The use of aziridinium ions in two different projects is described. First, the stereospecificity of the ring opening of aziridinium ions with MeNH2 as a route to chiral diamines has been explored. When the aziridinium ion contained a phenyl or para-methoxyphenyl substituent, stereospecific ring opening occurred. In contrast, switching the para-methoxy group to a para-N,N-dimethylamino group gave a racemic diamine product. Second, starting from N-Boc pyrrolidine, asymmetric lithiation-trapping-ring expansion (via an aziridinium ion) was used to synthesise a piperidine alcohol. In this way, a formal synthesis of (−)-swainsonine was completed.