Synthesis and conformational behavior of pseudopeptides containing δ-azaproline. A cis conformational preference for Xaa1–δ-azaPro bond

δ-Azaproline, a new bis-nitrogen proline surrogate has been used in order to control the conformation of an AA–ΨPro bond. Conformational analysis of Xaa1–δ-azaPro–Xaa3 performed by NMR, IR experiments, and molecular modeling revealed a preference for a trans conformation of the Xaa1–δ-azaPro bond when δ-azaPro is protected by a Boc group. The removal of the Boc protection leads to the establishment of a C10 pseudocycle via a hydrogen bond network favoring the cis conformation of the Xaa1–δ-azaPro bond.