cis-cyclononene conformational families and a crystallographic example of a skew-chair-boat type-2 conformation

Robert Glaser, Dror Shiftan, Ganit Levi-Roso, Itzhak Ergaz, Shimona Geresh, Marc Drouin

Research output: Contribution to journalArticlepeer-review

11 Scopus citations

Abstract

Cyclononanes from the 7 low-energy conformational archetypal families (twist-boat-chair, twist-chair-boat, twist-chair-chair, twist-chair-twist-chair, skew-chair-boat, skew-chair-chair, and skew-boat-boat) were transformed into 12 of 13 MM3 stochastically generated cis-cyclononenes. This was done by systematically converting single bond synclinal endocyclic torsion angles, one-at-a-time, into double-bond synperiplanar analogues, followed by geometry optimization [e.g. density functional theory B3LYP/6-31G(d)]. Torsion angles adjacent to the new double bond maintained their signs, while their magnitudes usually changed considerably to accommodate the new neighboring synperiplanar torsion angles. The six remaining torsion angles all maintained their signs and approximate magnitudes compared to corresponding values in the seven saturated parent structures. As a result, the same "twist"/"skew" conformational descriptors previously used for the saturated conformers can now also be applied to the corresponding unsaturated analogues. Three conformational families have multiple members (subtypes) in which the double bond is located at different positions on the same ring conformation. The solid-state structures of (±)-1-phenyl- 1,3,4,5,6,7-hexahydro-2,6-benzoxazonine-6-carbonitrile (22) and (1RS,3SR)-1-phenyl-3-methyl- 1,3,4,5,6,7-hexahydro-2,6-benzoxazonine-6-carbonitrile (23) were determined by single-crystal X-ray diffraction analysis. The asymmetric unit of the P1 unit cell for 22 contains two symmetry-unrelated molecules, both of which exhibit a skew-chair-boat (SCBtype-2) conformation identical to that found for crystalline 23. This subtype has yet to be found in the Cambridge Crystallographic DataBase. Crystal lattice packing considerations alone cannot explain the observation of the SCBtype-2 conformation since 1H NMR spectroscopy shows the same conformational bias when the crystals are dissolved in CDC13.

Original languageEnglish
Pages (from-to)5486-5496
Number of pages11
JournalJournal of Organic Chemistry
Volume67
Issue number16
DOIs
StatePublished - 9 Aug 2002

ASJC Scopus subject areas

  • Organic Chemistry

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