Anomers structural significance

The situation is completely different in the presence of 150 mol % LiNTf2- Indeed, whether isomer 110 or 111 is used, the formation of a anomer 110 predominates (a /i = >99 <1). It has been proposed that the intermediate oxocarbenium ion 1-3 is involved in the anomerization mechanism. In the presence of the trityl cation, the equilibrium between anomers 110 and 111 might shift to the more thermodynamically stable anomer 111. The significant role of perchlorate and bis(sulfonimide) anions has been consistently claimed. Thus, intermediate 1-3 would be efficiently stabilized by these anions located at the sterically less-encumbered / side of anomeric centers, achieving nucleophilic attack from the a side. It is also suggested that the Li cation is likely to effect the selective stabilization of product a anomer 110 by forming coordination complexes as exemplified by putative chelated structures C-1 and C-2 (Fig. 10). 

From infrared studies, Spedding concluded that only one of the hydroxyl groups is hydrogen-bonded in the /3-(d or l) anomer, and he therefore favored the HJA conformation (32). He pointed out that the different environmental conditions must be of significance when the fine structure of a given compound is considered. 

The modeled transition-state structures for hydrolysis by the a-glucosi-dase and the glucoamylase show significant oxocarbonium ion character with the D-glucosyl unit of each having a flattened 4C conformation consistent with a C-l-O-5 bond order of 1.92, even though opposite o-glucose anomers are formed from the substrate. The transition-state structures show modest differences but they do not predict the stereochemical outcome of the catalyzed reactions.157... 

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