Glycans are a ubiquitous, structurally diverse class of biopolymers, the biosynthesis of which is catalyzed by glycosyltransferases.1 These enzymes utilize nucleoside diphosphate saccharides as substrates for the transfer of a saccharide unit to acceptors such as (oligo)saccharides, lipids, or proteins.2 Eukaryotic glycosyltransferases employ a limited number of sugar nucleotides as glycosyl donors (UDP-Glc, UDPGlcNAc, UDP-GlcUA, UDP-Gal, UDP-GalNAc, GDP-Fuc, GDP-Man, and CMPNeu5Ac),3 while the structures of sugar nucleotides in prokaryotes are both numerous and diverse. For the assembly of glycans using glycosyl transferases, an ef¥cient synthesis of (non)natural nucleoside diphosphate saccharides is imperative. Numerous strategies for the synthesis of pyrophosphates have previously been reported in the literature,4,5 of which the morpholidate method of Moffatt and Khorana is the most frequently used.6 In this chapter we present a convenient approach for the preparation of sugar nucleotides using a nucleotide phosphoramidite, as exempli¥ed by the synthesis of UDP-N-acetylglucosamine (UDP-GlcNAc, 5). Phosphoramidites have long been used as powerful phosphorylating agents in the synthesis of nucleic acid oligomers.7 Utilizing this property in combination with a suitable phosphate nucleophile (i.e., glycosyl-phosphate) enables the facile construction of sugar nucleotides. As seen in the opening scheme of this chapter activation of phosphoramidite 18 in presence of glycosyl phosphate 28 leads to the formation of two diastereomeric phosphatephosphite intermediates 3. 31P NMR [δ = 130.3 (d, J = 5.8 Hz), 128.3 (d, J = 4.5 Hz), −10.9 (d, J = 4.5 Hz), −11.0 (d, J = 5.8 Hz) ppm]. Subsequent oxidation and ensuing

deprotection produces the required sugar nucleotide 5. The method described herein is attractive for a number of reasons: First, the high reactivity of phosphoramidites enables rapid and ef¥cient formation of sugar nucleotides. Second, the accidental presence of moisture does not give rise to homocoupling, under the used reaction conditions. This type of side reaction can occur when methods involving activated forms of phosphates are applied. Hydrolysis of the reactive phosphorous (V) species with concurrent homocoupling results in a complex reaction mixture, which makes puri¥cation cumbersome.9 Finally, the stereochemistry of the ¥nal product is ensured and retained throughout the reaction by the use of diastereomerically pure glycosyl phosphates.