Synthesis of Constrained Head-to-Tail Cyclic Tetrapeptides by an Imine-Induced Ring-Closing/Contraction Strategy

Making heads or tails of it: A strategy involving a head‐to‐tail imine‐captured ring closure followed by ring contraction was used to synthesize otherwise difficult cyclic tetrapeptides. Compared with the direct lactamization process, the estimated activation energies for the cyclic imine formation and the ring contraction were lowered by 7.3 and 7.6 kcal mol−1, respectively, which enables cyclization.