Anchimeric assistance Neighboring group

NEIGHBORING GROUP MEGHANISM ANCHIMERIC ASSISTANCE NEIGHBORING-GROUP PARTIGIPATION INTRAMOLECULAR CATALYSIS SYNARTETIC ACCELERATION N-END RULE... 

The most obvious way in which a protecting group can influence the stereoselectivity of a glycosylation is through anchimeric assistance (neighboring-group participation) [31]. When there is a carbonyl functionality present at C2 [32] e.g. ester, carbonate, phthalimide, etc.) it can attack the incipient oxonimn ion to form the thermodynamically more stable dioxonium species 5 (Figure 6). 

It is thus clear that P-aryl groups can function as neighboring groups. Much less work has been done on aryl groups located in positions farther away from the leaving group, but there is evidence that these too can lend anchimeric assistance. ... 

Anchimeric assistance may also explain slight changes in mechanism resulting from the presence of a neighboring group. One likely case is the acid-catalyzed hydrolysis of phenylglycosides. Raftery s group" found that the secondary kinetic isotope effect (/tr/ d) was 1.13 for acid hydrolysis of phenyl-4-0-(2-acetamido-2-deoxy-j8-D-glu-... 

Reactions on synthetic polymers often mimic similar reactions involving small molecules where size is the main difference. There are some exceptions where nearby groups may hinder or assist in the reaction in which differences occur. Here, the main differences are often kinetic, though some geometrical differences are found for specific cases. Reactions in which the rate of reaction in polymers is enhanced by the presence of neighboring groups are called anchimeric assistance. 

The direction of oxirane ring-opening in the stereoisomeric compounds 289 depends on several factors for example, favored axial attack of the nucleophile, anchimeric assistance of neighboring groups, and conformational control of the approaching nucleophile. The examples shown in Scheme 2 provide support for the importance of each factor. There are, however, exceptions to these rules (see, for example, Ref. 184). Therefore, the results of oxirane ring-opening in 289 cannot always be foreseen. 

All mechanistic pathways that generate product (H) represent examples of neighboring group participation, but anchimeric assistance is observed only if the rate-limiting step is that of ring closured Since each of the steps may be readily reversible under the conditions of the reaction, it is not always clear whether a given reaction was conducted under conditions of kinetic or thermodynamic control, which step was rate-limiting, or which step controlled the stereochemistry. 

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