Solid phase synthesis of aromatic oligoamides: application to helical water-soluble foldamers

Synthetic helical aromatic amide foldamers and in particular those based on quinolines have recently attracted much interest due to their capacity to adopt bioinspired folded conformations that are highly stable and predictable. Additionally, the introduction of water-solubilizing side chains has allowed to evidence promising biological activities. It has also created the need for methods that may allow the parallel synthesis and screening of oligomers. Here, we describe the application of solid phase synthesis to speed up oligomer preparation and allow the introduction of various side chains. The synthesis of quinoline-based monomers bearing protected side chains is described along with conditions for activation, coupling, and deprotection on solid phase, followed by resin cleavage, side-chain deprotection, and HPLC purification. Oligomers having up to 8 units were thus synthesized. We found that solid phase synthesis is notably improved upon reducing resin loading and by applying microwave irradiation. We also demonstrate that the introduction of monomers bearing benzylic amines such as 6-aminomethyl-2-pyridinecarboxylic acid within the sequences of oligoquinolines make it possible to achieve couplings using a standard peptide coupling agent and constitute an interesting alternative to the use of acid chloride activation required by quinoline residues. The synthesis of a tetradecameric sequence was thus smoothly carried out. NMR solution structural studies show that these alternate aminomethyl-pyridine residues do not perturb the canonical helix folding of quinoline monomers in protic solvents, contrary to what was previously observed in nonprotic solvents.