Glycals metallated

During an attempt to metalate a glycal with /-BuLi, it was discovered by deuterium labeling that a TBDMS ether can be deprotonated. °- ... 

Table 2 Transition metal-catalyzed coupling of different 1-substituted glycals... 

In the following we have broadly divided the available C(l) nucleophiles according to structure and reactivity. C(l)-Nucleophiles based on metallation at an sp -center are categorized in terms of the presence and, as appropriate, the nature of a hetero atom (O or N) substituent at C(2). C(l)-Lithiated glycals correspond to nucleophilic sp -centers, and variations on this theme (concerning the nature of the metal component) are covered in depth. 

Transition-metal-catalysed reactions of glycals with carbon nucleophiles provide an important route to C-glycosides.24 These reactions may proceed by two different pathways depending on the oxidation state of the transition metal used and the nature of the carbon nucleophile (Scheme 3.5c). For example,25 transmetallation of Pd(OAc)2 with pyrimi-dinyl mercuric acetate results in the formation of a pyrimidinyl palladium complex, which adds to the double bond of glycal derivatives. The resulting intermediate can undergo different transformations depending on the reaction conditions. 

Initially, the de novo synthesis of enantiomerically pure carbohydrates [110] and glycolipids [111] using transition metal complexes and chiral auxiliaries afforded only modest success. Ultimately, it was the use of enantiomerically pure aldehydes, such as the R and S enantiomers of 2-(phenylseleno)propionaldehyde, to convey facial selectivity upon the LACDAC reaction that enabled the synthesis of optically pure glycals. Syntheses of several complex monosaccharides such as the main sialic acid-type N-acetylneuraminic acid (Neu5Ac) and rac-3-deoxy-ma o-2-octulosonic acid (KDO) were accomplished with this technology [112, 113], The LACDAC... 

When considering sugar ring unsaturation removed from the 1,2 position, many methods are available in addition to methods analogous to those used in glycal formation. As shown in Scheme 6.39, the direct elimination of sulfonates under basic conditions provides a useful strategy [20]. Moreover, in the case of m/i5 -2,3-bis-sulfonates, initial displacement of one sulfonate with lithium iodide can be followed by metallation and tandem elimination, as shown in Scheme 6.40 [69]. 

The direct metallation of sugar derivatives, shown in Figure 7.20, is a useful method for the preparation of anomeric nucleophiles. Anomeric halides and glycals are excellent substrates for this reaction. Additionally, selective generation of a and 3 anomers is possible. 

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