We present an E-selective
ring-closing metathesis
reaction in α-helical stapled peptides at positions i and i + 4. The use of two chiral carbocyclic
α,α-disubstituted α-amino acids, (1S,3S)-Ac5c3OAll and (1R,3S)-Ac5c3OAll,
provides a high E-selectivity of a ≤59:1 E:Z ratio, while mixtures with E:Z ratios of 2.1–0.5:1 were produced
with standard acyclic (S)-(4-pentenyl)alanine amino
acids. A stapled octapeptide composed of (1S,3S)- and (1R,3S)-Ac5c3OAll amino acids showed a right-handed α-helical
crystal structure.