Synthesis and Structure of Symmetrical Bicyclic Hexapeptides Bridged by Metathesis Reaction

Bicyclic hexapeptides 1a−c were synthesized via an intramolecular ring-closing metathesis reaction on solid phase followed by an N- to
C-terminal cyclization in solution. Structural elucidation showed that these compounds assumed a C2-symmetrical structure with two β-turns.
The trans-ethylene plane was found to occupy two positions in rapid interconversion. One of the bicyclic hexapeptides crystallized with five
water molecules, which made an arch above the ethylene group.