Study of glycosylation with N-trichloroacetyl-D-glucosamine derivatives in the syntheses of the spacer-armed pentasaccharides sialyl lacto-N-neotetraose and sialyl lacto-N-tetraose, their fragments, and analogues

The syntheses of 2-aminoethyl glycosides of the pentasaccharides Neu5Ac-alpha(2-->3)-Gal-beta(1-->4)-GlcNAc-beta(1-->3)-Gal-beta(1-->4)-Glc and Neu5Ac-alpha(2-->3)-Gal-beta(1-->3)-GlcNAc-beta(1-->3)-Gal-beta(1-->4)-Glc, their asialo di-, tri-, and tetrasaccharide fragments, and analogues included a systematic study of glycosylation with variously protected mono- and disaccharide donors derived from N-trichloroacetyl-D-glucosamine of galactose, lactose, and lactosamine glycosyl acceptors bearing benzoyl protection around the OH group to be glycosylated. Despite the low reactivity of these acceptors, stereospecificity and good to excellent yields were obtained with NIS-TfOH-activated thioglycoside donors of such type, or with AgOTf-activated glycosyl bromides, while other promotors, as well as a trichloroacetimidate donor, were less effective, and a beta-acetate donor was inactive. In NIS-TfOH-promoted glycosylation with the thioglycosides, the use of TfOH in catalytic amount led to rapid formation of the corresponding oxazoline, but the quantity of TfOH necessary for further efficient coupling with an acceptor depended on the reactivity of the donor, varying from 0.07 equiv for a 3,6-di-O-benzylated monosaccharide derivative to 2.1 equiv for a peracetylated disaccharide one. In the glycosylation products, the N-trichloroacetyl group was easily converted into N-acetyl by alkaline hydrolysis followed by N-acetylation.

An alpha-L-fucosidase from Thermus sp. with unusually broad specificity

An alpha-L-fucosidase (E.C. 3.2.1.51) exhibiting a wide aglycon specificity expressed in ability of cleaving alpha1 --> 6-, alpha1 -->3-, alpha1 --> 4-, and alpha1 --> 2-O-fucosyl bonds in fucosylated oligosaccharides, has been isolated from culture filtrate of Thermus sp. strain Y5. The alpha-L-fucosidase hydrolyzes p-nitrophenyl alpha-L-fucopyranoside with V(max) of 12.0 +/- 0.1 microM/min/mg and K(m) = 0.20 +/- 0.05 mM and is able to cleave off about 90% of total L-fucose from pronase-treated fractions of fucosyl-containing glycoproteins and about 30% from the native glycoproteins. The purified enzyme is a tetramer with a molecular mass of 240 +/- 10 kDa consisting of four identical subunits with a molecular mass of 61.0 +/- 0.5 kDa. The N-terminal sequence showed homology to some alpha-L-fucosidases from microbial and plant sources. Hydrolysis of p-nitrophenyl alpha-L-fucopyranoside occurs with retention of the anomeric configuration. Transglycosylating activity of the alpha-L-fucosidase was demonstrated in reactions with such acceptors as alcohols, N-acetylglucosamine and N-acetylgalactosamine while no transglycosylation products were observed in the reaction with p-nitrophenyl alpha-L-fucopyranoside. The enzyme can be classified in glycosyl hydrolase family 29.

Synthesis of propyl and 2-aminoethyl glycosides of alpha-D-galactosyl-(1-->3')-beta-lactoside

Propyl and 2-aminoethyl alpha-D-galactopyranosyl-(1-->3')-beta-lactosides (1 and 2) were prepared from the corresponding perbenzylated trisaccharide allyl glycoside 6 which, in turn, was obtained by methyl triflate promoted alpha-galactosylation of benzylated allyl lactoside acceptor 4 with thiogalactoside 3. Transformation of the allyl moiety in compound 6 into 2-azidoethyl one was achieved by cleavage of the double bond followed by reduction into alcohol 9, subsequent mesylation, and mesylate-->azide substitution. Alternatively trisaccharide 2 was synthesized using alpha-galactosylation of selectively benzoylated 2-azidoethyl lactoside 19 with 3 as the key step.