Studies concerning the electrophilic amino-alkene cyclisation for the synthesis of bicyclic amines

The bromination of a series of cyclohexenyl substituted secondary amines 1a-i has been investigated using Br₂, PHT and NBS. In the case of Br₂ and NBS the secondary amines preferentially undergo N-bromination. In contrast, PHT cleanly affords the products of alkene dibromination. In the case of Br₂ the N-bromo species then give the products of alkene dibromination, albeit less efficiently. On subsequent treatment with K₂CO₃ these dibromides form the corresponding hexahydroindoles 2a-h and octahydroquinoline 2i. The presence of an N-substituent bearing a stereogenic centre (1h and 1i) was studied and the products 2h and 2i were isolated with no diastereoselectivity. When NBS was used a novel cyclisation, forming bromo-substituted octahydroindoles 9a,b and d, was observed. In relation to this sequence it was shown that these products were not intermediates in the former Br₂/PHT processes and that the reaction only proceeded in the presence of the succinimide by-product of N-bromination.