Front octants

Examples of zig- zag coupling paths Substituent X is separated from the carbonyl groups by 1, 2, 3, 4, and 5 C—C bonds, respectively, in structures (19)—(23). Structure (24) illustrates a coupling path which extends into a front octant region... 

See Figure 1. Back octant contributors are denoted by filled circles ( ), front octant contributors or suspected front octant contributors are denoted by open circles (o). 

However, some ketones, such as 1-keto and 11-keto steroids have some molecular components lying in front of the carbonyl carbon (even in front of its oxygen), or in front octant regions. Clarification of the boundary between front octants and back octants was the subject of intensive theoretical and experimental studies, [6-11] which led to a clarification on the shape of the third nodal surface as convex (B, Figure 7). For most applications of the Octant Rule it is sufficient to consider only the back octants, and the special circumstances which require consideration of front octants will be obvious from the structure of the ketone. In practical applications of the Octant Rule, the octant diagrams are used to... 

Figure 7. (Left) Classical octant rule diagram (ref 1) for the ketone carbonyl n - x transition. Local symmetry-derived, orthogonal octant planes XZ and YZ divide all space into quadrants, and a non-symmetry-derived third nodal surface (A) is approximated by an orthogonal plane bisecting the C=0 bond. "Front" octants are those nearer an observer along the +Z axis, while "back" octants lie towards -Z. (Middle) Octant contribution signs that perturbers make in back and front octants. (Right) Revised octant rule [6] with octant planes XZ and YZ unchanged and the third nodal surface defined theoretically as a concave surface (B). [Reprinted with permission from ref. 7. Copyright ° 1986 American Chemical Society.]...

Figure 13. (a)-(d). Chair conformation (upper), approximate Ae (middle), and octant projection diagrams (lower) for a and 0-methylcyclohexanones. The Ae values are from refs 6-8, 1 and 12. (e)-(h). (3R)-Methylcyclohexanone with CH3 in an axial configuration (e) (15,3R)-4(a)-methyladamantan-2-one (f) and octant diagrams for back octants (g) and front octants (h). 

Rings A and B have the same octant perturbers as the corresponding decalone (Table I, entry (b)), but (-) front octant contribution rings C and D, together with (-) back octant... 

The Octant Rule applied to corresponding A/B cis (5/3) ring A ketones easily predicts the observed CEs. It is interesting to note the weak (-) CE for the 4-keto-5/3-steroid is probably due to an accumulation of front octant contributions (mainly from C-7, C-8 and C-9) overcoming the (+) octant contribution of the C-19 angular methyl. Here, as with the 5a-l-ketosteroid, it would have been difficult to predict the CE sign from the octant diagram. 

If viewed from the oxygen, most optically active carbonyl compounds have their substituents only in the rear octants. The appearance of the plane that separates the rear octants from the front octants is not determined by the symmetry of the isolated chromophore. Calculations have shown that it has approximately the shape depicted in Figure 3.4b. Many examples have verified the validity of the octant rule, but there are also cases where it is not applicable, at least not in its original, simple form. This is true for ketones with a cyclopropane ring in the a, jS-position and for fluorosubstituted ketones, for which the experimentally observed sign can be reproduced only if the perturbation due to the fluorine atom is assumed to be smaller than that due to the hydrogen atom. More recent detailed calculations solved some of these problems. (Cf. Charney, 1979.)... 

Comparison of c.d. data for the des-D-(tricyclic) and D-homo-7-ketones (26) and (27) respectively showed that ring d makes a significant front octant contribution to the n— Ti transition (290 nm), the sign of its contribution reversing that of a group in the corresponding rear octant. The contribution of ring d... 

The related Trost cyclobutanone annelation procedure, in which 1-lithiophenyl cyclopropyl sulphide is condensed with a cyclic ketone and the product rearranged with an acid catalyst, can be controlled to give considerable stereoselectivity and regiospecificity. Application to the methyladamantanone (321) permits specific syntheses of the two ketones (322a) and (322b) to be accomplished, and these have been used in the first experimental verification of the Front Octant Rule. ... 

Lightner has reported the first simple cases of ketones with only a dissymmetric front octant perturber. The spiro-ketone (11) was made from (-f )-7,7-dimethoxybicyclo-[2,2,l]heptan-2-ol using a Trost spiroannelation procedure. The methyl group is subject to large downfield shifts in benzene or in presence of [Eu(dpm)3], confirming the relative stereochemistry. The ketone shows a strong positive Cotton effect near... 

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