The meso Helix: Symmetry and Symmetry-Breaking in Dynamic Oligourea Foldamers with Reversible Hydrogen-Bond Polarity.
Oligoureas (up to n=6) of meso cyclohexane-1,2-diamine were synthesized by chain extension with an enzymatically desymmetrized monomer 2. Despite being achiral, the meso oligomers adopt chiral canonical 2.5-helical conformations, the equally populated enantiomeric screw-sense conformers of which are in slow exchange on the NMR timescale, with a barrier to screw-sense inversion of about 70 kJ mol(-1) . Screw-sense inversion in these helical foldamers is coupled with cyclohexane ring-flipping, and results in a reversal of the directionality of the hydrogen bonding in the helix. The termini of the meso oligomers are enantiotopic, and desymmetrized analogues of the oligoureas with differentially and enantioselectively protected termini display moderate screw-sense preferences. A screw-sense preference may furthermore be induced in the achiral, meso oligoureas by formation of a 1:1 hydrogen-bonded complex with the carboxylate anion of Boc-d-proline. The meso oligoureas are the first examples of hydrogen-bonded foldamers with reversible hydrogen-bond directionality.
School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
Romina
R
Wechsel
School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
James
J
Raftery
ISM, UMR CNRS 5255, Université de Bordeaux, 351 cours de la Libération, 33405, Talence, France.
Dominique
D
Cavagnat
Université de Bordeaux, CBMN, UMR 5248, Institut Européen de Chimie et Biologie, 2 rue Robert Escarpit, 33607, Pessac, France.|CNRS, CBMN, UMR 5248, 33600, Pessac, France.
Gilles
0000-0002-2584-7502
G
Guichard
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK. j.clayden@bristol.ac.uk.
Jonathan
0000-0001-5080-9535
J
Clayden
NMR spectroscopy; chirality; foldamers; helices; structure elucidation