title: Pseudohelical and helical primary structures of 1,2-spiroannelated four- and five-membered rings: syntheses and chiroptical properties.
abstract: The pseudohelical hydrocarbons (R)-6, (S)-7, and (R)-8 and the helical hydrocarbon (P)-9, formally derived from the helical hydrocarbon (P)-4 by stepwise replacement of each of the four-membered rings by a five-membered ring, have been prepared. Their optical rotations vary systematically, both in magnitude and sign. Of the extremes, (P)-4 represents the usual case of a right-handed dextrorotatory helix, while (P)-9 represents the unusual case of a right-handed levorotatory helix. To rationalize these facts, DFT calculations of the rotatory power of (P)-helices of three-, four-, and five-membered rings have been performed. The results show a very good agreement with the experimental data for the rigid helices of three-membered rings and always show the correct sign and order of magnitude for the flexible helices of four- and five-membered rings for which Boltzmann-averaged optical rotations of up to six conformers had to be used. Within the conformers of the latter, a set of large dihedral angles for the bonds of the inner sphere correspond to a high specific rotation, and a set of small dihedral angles correspond to a low specific rotation. As a consequence, the Boltzmann-averaged values markedly depend on the geometry and weight of the conformers involved.
authors.affiliation: Institut für Organische und Biomolekulare Chemie, Universität Göttingen, Tammannstr 2, 37077, Göttingen, Germany.
authors.forename: Tien
authors.initials: T
authors.lastname: Widjaja
authors.forename: Lutz
authors.initials: L
authors.lastname: Fitjer
authors.forename: Aritra
authors.initials: A
authors.lastname: Pal
authors.forename: Hans-Georg
authors.initials: HG
authors.lastname: Schmidt
authors.forename: Mathias
authors.initials: M
authors.lastname: Noltemeyer
authors.forename: Christian
authors.initials: C
authors.lastname: Diedrich
authors.forename: Stefan
authors.initials: S
authors.lastname: Grimme