Novel design of bicyclic β-turn dipeptides on solid-phase supports and synthesis of [3.3.0]bicyclo[2,3]-Leu-enkephalin analogues

External bicyclic beta-turn dipeptide mimetics provide an excellent design approach that can offer a rich chiral ensemble of structures with different backbone conformations. We report herein a novel design of a convergent combinatorial synthetic methodology, which is illustrated by the solid-phase synthesis of a series of [3.3.0]-bicyclo([2,3])-Leu-enkephalin analogues. The reactions were optimized and the epimeric configurations were determined by 2D NMR spectroscopy. Biological assays show that these analogues have more potent delta binding affinity and bioactivity for delta vs mu opioid receptor, which may be related to the different conformations preferred by these analogues in our modeling studies.