Sialosides functionalities

Straightforward solutions to this problem have been addressed in the following publications describing the synthesis of sialoside dendrimers 554, where the glycans are interspaced by TRIS residues used as dummy functionalities (Fig. 68).335... 

Scheme 1 Sialoside precursors having varied functionalities for direct attachment to hyperbranched or dendritic scaffolds...

To facilitate accesses to suitably functionalized sialic acid derivatives and complex sialyloligosaccharides for other usehil neoglycoconjugates, phase transfer catalysis (PTC) has been exploited extensively [for reviews see 42]. This process provided a wide range of carbohydrate derivatives under essentially clean Sn2 transformations. In the case of acetochloroneuraminic acid 1, the PTC reactions always provided inverted a-sialic acid derivatives [43]. para-Formylphenyl sialoside 7 [44], together with many other sialoside derivatives such as 8-10 [43], including thioacetate 12 [45] and azide 14 [46], were thus obtained (Scheme 1). Aldehyde 7 and similar glycosides are of particular interest since they could be directly conjugated to protein by reductive amina-tion after suitable deprotection [44]. 

Another strategy for selective delivery of CA s at cancer cells exploits the intrinsic overexpression of sialic acid in them. Upon incubation of cancer cells with peracetylated N-levulinoylmannosamine the sialoside biosynthetic pathway affords N-levulinoyl sialic acid, the ketone group of which can be a target for an Gd3+ complex of an aminooxy-functionalized DTPA derivative [165]. 

Chemical synthesis of sialosides is considered one of the most difficult glycosylation reactions because of a hindered tertiary anomeric carbon and the lack of a participating auxiliary functionality in the carbon next to the anomeric carbon in sialic acids (55, 56). Sialyltransferase-catalyzed glycosylation is believed to be the most efficient approach for the production of sialic acid-containing structures. 

Yu H, Chokhawala HA, Huang S, Chen X. One-pot three-enzyme chemoenzymatic approach to the synthesis of sialosides containing natural and non-natural functionalities. Nature Protocols 2006 1 2485-2492. 

Our group has recently cloned a truncated Pd2,6ST containing 17-497 amino acid residues as N-hexohistine tagged protein and explored its application in the one-pot three-enzyme system for preparative synthesis of functionalized o2,6-sialosides (25). The tolerance of donor substrate modification by the purified Pd2,6ST was tested using the one-pot three-enzyme system, in which CMP-sialic acid derivatives were generated in situ from sialic acid precursors by the aldolase and NmCSS. An extremely relaxed donor substrate specificity was observed for Pd2,6ST. The preparative-sacle reactions were then carried out at... 

Several non-natural a2,6-linked sialosides 36-40 with azide or alkyne-modified sialic acid residues were also prepared in excellent yields (86%-93%) from their C2- or C6- modified ManNAc or mannose bearing corresponding azide or alkyne functional groups 30-34 using the one-pot three-enzyme approach and GaipOMe (35) as an acceptor for Pd2,6ST (Scheme 5). 

Scheme 4. Conjugation of functionalized sialosides with a terminal azido group to other biomolecules.

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