Enantiopure 3-alkyl substituted piperidines are prepared by diastereoselective 6-exo-trig cyclisation of perhydro-1, 3-benzoxazines derived from (−)-(8)-amino menthol. The diastereoselective cyclisation is promoted by tributyltin hydride, and the competitive 1, 5- hydrogen migration depends on the position of the acceptor double bond and the radical site.
