Endoanomeric effect

The conformational effects arising from the endoanomeric effect are for furanoses much less profound and as a result relatively little research has been performed in this area. The puckering of the furanose ring of an a and a P anomer usually adjusts the anomeric substituent in a quasi-axial orientation and hence both anomers experience a similar stereoelectronic effect. On the other hand, the conformational preference of the exocyclic C—O bond is controlled by the exoanomeric effect in the usual way. 

A major breakthrough in a-glycosidic bond synthesis came with the introduction of the in situ anomerisation procedure.1 Lemieux and coworkers observed that a rapid equilibrium can be established between a and (3 halides by the addition of tetra-w-butyl ammonium bromide (Scheme 4.6). The anomerisation is believed to proceed through several intermediates. At equilibrium, there is a shift towards the a bromide since this compound is stabilised by an endoanomeric effect. Because, the p bromide is much more reactive towards nucleophilic attack by an alcohol,... 

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