expression.

Phylogenetic and mutational analyses reveal key residues for UDP-glucuronic acid binding and activity of beta1,3-glucuronosyltransferase I (GlcAT-I).

The beta1,3-glucuronosyltransferases are responsible for the completion of the protein-glycosaminoglycan linkage region of proteoglycans and of the HNK1 epitope of glycoproteins and glycolipids by transferring glucuronic acid from UDP-alpha-D-glucuronic acid (UDP-GlcA) onto a terminal galactose residue. Here, we develop phylogenetic and mutational approaches to identify critical residues involved in UDP-GlcA binding and enzyme activity of the human beta1,3-glucuronosyltransferase I (GlcAT-I), which plays a key role in glycosaminoglycan biosynthesis. Phylogeny analysis identified 119 related beta1,3-glucuronosyltransferase sequences in vertebrates, invertebrates, and plants that contain eight conserved peptide motifs with 15 highly conserved amino acids. Sequence homology and structural information suggest that Y84, D113, R156, R161, and R310 residues belong to the UDP-GlcA binding site. The importance of these residues is assessed by site-directed mutagenesis, UDP affinity and kinetic analyses. Our data show that uridine binding is primarily governed by stacking interactions with the phenyl group of Y84 and also involves interactions with aspartate 113. Furthermore, we found that R156 is critical for enzyme activity but not for UDP binding, whereas R310 appears less important with regard to both activity and UDP interactions. These results clearly discriminate the function of these two active site residues that were predicted to interact with the pyrophosphate group of UDP-GlcA. Finally, mutation of R161 severely compromises GlcAT-I activity, emphasizing the major contribution of this invariant residue. Altogether, this phylogenetic approach sustained by biochemical analyses affords new insight into the organization of the beta1,3-glucuronosyltransferase family and distinguishes the respective importance of conserved residues in UDP-GlcA binding and activity of GlcAT-I.

Synthesis of a set of sulfated and/or phosphorylated oligosaccharide derivatives from the carbohydrate-protein linkage region of proteoglycans.

The synthesis of a set of various sulfoforms and/or phosphoforms as 7-methoxy-2-naphthyl glycosides of beta-D-Xylp, beta-D-Galp-(1-->4)-beta-D-Xylp, and beta-D-Galp-(1-->3)-beta-D-Galp-(1-->4)-beta-D-Xylp, structures encountered in the common carbohydrate-protein linkage region of proteoglycans, is reported for the first time. These molecules will serve as probes for systematic studies of the substrate specificity of the glycosyltransferases involved in the early steps of the biosynthesis of proteoglycans. A straightforward divergent preparation was achieved using key intermediates, which were designed as common precursors.

Tandem mass spectrometry for the characterisation of sulphated-phosphorylated analogues of the carbohydrate-protein linkage region of proteoglycans.

Carbohydrate-protein linkage region of proteoglycans is a key oligosaccharide structure because their sulphated and/or phosphorylated analogues control the biosynthesis of glucosaminoglycans or galactosaminoglycans. Therefore, synthesised sulphated and/or phosphorylated analogues were characterised by tandem mass spectrometry in the negative-ion mode. Results demonstrated that the product ion profile was characterised by glycosidic and cross-ring cleavages depending on the position and the type of the charged group (sulphate, phosphate or carboxylate). When the above compounds were sulphated and phosphorylated, the ion found at m/z 79 was the only one that demonstrated a phosphate group on the structure. The data also suggested that when a sodium cation was present in a sulphated and phosphorylated structure, the phosphate group in most cases was neutralised by the sodium cation, and therefore cleaved off the molecule, while the sulphate group was carrying the negative charge.

Stereocontrolled preparation of biotinylated chondroitin sulfate E di-, tetra-, and hexasaccharide conjugates.

The synthesis of biotinylated conjugates of oligomers of the basic repeating unit of chondroitin sulfate E (CS-E) with the sequence [GlcA-4,6-disulfated GalNAc]n is reported herein for the first time. An efficient and stereocontrolled preparation of di-, tetra-, and hexasaccharide derivatives was achieved using a common key disaccharide intermediate in an iterative way. An unexpected and never reported side reaction on the carbonyl group of the levulinate ester was observed during a coupling reaction. These complex molecules should be useful to study their interactions with various proteins.

An expeditious preparation of various sulfoforms of the disaccharide beta-D-Galp-(1-->3)-D-Galp, a partial structure of the linkage region of proteoglycans, as their 4-methoxyphenyl beta-D-glycosides.

An expeditious preparation of various sulfoforms of the disaccharide 4-methoxyphenyl O-(beta-D-galactopyranosyl)-(1-->3)-beta-D-galactopyranoside, namely the 4(I)- and 6(I)-sulfate, the 4(II)- and 6(II)-sulfate, and the 6(I),6(II)-disulfate derivatives, is reported for the first time. These molecules will be useful for the study of the early steps of the biosynthesis and sorting of proteoglycans. All target compounds were readily obtained from the common key intermediate 4-methoxyphenyl O-(2,3-di-O-benzoyl-4,6-di-O-levulinoyl-beta-D-galactopyranosyl)-(1-->3)-2-O-benzoyl-4,6-O-benzylidene-beta-D-galactopyranoside, easily prepared from the common starting material 4-methoxyphenyl 4,6-O-benzylidene-beta-D-galactopyranoside. Noticeable is the possible preparation of the different 6-O-sulfonated species through a one-pot procedure starting from a tetrol precursor.

An access to various sulfation patterns in dermatan sulfate: chemical syntheses of sulfoforms of trisaccharide methyl glycosides.

The syntheses are reported for the first time of alpha-L-IdopA2SO(3)-(1-->3)-beta-D-GalpNAc4SO(3)-(1-->4)-alpha-L-IdopA2SO(3)-(1-->OMe), its disulfated analogue alpha-L-IdopA2SO(3)-(1-->3)-beta-D-GalpNAc-(1-->4)-alpha-L-IdopA2SO(3)-(1-->OMe), and of beta-D-GalpNAc4SO(3)-(1-->4)-alpha-L-IdopA2SO(3)-(1-->3)-beta-D-GalpNAc4SO(3)-(1-->OMe), which represent structural fragments of dermatan sulfate, unavailable directly by chemical or enzymatic degradation of the glycosaminoglycan polymer. These molecules were readily obtained from a pair of key disaccharide intermediates, in which the relative difference of stability of the D-GalNAc 4-hydroxy protecting groups (acetate or pivalate) toward saponification conditions allowed access to various sulfoforms from a common precursor. For the preparation of these blocks, the 4-O-pivaloyl-D-galacto moiety was readily obtained through a one-pot stereospecific intramolecular nucleophilic displacement on an easily available 3-O-pivaloyl-D-gluco precursor, and the L-IdoA moiety through selective radical oxidation at C-6 of a L-ido 4,6-diol derivative with oxoammonium salts.