Solution-Phase Synthesis of Aib-Containing Cyclic Hexapeptides

The 18‐membered Aib‐containing cyclohexapeptides, cyclo(‐Gly‐Aib‐Aib‐Gly‐Aib‐Phe‐) (22), cyclo(‐Gly‐Aib‐Phe(2Me)‐Gly‐Aib‐Aib‐) (24a), cyclo(‐Gly‐Phe(2Me)‐Aib‐Gly‐Aib‐Aib‐) (24b), and cyclo(‐Gly‐Phe(2Me)‐Aib‐Gly‐Aib‐Phe‐) (25), have efficiently been synthesized by solution‐phase techniques. The linear precursors 1a–1d were prepared by combining the ‘azirine/oxazolone method’ for incorporation of α,α‐disubstituted α‐amino acids (Aib, Phe(2Me)) into the peptide chains by classical peptide coupling methods for segment condensations. Deprotection of the amino and carboxy termini of 1a–1d, followed by cyclization with DEPC as the coupling reagent, gave the above‐mentioned cyclic hexapeptides 22, 24a, 24b, and 25 in good yields (26–57%). The solid‐state conformations of the linear hexapeptides 1d, 16 and 27, and of the cyclohexapeptides 22 and 25 have been established by X‐ray crystallography.