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17.2 A Generic Strategy for the Synthesis of Sialoconjugate Libraries 365

OH OH OH
HO OH HO OH HO OH
O CO H O CO 2 H O CO H
AcNH 2 HO NH HO 2
HO OH HO OH HO OH
1 O 2 3

R 9 R 8 CO 2 H
R 5 O O Acceptor
R 7
R 4
4
R = OH, O-acetyl, O-Fuc, O-Gal
5
R = OH, NH , NH-acetyl, NH-glycolyl, NH-(O-acetyl)glycolyl, NH-(O-methyl)glycolyl, NH-(O-α2Neu5Gc)glycolyl
2
7
R = OH, O-acetyl, NH 2 , NH-acetyl
8
R = OH, O-acetyl, O-methyl, O-sulfate, O-α2Sia, O-Glc
9
R = OH, O-acetyl, O-lactoyl, O-phosphate, O-sulfate, O-α2Sia, H
Figure 17.2 The three basic forms of sialic all known sialic acids is shown in the 2α-
2
acids, and the natural sialic acid diversity configurated C chair conformation that is
5
of sialoconjugates [4, 12]. The standard typical for sialoconjugates. Conjugates vary
4
9
nine-carbon sugar backbone common to in the combination of substituents R to R .
acid in humans and bacteria, occurrence of variations in the Neu5Ac structure is
species and tissue specific, is developmentally regulated, and is thought to arise
by post-synthetic modification of Neu5Ac units at the stage of sialoconjugates.
Most sialic acid variations reported to date have been found in vertebrates and
only a few in bacteria. A multitude of specific enzyme activities are involved,
such as required for regiospecific O-methylation or esterification to furnish unique
O-ester (acetyl, lactoyl, phosphate, sulfate) derivatives. Other variations depend
on hydroxylation of CMP-Neu5Ac to furnish the corresponding N-glycolyl inter-
mediate (2) or require entirely independent upstream pathways, for example, for
conjugation of the non-aminated analog KDN (3) [4, 12, 13]. Therefore, synthetic
access to the manifold of sialoconjugates incorporating structural modifications
in the sialic acid moiety is highly complex and, when based on natural biosyn-
thetic pathways, will usually require a plethora of individual multistep synthetic
strategies (Scheme 17.2) [1]. Currently, such an approach is impractical for in
vitro synthesis owing to the fact that, except for very few exceptions (listed in
Table 17.1), most of the enzymes involved in sialic acid modification are yet to be
discovered.
While Nature’s strategy is to build a core motif first before diversification takes
place, an alternative in vitro strategy can be envisaged by building a synthetic pool of
diversified sialic acids first, which are subsequently transferred along the common
Leloir-type pathway to yield the sialoconjugate variety, dramatically cutting down the
number of enzymes involved (Scheme 17.2). Structural modifications for the entire
variety of known and novel sialoconjugates can be efficiently realized at the very
beginning because methods for the preparation of natural sialic acids, non-natural
derivatives, and functional analogs, or their immediate precursors, are highly
developed by purely chemical or chemoenzymatic routes (Scheme 17.3) [14–17].
At this stage, also interesting functionalities can be easily incorporated that, along