Conformational effects of chiral α,α‐dialkyl amino acids I. C‐Terminal tetrapeptides of emerimicin containing α‐ethylalanine

The syntheses and the crystal structures of the C‐terminal tetrapeptide fragments of emerimicin IV and III, Boc‐r‐EtA‐Hyp(Bzl)‐Ala‐Phol and Boc‐r‐EtA‐Hyp(Bzl)‐MeA‐Phol, containing the chiral α,α‐dialkyl amino acid, r‐α‐ethylalanine (r‐EtA) are reported. The two peptides are isomorphous and assume a 3₁₀‐helical conformation in the crystal. A comparison of the crystal data on α,α‐dialkyl amino acids indicates that alkyl substituents larger than a methyl group do not preclude peptides containing these amino acids from assuming the conformations associated with minima which have been well characterized for α‐methylalanine.