Synthesis and characterisation of helical beta-peptide architectures that contain (S)-beta(3)-HDOPA (crown ether) derivatives

A new set of β-amino acids that carry various crown ether receptors on their side chains of the general formula (S)-β3-HDOPA(crown ether) (HDOPA: homo-3,4-dihydroxyphenylalanine; (crown ether): [15]crown-5 ([15-C-5]), [18]crown-6 ([18-C-6]), [21]crown-7 ([21-C-7]), 1,2-Benzo-[24]crown-8 ([Benzo-24-C-8]) and (R)-Binol-[20]crown-6 ([(R)-Binol-20-C-6])) was prepared. Peptides that are based on these new crowned β-amino acids combined with (1S,2S)-ACHC (2-aminocyclohexanecarboxylic acid), which is known to be a potent 314-helix inducer, to the hexamer level, with two crowned residues at the i and i+3 positions of the main-chain, were synthesized in solution by stepwise coupling using Boc-Nα-protection (Boc: tert-butoxycarbonyl) and the EDC/HOAt C-activation method. Their conformational analysis was performed by using FTIR absorption, NMR and CD spectroscopy techniques. Our results are in full agreement with a 314-helix conformation.