title: Tuning Helical Chirality in Polycyclic Ladder Systems.
abstract: Conceptually and experimentally, a new set of helical model compounds is presented herein that allow correlations between structural features and their expression in the secondary structure to be investigated. A cross-linked oligomer with two strands of mismatching lengths connected in a ladder-type fashion serves as a model system. Compensation for the dimensional mismatch leads to the adoption of a helical arrangement. A strategically placed relay ensures the continuity and uniformity of the helix. Upon exchanging the heteroatomic linkage, the helix responds by increasing or decreasing the torsion of the backbone. Inversion of the relay's substitution pattern causes a distortion of the structure, while maintaining the directionality of the helix. Based on a short synthetic protocol with a modular precursor, four closely related "Geländer" oligomers (Geländer is the German word for bannister) were accessed and fully characterized. XRD analysis for one representative of each helical arrangement and complementary computational studies for the remaining derivatives allowed the impact of the alterations on the secondary structures to be studied. Isolation of pure enantiomers of all new Geländer oligomers provided insight into the racemization kinetics and estimation of the racemization barrier. In silico simulation of the electronic circular dichroism spectra of the model compounds enabled the helicity of the isolated samples to be assigned.
authors.affiliation: Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel (Switzerland).
authors.forename: Michel
authors.initials: M
authors.lastname: Rickhaus
authors.affiliation: Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel (Switzerland).
authors.forename: Oliver T
authors.initials: OT
authors.lastname: Unke
authors.affiliation: Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel (Switzerland).
authors.forename: Rajesh
authors.initials: R
authors.lastname: Mannancherry
authors.affiliation: Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel (Switzerland).
authors.forename: Linda M
authors.initials: LM
authors.lastname: Bannwart
authors.affiliation: Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel (Switzerland).
authors.forename: Markus
authors.initials: M
authors.lastname: Neuburger
authors.affiliation: Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel (Switzerland).
authors.forename: Daniel
authors.initials: D
authors.lastname: Häussinger
authors.affiliation: Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel (Switzerland). marcel.mayor@unibas.ch.|Institute for Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe (Germany). marcel.mayor@unibas.ch.|Lehn Institute of Functional Materials (LIFM), Sun Yat-Sen University, Guangzhou (P.R. China). marcel.mayor@unibas.ch.
authors.forename: Marcel
authors.initials: M
authors.lastname: Mayor
keywords: atropisomerism; chirality; conformation analysis; helical structures; oligomers