Sialylated trisaccharides, synthesis using

SCHEME 5.16 Use of an ALacetyl-4-0,5-ALoxazolidinone-protected sialyl phosphate in a one-pot trisaccharide synthesis. 

Synthesis of Sialyated Trisaccharides. We have used a combination of glycosidases and glycosyltranferases to prepare sialylated trisaccharides (Equations 8-9). The enzyme P-galactosidase from bovine... 

The immobilized enzyme was used for repeated synthesis of the above sialylated trisaccharides without adding detergent to the system, which was used in previous synthesis with the soluble enzyme (42), The use of detergent complicates isolation of the products. The stability of the enzyme preparation was good and no decrease of activity was observed on repeated preparative synthesis. Recently, immobilization to CNBr-activated agarose of the a2-6sialyltransferase was reported (50),... 

Based on the abiHty of the E. coli NeuA to produce 9-glycosylated KDN 47 from a disaccharide 46 aldol acceptor, the synthesis of the unusual a-2,3- and a-2,6-linked sialylated trisaccharide 48 and tetrasaccharide 49 was reported, in which sialic acids are hnked in both the terminal and reducing positions (Scheme 17.17) [43]. Each transformation was conducted as a one-pot two-step cascade using a combination of CSS and one of the complementary SiaT types. 

Scheme 13. Enzymatic synthesis of a sialylated trisaccharide derivative, by stepwise sugar-chain elongation, using a water-soluble chymotrypsin-sensitive glycopolymer. Reagents i) 1. HEPES buffer pH 6, UDP-Gal (1.5 eq.), GT, 37 °C, 24 h 2. cacodylate buffer pH 7.4, ST6, CIAP, CMP-NeuAc (1.5 eq.), 37°C, 48 h, gel filtration on Sephadex G-25, q.y. ii) 1. a-chymotrypsin, Tris HCl buffer pH 7.8, 40 °C, 24 h 2. gel filtration on Sephadex G-15, 87%.

Following the enzymatic synthesis of CMP-Neu5Ac, isolated a2-6-sialyl-transferases could be used to introduce sialic acid to the N-acetyl lactosamine derivates a-f in position 6. This afforded a series of trisaccharide derivates Neu5Aca2-6 Gal l-4GlcNAc l-R. The unblocked trisaccharide a [55], the methyl glycoside b [56], the -Asn derivative c [57], the pentapeptide derivative d [58], the allyl e [59], and the pent-4-enyl glycoside f [48] can be prepared in convincing yields (Fig. 6). 

Recently, Takahashi reported the use of 37 for the synthesis of 2,6-sialyl T antigen by one-pot glycosylation with acceptor 43 and subsequent addition of donor 44. The desired trisaccharide 45 was obtain in 77% overall yield (Scheme 7). 

Recently, De Meo reported the application of the SBox approach to stereoselective a-sialylation and to the convergent synthesis of a GM3 analog (74). In the latter synthesis, selective activation of the SBox moiety of the sialosyl donor over the S-ethyl moiety of the galactosyl acceptor was conveniently achieved in the presence of AgOTf The obtained disaccharide was used in subsequent coupling directly to afford the desired GM3 trisaccharide sequence in good overall yield. 

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