Synthesis of Bicyclic Peptides

Bicyclic peptides have received interest as anticancer agents, protease inhibitors, antibiotics, receptor antagonists, artificial receptors and models of various structural motives of proteins. Their common characteristic feature, limited conformational freedom of the backbone, plays a central role in all these applications. The same feature poses a challenge for synthetic organic chemists. Additional levels have to be included in the protecting group strategy to ensure the desired regioselectivity of cyclization and still the yields tend to be low, owing to increasing rigidity that cyclizations, in particular the second one, result in. The present review is aimed at summarizing the strategies described for the synthesis of various types of bicyclic peptides, originally in solution-phase and nowadays more often on a solid-support.