Anomeric interactions competition

The idea of competition between the anomeric interactions is strongly supported by an ab initio study (67) on proton affinities of oxygens in the sc, sc and ap, sc conformers of dimethoxymethane (11) (Figure 3). Deslong-champs (7a) suggested that oxygens involved in o hyperconjugative... 

Up Op, p interactions are present instead (Figure 6.65). The competitiveness of homoanomeric donation (vide infra) from the F lone pairs relative to the classic anomeric donation from the N lone pairs is further increased by the larger s-character (and the poor donor ability) of the N lone pairs as a consequence of Bent s rule. 

Cooperativity/competUion among acceptors (armed/disarmed carbohydrates) Cleavage of the C-X bond at the anomeric position proceeds via evolution of the anomeric n o C-X interaction that culminates in the oxocarbenium ion. This carbocation is so strongly stabilized by n j- p interaction that this interaction is commonly described as a dative it-bond. Because oxacarbenium ions are strongly stabilized, their formation is relatively facile. In cross-coupling glycosylalion reactions, however, it is beneficial to differentiate the reactivity of two saccharides for the selectivity reasons. The competition and cooperativity between stereoelectronic effects can be used to fine-tune the reactivity of anomeric systems. 

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