Iterative, aqueous synthesis of beta3-oligopeptides without the use of coupling reagents

The discovery that oligomers of β-amino acids fold into highly defined ordered structures and their resistance to proteolysis has attracted considerable attention in the synthesis, study and application of these peptidomimetics. Although the synthesis of short β-peptides is a well-established method, difficulties in the preparation of longer poly-β-peptides include problems associated with hydrophobic collapse, deprotection, and insolubility of the growing peptide. Based on our recently reported amide-bond forming reaction between α-ketoacids and hydroxylamines, we have developed a new approach to the synthesis of poly-β-peptides. This unprecedented amide formation proceeds in water, produces only carbon dioxide and methanol as by-products and does not require coupling reagents or protecting groups. Our approach towards the synthesis of β 3-oligopeptides takes advantage of the chemoselective amide bond formation of the growing peptide chain with enantiopure isoxazolidine monomers. Amide bond formation results in N–O bond cleavage rendering and α-ketoester which following hydrolysis is poised for iterative chain elongation. A major challenge in adapting our methodology for the preparation of β 3-oligopeptides was the synthesis of the enantiopure isoxazolidine monomers. We overcame this difficulty by employing a sugar-assisted 1,3-dipolar cycloaddition offering access to a variety of enantiopure monomers. To address the synthesis of longer β3-oligopeptides we turned towards a solid-phase implementation of our method. Several notable challenges needed to be overcome: the peptide would be assembled in an inverse N→C direction requiring the design and synthesis of a new linker, and identification of a water-compatible solid support was crucial as aqueous conditions would be employed. We developed a seven-step protocol to the synthesis of a linker for inverse peptide synthesis and identified a water-compatible solid support. Our approach towards the iterative solid phase synthesis of β 3-oligopeptides begins by attaching the new ketoacid linker. Subsequent hydrolysis unmasks the α-ketoacid that can be directly coupled with the unprotected isoxazolidine monomers. We have demonstrated that this two-step coupling/hydrolysis iteration can be executed to assemble short sequences β 3-pentamers. This new approach to peptide synthesis offers great potential in facilitating the preparation of β3-oligopeptides promoting their study and applications as peptidomimetics