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‐OCH3and N‐Ac‐Dehydro‐Phe‐NorVal‐OCH3