Bicyclic aziridinium ions

An Sn2 reaction with anchimeric assistance (via formation of a bicyclic aziridinium ion intermediate) has been proposed for the nucleophilic substitution of trans-2-substituted 3-piperidinol mesylates by nitrogen nucleophiles with retention of trans... 

Ring expansion reactions of 2-substituted pyrrolidines, with the intermediacy of a bicyclic aziridinium ion, has provided new piperidines. Treatment of the chiral olefmic imine 264 with Br2, followed by reduction of the resultant ion 265 results in the formation of a separable mixture of spiromethylpyrrolidines and a single spiropiperidine 266 (23% yield), a precursor of (-)-nitramine 267 (Scheme 80) <05SL1726>. 

Closure of 5-halo-amines by nucleophilic displacements of halogen, is complemented by the conversion of iV-chloro-pent-4-enylamines into 3-chloropiperidines by exposure to tetra-n-butylammonium iodide. The process is believed to involve conversion to an A -iodo-amine, which serves as an electrophilic source of iodine, cyclisation to a 2-(iodomethyl)-pyrrolidine, ring closure to a bicyclic aziridinium ion and ring opening via attack by chloride to produce the six-membered heterocycle. 

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