1,3-Diaxial repulsion

The greater stability of an equatorial methyl group compared with an axial one IS another example of a steric effect (Section 3 2) An axial substituent is said to be crowded because of 1,3 diaxial repulsions between itself and the other two axial sub stituents located on the same side of the ring... 

Less than 0 01 % (Serious 1 3 diaxial repulsions involving tert butyl group)... 

Diaxial repulsion (Section 3 10) Repulsive forces between axial substituents on the same side of a cyclohexane nng... 

The erosion in regioselectivity observed for ketones may be due to 1,3-diaxial repulsion in an intermediate 45 (indicated by shading in Scheme 12), that is inevitable when isoprene reacts with ketones at Cl position. Accordingly, the reaction may optionally proceed through an intermediate 45", which is formed by the reaction of a ketone with isoprene at the C4 position and may be sterically less congested than 45. ... 

The preference for the methyl group to assume an equatorial orientation is presumably reinforced by the necessity to avoid a 1,3-diaxial repulsion with the axial hydroxyl group. Similar routes to monofluorocyclohexanes [378] and difluorocyclopentanes [379] have been described by these authors. Recently, difluoroallylic radicals were investigated (Eq. 153) products from 6-exo and... 

Considering the anomeric effect, and allowing for the effect of 1,3-diaxial repulsions, predict the lowest energy conformation for the dimethylacetal of formaldehyde (MeO)2CH2. 

The olefin octant rule has been applied to the ti <— n transition of diterpene olefins near 200 nm. The o.r.d. and c.d. curves of levopimaric acid have been interpreted in terms of a folded conformation (4) of ring c which affords relief of the 1,3-diaxial repulsion between the angular methyl group and the 11) -hydrogen atom. 

Another way to use the 1,3-diaxial repulsion for the destabilization of a chair-chair conformation is the introduction of two bulky substituents in exo-2 and exo-4 positions (12), which should and does lead to a preference for the chair-boat conformation 12b (26). 

From the analysis of the factors thus far considered, boat-boat conformation 2c should be stabilized by 1,3-diaxial repulsion. The introduction of bulky substituents both in endo-3 and in endo-1 positions (13) leads to the predominance of the double boat conformation 13c in the equilibrium (27,28). Analogously, the introduction of four bulky substituents in exo-2,4,6, and 8 positions (14) could lead to the destabilization of the chair form for each of the six-membered rings and to a preference for boat-boat conformation 14c because of the strong 2 4 and 6 8 repulsion of the substituents. However, compounds of this kind have not been studied yet. It is necessary to mention that the boat-boat conformation should correspond in fact to a maximum on the energy curve, while a twist-twist, TT, form approximated by 15 would be expected to correspond to a minimum. In the TT conformation the flagpole and eclipsed interactions are diminished (the BB conformation is significantly more flexible than the CB one and is easily twisted). 

As demonstrated by X-ray diffraction studies, salts 204-208 adopt the chair- chair conformation in the crystal state. This conformation is stabilized by intramolecular N-H - X bonding (148,149,151). However, for steric reasons, compound 209 having two methyl groups at nitrogen adopts the CB conformation in the crystal state (from X-ray data) (152). The alternative BC conformation would be destabilized by 1,3-diaxial repulsion. 

The epimeric tetrol had a molecular rotation of —12°. The configuration l(125/3) should exist as a mixture of the aaee conformations (147 and 148) in about equal proportions. The rotation predicted is +45° for (148), and —90° for (147). The apparent conformational ratio for (148)/(147) is about 3 2. In any event, the more levorotatory epimer must have the (12/35) configuration (146). The observed conformational ratio of 3 2 for (148)7(147) probably results from competition between 1,3-diaxial repulsion and 1,3-diaxial hydrogen-bonding. ... 

Diaxial repulsion, 104 Diazonium salts, 890-897, 904-905 azo coupling of, 895-897, 936 conversion to... 

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