Anti-octant effects

Circular dichroism data for a series of a-ketols and their acetates show that axial acetoxy-groups frequently make anti-Octant contributions to the ketone (n 7T ) Cotton effect. Axial hydroxy-groups may produce either an Octant or anti-Octant effect. It seems probable that the conformation of the acetoxy-or hydroxy-group itself may be the decisive factor, the orientations of oxygen lone-pairs in relation to the carbonyl group probably being critical. 

P. Vogel s group studied exhaustively the 5,6,7,8-tetramethylidenebicyclo[2.2.2]octane system and its metal carbonyl complexes. The preparation and CD spectra of tricarbonyl-iron complexes (144-147) were reported333. The chirality of complexes 144 and 146 is due uniquely to the coordination of Fe(CO)3 moieties. The signs of the Cotton effects for (+)-144 and (+)-146 obey the octant rule, as the endo-Ft(CO)j, of 144 and 146 fall in a positive octant, while the second exo-Fe(CO)3 (syn to the carbonyl) lies almost on the XY nodal plane, and thus its contribution is expected to be small. The deuterium-substituted free tetraenone 148, however, showed an anti-octant behavior. The CD spectra of 144 and 146 are strongly temperature and solvent dependent. 

A strong positive Cotton effect at 320 nm in the CD spectra of uvidin A (3.5) and B (3.6) indicated the P-configuration of the epoxy ring, according to the anti-octant rule for ocP-epoxyketones (20). 

The influence of a - and )S -substituents on the Cotton effects of saturated ketones is well known, and is expressed in the familiar octant rule . Considerable effects due to suitably oriented y - and -substituents have now been demonstrated. Although no convenient series of steroidal ketones with y -substituents was available, a wide range of anti-Ti-substituted camphor derivatives (9) reveals effective perturbation of the carbonyl n—> n transition by... 

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