An interesting dichotomy in the cyclisation of exocyclic enamines with protected dehydroamino acids leading to different β-turn templates

Two templates for the preparation of external β-turns have been synthesised. In the course of the synthetic studies an interesting dichotomy was observed in the PCl3 catalysed reaction of exocyclic enamines such as 6 and 14 with protected dehydroamino acids. When amide protected dehydroamino acids were condensed with 6 and 14 the expected 6/6 and 6/5 fused bicyclic compounds such as 7 and 15 respectively were obtained, whereas when urethane protected dehydroamino acids were used, the 5/6 and 5/5 fused products 9 and 18 were obtained.