Synthesis of 3,4-disubstituted piperidines by ene cyclisation of 4-aza-1,7-dienes

Ene cyclisation of a variety of 4-aza-1,7-dienes affords 3,4-disubstituted piperidines. In particular, cyclisation of diesters 14 and 20 catalysed by MeAlCl2 gives the corresponding trans 3,4-disubstituted piperidines with diastereomeric ratios of >200:1.