Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

The first single-step random-glycosylation methodology for fully unprotected glycosyl acceptors is reported by random glycosylation leading to all possible regioisomers. For such systems conventional glycosylation methods such as Koenigs–Knorr glycosylation, Schmidt's trichloroacetimidate glycosylation and reactions employing glycosyl fluoride donors fail entirely. Starting from unprotected nonreducing saccharides, the glycosylation of β-glucosylated and β-galactosylated monosaccharides (Glc, Gal), symmetric disaccharides (e.g. α,α-trehaloses) as well as unsymmetric disaccharides (e.g. sucrose) were studied. The influence of base type and concentration were examined. Several libraries of di- and trisaccharides were generated. All regioisomers were formed in approximately equal proportions, and their partial separation was achieved by flash column chromatography. Even though it appears that overall yields are lower when comparing to classical protecting-group chemistry, this synthetic effort may be superior especially for access to higher saccharides. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Recombinant transsialidase from Trypanosoma cruzi (TcTS) was used for the sialylation with natural and non-natural derivatives of neuraminic acid. Neu5Ac-α(23)-Gal-β(16)-Glc-αOMe was prepared in 80 % yield. Correspondingly, the modified trisaccharide derivatives Neu5Prop-α(23)-Gal-β(16)-Glc-αOMe (32 %) and Neu5Gc-α(23)-Gal-β(16)-Glc-αOMe (Prop=propanoyl, Gc=glycolyl) were obtained in 60 % yield, respectively.

Snails from the family Helicidae produce in their albumen glands a highly branched galactan, which consists almost exclusively of D- and L-galactose. The D-Gal residues are glycosydically β(16)- or β(13)-linked, whereas the L-Gal moieties are attached α(12). Up until the present time, two β(16)-D-galactosyl transferases and one α(12)-L-galactosyl transferase have been identified in a membrane preparation of these glands. These were used to synthesise various oligosaccharides by successive addition of the NDP-activated (NDP=nucleoside-5′-diphosphate) D-Gal or L-Fuc moieties, up to a heptasaccharide by starting from the disaccharide D-Gal-β(13)-D-Gal-β(1OMe. Even larger oligosaccharides up to a tridecasaccharide were obtained by starting with the hexasaccharide D-Gal-[β(13)-D-Gal]₄-β(14)-D-Glc as an acceptor substrate. This tandem exploitation process has high potential for the easy introduction of D-Gal and L-Fuc residues into a great variety of oligosaccharides, which can be used in ligand/acceptor studies.

Sialyloligosaccharides of the type Neu5 Acα2-3Galβ1-3GalNAc and a range of corresponding motifs play an important role in nature and are found in gangliosides, Lewis type I structures and the sialyl-Thomsen–Friedenreich antigen occurring in higher animals, viruses, bacteria, protozoa and pathogenic fungi. There is considerable interest to evaluate the significant functionalities of these glycostructures, and here we present a chemoenzymatic approach by a facile synthesis of such motifs. Employing chemoenzymatic methods, several modified Galβ1-3GalNAc derivatives were synthesized. Modifications were introduced at the stage of the monomeric building blocks prior to formation of the disaccharides by means of four different β-galactosidases from bovine testes, Bacillus circulans and Xanthomonas manihotis as well as the phosphorylase from Bifidobacterium bifidum. Finally, the modified disaccharide derivatives could be efficiently sialylated using the recombinant trans-sialidase from Trypanosoma cruzi.

Atrans-sialidase from Trypanosoma cruzi was used to prepare several sialic acid-containing oligosaccharides with a truncated glycerol side chain. STD NMR was used to characterise the binding constant and binding epitope of the oligosaccharides to their neural receptor protein MAG.

Carbohydrate-modified polysiloxanes have been presented several times within the last decade. In this work, a new route to carbohydrate-segmented polysiloxanes is presented. A series of allyl-group-containing bifunctional carbohydrate derivatives was synthesized and reacted with hydrodimethylsilyl-terminated polysiloxane in hydrosilylation reactions with Speier's catalyst. The carbohydrate monomers and the resulting materials were fully characterized with ¹H and ¹³C NMR spectroscopy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3814–3822, 2005

Summary: In the 1990's, carbohydrate modified silicones were developed as a novel class of hydrophilic modified silicones which are now ready for marketing and industrially available, mainly as ingredients of cosmetic formulations. This report focuses on the synthesis of α,ω-carbohydrate modified linear poly(dimethylsiloxane)s by hydrosilylation reactions. Thus, a series of carbohydrate derivatives was synthesised and reacted with a hydrodimethylsilyl-terminated poly(dimethylsiloxane) applying Speier's catalyst. The resulting oils were characterised by MALDI TOF MS and NMR spectroscopy. Deprotection gave a series of novel amphiphilic modified silicones, which were more viscous than their protected analogues, due to hydrogen bonding.

The focus of this work is the synthesis of bidesmosidic saponin mimetics. Therefore, dihydrodiosgenin derivatives, which differ from the natural compounds by reduction of the 22-(hemi) acetal were used as glycosyl acceptors. In preliminary studies, the dihydrodiosgenin glycosides 16, 17 and 19, as well as trisaccharide 22, were synthesized. The acceptors 10 and 14 were subjected to DMTST-mediated glucosylation for the synthesis of the chacotriose-substituted compound 3. For a selective 2,4-di-rhamnosylation of the dihydrodiosgenin glucopyranoside, differentiation of the glucose OH groups was achieved by selective benzoylation with 1-(benzoyloxy)benzotriazole. Reaction of the 3,6-di-O-benzoate 32 with the perbenzoylated ethyl thiorhamnopyranoside donor 15 gave the 2,4-di-rhamnosylated compound 33, together with the mono-rhamnosylated derivative. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

The synthesis of novel polyurethanes and polyureas based on modified glycosylamines and glucosamines has been successfully accomplished by catalytic polymerizations. Several modified glucosamine monomers were synthesized to study their reactivities during these polymerization reactions. It was found that the anomeric hydroxyl groups are more reactive than the amino groups. The resulting polymers were characterized by NMR and IR spectroscopy, elementary analysis, viscosimetry, and gel permeation chromatography. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2332–2341, 2001

Galβ14 disaccharide structures are vital core units of the oligosaccharide components of glycoconjugates. β-Galactosidase from Bacillus circulans (E.C.3.2.1.23) catalyses the transfer of galactose from a donor structure such as GalβOpNP to various GlcNAc and galactose derivatives, forming β14 linkages. The synthesis of several biologically relevant disaccharides {Galβ14GlcNAcαOAll (3), Galβ14GlcNAcβOAll (5), Galβ14GalαOAll (10), Galβ14GalβOAll (12), Galβ14GalβSPh (14), Galβ14GalβOpNP (16) and the trisaccharide Galβ14Galβ14GalβOpNP (18)} has been achieved in 30–66 % yield by application of this enzyme.

A combined sequential use of β-galactosidase from bovine testes together with α2–3-sialyltransferase from porcine liver including cofactor regeneration transforms GalNAcα1–OThr by a multistep one-pot reaction into the sialylated Thomsen–Friedenreich antigen determinant Neu5Acα2–3Galβ1–3GalNAcα1-OThr.

Several 4- protected D-glucopyranosides were synthesized and cleaved by sodium metaperiodate. Depending on the protection pattern the resulting dialdehydes showed different types of structures. Predominantly dioxane structures of the hemiacetal or hemialdal type were obtained with preferred trans orientation of the exo oxygen groups at C-1 and C-2. The dialdehyde derived from methyl 4-O-methyl-α-D-glucopyranoside was treated with various C-nucleophiles. A higher reactivity of the aldehyde function at the former C-3 in comparison to C-2 and a special stability of dioxane type structures for glycol cleavage dialdehydes derived was observed.

The 2-azidofucosyl donor 4 was synthesised via a azidonitratisation and stereoselectively α-linked to the benzyl fucoside acceptor 5 by methyl triflate catalysis. The resulting disaccharide 6 could be transformed to the corresponding acetamido derivative and deprotected to 2′-(acetamido)fucobioside 10