cis-Cyclononene Conformational Families and a Crystallographic Example of a Skew-Chair−Boat Type-2 Conformation

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 P (1) over bar 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 H-1 NMR spectroscopy shows the same conformational bias when the crystals are dissolved in CDCl3.