Synthesis of external β-turn templates by reaction of protected dehydroamino acids with cyclic enaminoesters

Two external β-turn templates have been synthesised and one of them has been derivatised as a GLDV-tetrapeptide. In the course of the synthesis an interesting dichotomy was observed in the condensation of exocyclic enamines such as and with protected dehydroamino acids using phosphorus trichloride. When dehydroamido acids were condensed with the enamines and then 6/6 and 6/5 fused bicyclic compounds such as and , respectively, were obtained, whereas, when dehydroamino acid urethanes were used, the 5/6 and 5/5 fused products and were obtained. The bicyclic template was converted to the GLDV-tetrapeptide derivative but the sensitivity to base of the acyl–enamine system of the template reduced the yield in the synthesis of the external turn .