Robust Helix Formation in a New Family of Oligoureas Based on a Constrained Bicyclic Building Block
Résumé
The field of foldamers has become an important area of chemistry over the years because of the particular structural and functional properties that foldamers display. The design, structural properties, and activities of different families of foldamers, including aromatic polyamides or structures closely related to peptides such as peptoids, b-peptides, and g-peptides, have been exhaustively reviewed.[1–8] Among the foldamers based on natural peptide sequences, b-peptides are the most widely studied system. In this system, cyclic b-amino acids are used as building blocks, allowing stabilization of various secondary structures in oligomers by strongly pro- moting gauche-type torsion angles.[3,9–12] Our group has recently described a highly constrained bicyclic b-amino acid, named (S)-ABOC 1, (S)-aminobicyclo[2.2.2]octane-2- carboxylic acid (Figure 1).[13] This b2,3,3-trisubstituted bicyclic amino acid, able to induce a turn in peptides both in solution and in the solid state,[14] displays drastically reduced con- formational freedom and a q1 angle locked at approximately 558. Therefore, this motif is particularly attractive for the design of new foldamers. To stabilize the helical system, other parameters should be considered. Indeed, Guichard and co- workers[15,16] provided a useful tool by introducing urea linkages for oligourea foldamers that are g-peptide mimet- ics.[17, 18] The presence of additional nitrogen atoms in the urea linkage promotes helix stabilization by introducing additional conformational restriction to the backbone and hydrogen- bond donor sites.