α,β‐Dehydro‐amino acid residues in the design of peptide structures: Synthesis, crystal structure, and molecular conformation of two homologous peptides—N‐Ac‐Dehydro‐Phe‐ L ‐Leu‐OCH 3 and N‐Ac‐Dehydro‐Phe‐NorVal‐OCH 3

The dehydro‐residue containing peptides N‐Ac‐dehydro‐Phe‐ L ‐Leu‐OCH 3 ( I ) and N‐Ac‐dehydro‐Phe‐NorVal‐OCH 3 ( II ) were synthesized by the usual workup procedures. The peptides crystallize from their solutions in methanol in space group P6 5 : ( I ) a = b = 12.528(2) Å, c = 21.653(5) Å; ( II ) a = b = 12.532(2) Å, c = 21.695(4) Å. The structures were determined by direct methods. Both peptides adopt similar conformations with ϕ,ψ of dehydro‐Phe as follows: ( I ) −57.0(5)° and −37.0(5)°; ( II ) −56.0(5),° and −37.5(5)°. The observed data on dehydro‐Phe when placed at the ( i + 1) position show that the ϕ,ψ values of dehydro‐Phe are either −60°, 140° or −60°, −30°. The conformation of −60°, 140° can be accommodated only with a flexible residue at the ( i + 2) position while the ϕ,ψ values of −60°, −30° are obtained with a bulky residue at the ( i + 2) position as in the present structures. The molecules are packed in a helical way along the c axis. These are held by two strong intermolecular hydrogen bonds involving both NH as donors and acetyl group and dehydro‐Phe oxygen atoms as acceptors. © 1994 John Wiley & Sons, Inc.