Circular Dichroism of Planar, Exocyclic s-cis-Butadienes Remotely Perturbed

Article abstract of DOI:10.1002/hlca.19880710215

Optically pure 5,6-dimethylidenebicyclo<2.2.1>hept-2-yl derivatives have been prepared.The sign of the Cotton effects associated with the lowest-energy transition of 2-(dicyanomethylidene)-((-)-(1S,4S)-15), (E)-2-(methoxyimino)-((+)-(1S,4S)-16), (Z)-2-(methoxyimino)-5,6-dimethylidenebicyclo<2.2.1>heptane ((-)(1S,4S)-17), and 2,3,5-trimethylidenebicyclo<2.2.1>heptane ((-)-(1R,4S)-18) is opposite to the chirality constituted by the coupling of the electric transition moments of the two homoconjugated ?-chromophores (Kuhn-Kirkowood dipole-coupling mechanism).When the substituents at C(2) are not ?-functions, no general rule can be retained for the chiroptical properties of the 5,6-dimethylidenebicyclo<2.2.1>hept-2-yl systems as shown for dimethyl acetal (-)-(1S,4S)-19, ethylene acetal (+)-(1R,4R)-20, exo and endo methyl ethers (+)-(1R,2S,4R)-20,and (+)-(1R,2R,4R)-22, and for spiro<5,6-dimethylidenebicyclo<2.2.1>heptane-2,2'-oxiranes>(-)-(1S,2S,4S)-23 and (-)-(1S,2R,4S)-24.