UDP-Activated Donors

The simultaneous replacement of uridine by deoxythymidine and galactose by a number of deoxy sugars to give non-natural donor substrates has also been reported [50], Partially protected a-sugar-1-phosphates were coupled to morpholidate-activated deoxythymidine-5-phosphate and subsequently deprotected in the presence of lithium hydroxide to give the corresponding dTDP sugars (see Table 1). 

The stereochemical preparation of CMP-sialic acid is quite a synthetic challenge. 

This is because of the absence of neighboring group participation to assist the 

Application of strategy B also made accessible nucleotide monophosphosialic acid derivatives with the cytidine replaced by other pyrimidine and purine bases, although in modest overall yields only [55]. By following pathway B a CMP- disialyl derivative could be synthesized. This compound has a second sialic acid unit a-linked to the 8-OH group of the CMP-activated sialic acid [56]. 

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All the UDP-activated donors (Scheme 2) are a-pyrophosphate sugars and thus are inherently prone to degradation in acidic media, leading to monosaccharide units and uridine diphosphate [31]. This lability must always be taken into account at the outset of an efficient synthetic strategy. In addition the separation of the highly polar target compounds from side-products in the reaction mixture necessitates the... 

Whitesides and coworkers extensively studied various enzymatic routes offered by nature to prepare UDP-activated donors on a gram scale (see Scheme 8) [74, 75]. 

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