Synthesis of branched poly-N-acetyl-lactosamine type pentaantennary pentacosasaccharide: Glycan part of a glycosyl ceramide from rabbit erythrocyte membrane

Explorations into New Reaction Chemistry

This Review describes the basic concepts that have guided our exploration of new chemical reactions by giving examples of results from my research group. Our strategy of carrying out research is to investigate three to four different topics at a time so we can gather as many results as possible. These may at first appear unrelated to each other but may have the potential to be united into a greater hypothesis after repeated feedback. Three scenarios from our research are presented: the "oxidative-reductive condensation reaction" devised in 1960, which after an interval of nearly 40 years brought forth the new concept of using compounds of structure Ph(2)POR as reducing reagents; the "TiCl(4)-aldol reaction" of 1973 that eventually led to the present "base-promoted aldol reaction" through a chain of ideas; and the "glycosylation reaction using fluorosugars" from 1984 which recently bloomed into "stereocontrolled glycosylation". Thus, it can be said that by reviewing what we had done before, we were able to expand on it to achieve new outcomes.

Synthesis and characterization of a novel glycopolymer with protective activity toward human anti-alpha-Gal antibodies

An efficient and rapid synthesis of the derivative of the biocompatible polymer poly(styrene co-maleic acid) with Linear B disaccharide (Galili antigen) was achieved. The oligosaccharide portion was obtained by a transglycosylation reaction catalyzed by coffee bean alpha-D-galactosidase using p-nitrophenyl-alpha-D-galactopyranoside both as donor and as acceptor. The reaction was carried out in aqueous buffer without any organic cosolvent. The molar yield (30%) and the regioselectivity (82%) were significantly improved with respect to the data so far reported in the literature. The selective reduction of the p-nitrophenyl group afforded the p-aminophenyl derivative of Linear B disaccharide. Linkage of this derivative via an amidic bond to the poly(styrene co-maleic acid) was obtained by using N'-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide. The products were chemically characterized by ionspray mass spectrometry, infrared, (13)C- and (1)H-nuclear magnetic resonance. The glycopolymer specifically reacts with human serum containing antibodies and with a mixture of partially purified human IgG and IgM anti-Linear B. It efficiently protects pig kidney PK15 cells from cytotoxic effects of human serum.

Changing the donor cofactor of bovine alpha 1, 3-galactosyltransferase by fusion with UDP-galactose 4-epimerase. More efficient biocatalysis for synthesis of alpha-Gal epitopes

Two fusion enzymes consisting of uridine diphosphogalactose 4-epimerase (UDP-galactose 4-epimerase, EC ) and alpha1, 3-galactosyltransferase (EC ) with an N-terminal His(6) tag and an intervening three-glycine linker were constructed by in-frame fusion of the Escherichia coli galE gene either to the 3' terminus (f1) or to the 5' terminus (f2) of a truncated bovine alpha1, 3-galactosyltransferase gene, respectively. Both fusion proteins were expressed in cell lysate as active, soluble forms as well as in inclusion bodies as improperly folded proteins. Both f1 and f2 were determined to be homodimers, based on a single band observed at about 67 kDa in SDS-polyacrylamide gel electrophoresis and on a single peak with a molecular mass around 140 kDa determined by gel filtration chromatography for each of the enzymes. Without altering the acceptor specificity of the transferase, the fusion with the epimerase changed the donor requirement of alpha1, 3-galactosyltransferase from UDP-galactose to UDP-glucose and decreased the cost for the synthesis of biomedically important Galalpha1,3Gal-terminated oligosaccharides by more than 40-fold. For enzymatic synthesis of Galalpha1,3Galbeta1,4Glc from UDP-glucose and lactose, the genetically fused enzymes f1 and f2 exhibited kinetic advantages with overall reaction rates that were 300 and 50%, respectively, higher than that of the system containing equal amounts of epimerase and galactosyltransferase. These results indicated that the active sites of the epimerase and the transferase in fusion enzymes were in proximity. The kinetic parameters suggested a random mechanism for the substrate binding of the alpha1, 3-galactosyltransferase. This work demonstrated a general approach that fusion of a glycosyltransferase with an epimerase can change the required but expensive sugar nucleotide to a less expensive one.

Enzymatic synthesis of natural and 13C enriched linear poly-N-acetyllactosamines as ligands for galectin-1

As part of a study of protein-carbohydrate interactions, linear N-acetyl-polyllactosamines [Galbeta1,4GlcNAcbeta1,3]nwere synthesized at the 10-100 micromol scale using enzymatic methods. The methods described also provided specifically [1-13C]-galactose-labeled tetra- and hexasaccharides ([1-13C]-Galbeta1,4GlcNAcbeta1,3Galbeta1,4Glc and Galbeta1, 4GlcNAcbeta1,3[1-13C]Galbeta1,4GlcNAcbeta1,3Galbeta 1,4Glc) suitable for NMR studies. Two series of oligosaccharides were produced, with either glucose or N-acetlyglucosamine at the reducing end. In both cases, large amounts of starting primer were available from human milk oligosaccharides (trisaccharide primer GlcNAcbeta1,3Galbeta1, 4Glc) or via transglycosylation from N-acetyllactosamine. Partially purified and immobilized glycosyltransferases, such as bovine milk beta1,4 galactosyltransferase and human serum beta1,3 N- acetylglucosaminyltransferase, were used for the synthesis. All the oligo-saccharide products were characterized by1H and13C NMR spectroscopy and MALDI-TOF mass spectrometry. The target molecules were then used to study their interactions with recombinant galectin-1, and initial1H NMR spectroscopic results are presented to illustrate this approach. These results indicate that, for oligomers containing up to eight sugars, the principal interaction of the binding site of galectin-1 is with the terminal N-acetyllactosamine residues.

Solid-phase oligosaccharide synthesis and related technologies

Research efforts directed at the development of methodologies effective for solid-phase synthesis of oligosaccharides have resulted in a number of impressive achievements. In addition, closely related technologies, such as soluble polymer-supported synthesis and fluorous synthesis of the same class of molecules, have proved to be quite promising.

Improved enzymatic synthesis of a highly potent oligosaccharide antagonist of L-selectin

The polylactosamine sLex beta1-3'(sLex beta1-6')LacNAc beta1-3'(sLex beta1-6')LacNAc beta1-3'(sLex beta1-6')LacNAc (7) (where sLex is Neu5Ac alpha2-3Gal beta1-4(Fuc alpha1-3)GlcNAc and LacNAc is Gal beta1-4GlcNAc) is a nanomolar L-selectin antagonist and therefore a potential anti-inflammatory agent (Renkonen et al. (1997) Glycobiology, 7, 453). Here we describe an improved synthesis of 7. The octasaccharide LacNAc beta1-3'LacNAc beta1-3'LacNAc beta1-3'LacNAc (4) was converted into the triply branched undecasaccharide LacNAc beta1-3'(GlcNAc beta1-6')LacNAc beta1-3'(GlcNAc beta1-6')LacNAc beta1-3'(GlcNAc beta1-6')LacNAc (5) by incubation with UDP-GlcNAc and the midchain beta1,6-GlcNAc transferase activity of rat serum. Glycan 5 was enzymatically beta1,4-galactosylated to LacNAc beta1-3'(LacNAc beta1-6')LacNAc beta1-3'(LacNAc beta1-6')LacNAc beta1-3'(LacNAc beta1-6')LacNAc (6). Combined with the enzymatic conversion of 6 to 7 (Renkonen et al., loc. cit.) and the available chemical synthesis of 4, our data improve the availability of 7 for full assessment of its anti-inflammatory properties.

Chemoenzymatic synthesis of the branched oligosaccharides which correspond to the core structures of N-linked sugar chains

Synthetic routes are described to a partial structure common to all high mannose-type sugar chains and complex-type sugar chains based on a chemoenzymatic strategy which incorporates, (a) enzymatic synthesis of oligosaccharide blocks using glycosidases, and (b) chemical synthesis of the branching oligosaccharides via regioselective coupling. All reaction products correspond to key intermediates necessary for the construction of N-linked oligosaccharides and we have synthesized the branched tetra-manno-oligosaccharide high mannose-type sugar chain and the branched hexa-oligosaccharide complex-type sugar chain using this simple and direct method.

N-trichloroethoxycarbonyl-glucosamine derivatives as glycosyl donors

D-Glucosamine can be readily transformed into 1,3,4,6-tetra-O-acetyl-2-deoxy-2-(2,2,2-trichloroethoxycarbonylamino+ ++) -D-glucopyranose (2). From this intermediate valuable glycosyl donors can be obtained; reaction with ethanethiol in the presence of boron trifluoride etherate afforded ethyl 3,4,6-tri-O-acetyl-2-deoxy-1-thio-2-(2,2, 2-trichloroethoxycarbonylamino)-beta-D-glucopyranoside (4) which gave, upon N-acetylation, the N-acetyl-N-trichloroethoxycarbonyl derivative (5). Selective removal of the 1-O-acetyl group in 2 followed by treatment with trichloroacetonitrile in the presence of base afforded 3,4,6-tri-O-acetyl-2-deoxy-2-(2,2,2-trichloroethoxycarbonylamino)- alpha -D-glucopyranosyl trichloroacetimidate (6). Reaction of 5 with five selectively protected glycosides as glycosyl acceptors in the presence of N-iodosuccinimide/trifluoromethanesulfonic acid as the promoter system furnished the corresponding beta-glycosides in good yields, thus exhibiting the valuable glycosyl donor properties of 5. Reductive removal of the trichloroethoxycarbonyl (Teoc) group afforded the corresponding N-acetyl-protected saccharides in high yields. The imidate 6 reacted with three of the above acceptors in the presence of catalytic amounts of trimethylsilyl trifluoromethanesulfonate to give the beta-linked disaccharides in even better yields. The direct replacement of the N-Teoc group by the N-acetyl group using zinc/acetic anhydride, via the free amines as transient intermediates, adds to the high efficiency and convenience of this methodology.

Stereocontrolled syntheses of O-glycans of core class 2 with a linear tetrameric lactosamine chain and with three lactosamine branches

Synthetic routes to O-glycans of core class 2 with a linearly extended tetralactosamine moiety and with three lactosamine branches are discussed. By a glycosyl fluoride-method, the mono- and the di-lactosaminyl units were attached to the core disaccharide (Gal beta 1-->3GalN3) derivatives in high stereoselectivity.

N-Tetrachlorophthaloyl (TCP) for ready protection/deprotection of amino sugar glycosides

The tetrachlorophthaloyl (TCP) group can be utilized when imidic protection of an amine is desired and durability of the protecting group to conditions ranging from mildly basic to harshly acidic is required. Installation can be accomplished in two steps by treating the free base with the commercially available TCP anhydride, and then closing the imidic ring with acetic anhydride and pyridine. Cleavage is effected by 2-4 eq of ethylenediamine under very mild conditions under which esters and glycopeptides have been shown to be stable, and racemization of amino acid residues does not occur. Unsubstituted phthalimides, even within the same molecule, are also unaffected during TCP cleavage. TCP protecting groups serve as beta-directors on donors and can also be present on acceptor species during electrophilic couplings in oligosaccharide synthesis.

Glycosylation reactions--present status future directions

A short review of the present status of glycosylation reactions is presented. The reactivity of both proven and newer glycosylation methods are briefly discussed. Emphasis is placed on the control of stereochemistry and regiochemistry. As well, the identification and avoidance of side reactions is covered. Polymer-supported synthesis of oligosaccharides is noted as a promising direction for eliminating some of the problems associated with purification. It is suggested that a better understanding of the mechanism of glycosylation reactions is necessary for future improvements to stereoselectivity and regioselectivity. A key advance would be methods for enhancing the reactivity of weakly nucleophilic hydroxyls.