Neighboring group assisted opening

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. 

How much anchimeric assistance there is. then, depends on how nucleophilic the neighboring group is. It also depends on how badly anchimeric assistance is needed. The more nucleophilic the solvent, the more assistance it gives, and the less the neighboring group participates. Or, if the open cation is a relatively stable one—tertiary or benzylic—it may need little assistance of any kind, either from the solvent or from the neighboring group. 

O bond assists in ionization (a neighboring group effect), a cyclic, delocalized ion can result. Opening of this ion by acetic acid, followed by deprotonation, would give the observed stereoisomer of the product. 

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