The Octant Rule. 15. Antioctant Effects: Synthesis and Circular Dichroism of 2-exo- and 2-endo-Alkylbicycloheptan-7-ones and -bicyclooctan-8-ones

Article abstract of DOI:10.1021/jo00220a037

Circular dichroism spectra of the n -> ?^* transition of alkylbicyclo<2.2.1>heptan-7-ones and alkylbicyclo<3.2.1>octan-8-ones have helped to define the boundary separating front and back octants in the ketone octant rule.Series of (1S,4R)-exo-2(R)-alkylbicyclo<2.2.1>heptan-7-ones and (1S,4R)-endo-2(S)-alkylbicyclo<2.2.1>heptan-7-ones were prepared and their circular dichroism (CD) spectra measured and compared with (1S,3R)-4(S)(a)- and -4(R)(e)-methyladamantan-2-ones.The alkyl groups include methyl, ethyl, and n-hexyl.Similarly, (1R,5S)-exo-6(R)- and -endo-6(S)-methylbicyclo<3.2.1>octan-8-ones were synthesized and their CD spectra determined.The CD Cotton effects for the n -> ?^* transitions of these ketones aid in defining the third nodal surface of the octant rule as convex, bending outward from behind the carbonyl carbon toward oxygen.The ring skeletal symmetry was examined by molecular mechanics (MM2) calculations, which located the coordinates (positions) of lone dissymetric methyl groups relative to the carbonyl group. 13C NMR spectroscopy confirmed the methyl group locations and provided additional new examples of the γ-gauche effect.