Electron-withdrawing anomeric

The incorporation of heteroatoms can result in stereoelectronic effects that have a pronounced effect on conformation and, ultimately, on reactivity. It is known from numerous examples in carbohydrate chemistry that pyranose sugars substituted with an electron-withdrawing group such as halogen or alkoxy at C-1 are often more stable when the substituent has an axial, rather than an equatorial, orientation. This tendency is not limited to carbohydrates but carries over to simpler ring systems such as 2-substituted tetrahydropyrans. The phenomenon is known as the anomeric ect, because it involves a substituent at the anomeric position in carbohydrate pyranose rings. Scheme 3.1 lists... 

The different reaction-periods needed57 for eliminations of the anomeric methyl D-glucofuranosidurono-6,3-lactones (see Table IV) must be attributed to differences in C-5-H acidity. For a possible explanation,89 it may be assumed that the electron-withdrawing property of the molecular vicinity also depends on the degree of electron... 

Chemical entities discussed in this chapter as glycosyl donors share the principal structural feature C(anomeric)—sulfur atom bond with thioglycosides, discussed earlier. However, the electron density on the sulfur atom is diminished, and consequently its chemical reactivity differs considerably, because of substitution with electron-withdrawing groups such as carboxylic or phosphoric acid residues. This... 

The addition of CF3TMS on a lactone group in position 1 allows introduction of CF3 on the anomeric position. Due to its electron-withdrawing effect, the CF3 group inhibits the possible formation of the oxocarbenium ion intermediate of the anomeric substitution. Consequently, the substiment introduced on the anomeric position (OAc,... 

Figure 3.9a may also represent the interaction of a nonbonded ( lone-pair ) orbital with an adjacent polar n or a bond [67]. If a polar n bond, one can explain stabilization of a carbanionic center by an electron-withdrawing substituent (C=0), or the special properties of the amide group. If a polar a bond, we have the origin of the anomeric effect. The interaction is accompanied by charge transfer from to A, an increase in the ionization potential, and a decreased Lewis basicity and acidity. These consequences of the two-electron, two-orbital interaction are discussed in greater detail in subsequent chapters. 

Substitution at the terminal position of the allylstannane, as in crotonyltributyl stannane, however, is not tolerated, because hydrogen abstraction from the allylic position is a competing reaction [21], An extension of the method involves the coupling of the anomeric radical precursors 28 with the allyltributyltin reagent 29 [14], In the reagent 29 the double bond is activated toward addition of nucleophilic radicals by the electron-withdrawing t-butoxy carbonyl group. The obtained product 30 has been useful en route to 3-deoxy-D-marmo-2-octulosonic acid (KDO). 

In comparison, the stereoselectivity of the radical reduction slightly decreases if the anomeric center is substituted by an electron withdrawing substituent (CN) using AIBN as radical initiator [17 b]. The corresponding radical is supposed to be more planar than pyramidal to explain the lower degree of stereoselectivity for the reduction process of 8 b into 9. 

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