Diene rule

The diene rule has had much importance in the interpretation of the optical activity of distorted dienes, as in the case of occidentalol, epi-occidentalol and fraws-occidentalol, 10, 11 and 12, respectively17. 

This means that R is proportional to the sine of the dihedral angle between the two double bonds, with the positive sense defined in Scheme 2. Such a dependence confirms the diene rule because positive angles define positive helicity and imply positive R. 

Dienes in quasi-s-fraws conformation are found only in cyclic structures where perfect planarity is hindered. The DR also holds valid for this kind of conformation, as demonstrated by the considerations of Section II.D.l.a and also confirmed by all the reported calculations. Indeed, contrary to what is sometimes found for cisoid systems, the rotational strength evaluated by many types of calculation is invariably found to follow the diene rule for transoid systems. However, very small skew angles are usually found in real molecules and this implies that the main contribution to the observed optical activity cannot come from the weak intrinsic distortion, but is more likely to stem from the dissymmetric perturbations, notably of the allylic axial substituents. 

The intrinsic contribution, accounted for by the diene rule, seems to be easily outweighed by the perturbations arising from the allylic axial substituents, which in turn define the allylic axial chirality rule. This latter can be formulated in two ways The olefin-picture , where chirality is referred to the nearest double bond, and the diene-picture ,... 

In this situation the two end-ring double bonds define a positive chirality. Application of the twisted diene rule gives a positive sign for the lowest-energy 1A - 1B transition, and in the case of 15-cA-(3R,3 R)-zeaxanthin this is confirmed by experiment. 

Walborsky and coworkers206 have reported the CD of series conjugated cyclohexylidene aldehydes and ketones. These compounds served as additional examples supporting the planar diene rule for CD jt tt transition208. 

The influence of allylic oxygen functions on application of the transoid diene rule in c.d. spectra of Erythrina bases has been discussed. ... 

The ultraviolet spectra of crowded olefins and substituted butadienes have been studied the sign of the Cotton effect has been related to the chirality of a series of w-molecular complexes of olefins with tetracyanoethylene the sign of the chiral-optical effects of non-planar heteroannular cisoid dienes is opposite to that predicted from the diene rule. Studies of linear and circular dichroism in mono-olefins and magnetic circular dichroism of conjugated olefins have been made, and the experimental ionization potentials of fourteen alkenes have been correlated with the inductive effects of substituents. ... 

---