Rearrangements in the course of nucleophilic substitution reactions

Nucleophilic substitutions in polyfunctional heterocyclic compounds such as carbohydrates are accompanied by a series of side reactions including rear rangements. As the mechanistic interpretation for their different types has disclosed the same steric prerequisites and even common intermediates, in this compilation the whole set of side reactions observed in each single ca se is presented. The neighbouring group (NG) attacks either in a "simple" w ay [by the one atom which is directly bound to the sugar chain; this form r equires antiperiplanar orientation of the NG to the leaving group (LG)] or in a "complex" type [by its third atom (usually a carbonyl oxygen), for whi ch, in pyranoid systems, a trans-diequatorial arrangement of LG and NG is e ssential]. Possible reactions, besides straight S(N)2 by an external nucleo phile (including the solvent) or internal nucleophilic atom under formation of a new ring system (IS) as well as elimination (E), arise from anchimeri c assistance by a NG and the ring heteroatom, respectively. The latter eith er causes substitution with retention of configuration (AS) or migration, t o the reaction centre, of the participating group (MS). Special types resul t when in the "complex" case the intermediate acyloxonium ion reacts to giv e derivatives of an orthoester (ISC) and, generally, besides the anchimeric ally assisting atom, also the attacking group is an internal one (AIS/AISC and MIS), respectively. Other paths are opened by the involvement of carbon atoms from the sugar chain, which gives rise either to ring contraction wi th formation of a formyl side chain (MS) or causes fragmentation (F).