Antioctant rule

From the very beginning the octant rule ran into difficulties because several (mostly well-defined) observations did not agree with its predictions. Striking examples where the octant rule is in serious conflict with the observations for a whole class of compounds are provided by the conjugated (a, )5)-cyclopropyl ketones. Here, the predicted signs of the (n, 7T ) CE were often opposite to the experimental findings. Therefore, for these types of molecules as an empirical finding an antioctant rule was assumed to be applicable. 

The above discussions about octant and antioctant behavior of cyclopropyl ketones has revealed that often there exists ambiguities in the interpretation of the CD spectra of these compounds. Therefore, sometimes the discussions of CD spectra of cyclopropyl ketones are a posteriori explanations of the experimental findings rather than a priori predictions of the outcomes of the experiments. This means that, in particular, the antioctant rule is viewed as a regularity and discrepancies have to be explained in terms of deviations from the foundations of that rule (such as conformer populations and/or special electronic situations of the chromophore). 

For geometrically rather well defined situations for the application of the antioctant rule two basic situations are differentiated, namely condensed systems (fused rings) XXV and spiro-compounds XX VI. The geometrical arrangements in these systems can be defined... 

For the bicyclo[4.1.0]-type of cyclopropyl ketones compounds 140 and 141, for instance, present support for the antioctant rule °. ... 

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