Octant extended rule

An extended study of the optical behavior of this class of products has been performed in connection with investigations on the conformations of lycorine (1) and related compounds. An empirical rule, similar to the octant rule, which allows the prediction of both the sign and magnitude of the Cotton effect at 290 nm from the steric orientation of the atoms about the aromatic chromophore, has been deduced from considerations on the ORD and CD curves of lycorine (1) and derivatives. When the benzene ring is viewed along the — z —+ z axis (the coordinates are fixed on the aromatic ring as depicted in Fig. 1), four back octants are defined which contribute, with the signs indicated, to the Cotton effect. 

In order to use its CD to determine the AC of a chiral molecule, a theory is required which predicts the sign of the CD of a given enantiomer. The utilization of CD by organic chemists was greatly stimulated by the development of the Octant Rule, which predicts the CD of the n-tt electronic excitation of carbonyl functional groups [3], Subsequently, so-called Sector Rules were developed for many other electronic chromophores, extending the applicability of CD [4],... 

There is already substantial experimental evidence and theoretical justification for electronic interaction between cyclopropyl and carbonyl groups. A charge-transfer band in the vapour-phase u.v. spectra of several cyclopropyl ketones has been detected at ca. 180 nm (s ca. 4000). The circular dichroism of several optically active cyclopropyl ketones, together with conformational analysis based upon n.m.r. spectroscopy results and INDO theory, has been used to extend the octant rule for such compounds. ... 

Snatzke has extended the useful analysis of the effect of 3-substituents upon the circular dichroism of adamantanones and substituted bicyclo[3,3,l]-nonan-9-ones. With earlier studies the effect of a wide range of substituents is now established, and the failure of the octant rule to account for the observed results is clear, but a satisfactory general analysis of substituent effects is still awaited. 

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