Hydrogen atom axial

FIGURE 1.6 The two chair conformations of cyclohexane a = axial hydrogen atom and e = equatorial hydrogen atom. 

The presence of 1,3-diaxial interaction between the C-2 alkyl group and the C-4 axial hydrogen atom is reflected in the rate of enamine formation of 2-substituted cyclohexanone. It has been shown by Hunig and Salzwedel (20) that even under forcing conditions, the yield of pyrrolidine and morpholine enamines of 2-methylcyclohexanone does not exceed 58%, whereas the C-2 unsubstituted ketones underwent enamine formation under rather milder conditions in better than 80 % yield. 

Figure 4.14 The origin of 1,3-diaxial interactions in methylcyclohexane. The steric strain between an axial methyl group and an axial hydrogen atom three carbons away is identical to the steric strain in gauche butane. Note that the -CH3 group in methylcyclohexane moves slightly away from a true axial position to minimize the strain.

Figure 4.18 The chair conformation of cyclohexane. The axial hydrogen atoms are shown in color.

Study of this reaction has not been limited to cyclic acetals of aldoses and aldosides, and synthetic applications in the ketose series have been developed,273,276,278,278 particularly for isopropylidene acetals of D-fructose. As an illustration of these reactions, two examples are given here. The action of butyllithium on 2,3 4,5-di-0-isopropyli-dene-l-0-methyl-/3-D-fructopyranose (280) gave273 a 30% yield of the 5-enopyranose 282. Abstraction by the base of the axial hydrogen atom from the 6-methylene group, giving the anion 281, was invoked... 

FIGURE 1.7 The two chair conformations of cyclohexane a = axial hydrogen atom and e = equatorial hydrogen atom. The middle and bottom panels show methylcyclohexane in the chair form with the methyl group equatorial (middle) and axial (bottom). 

XIV n=6) are extremely low (0-5%) in either method. This is doubtless due to the severely increased steric interaction of nonbonded groups attached to the ring, i.e., an axial methyl group and three axial hydrogen atoms on either side of the ring, in addition to a lowering of the probability of intramolecular collision of the two reactive atoms at the chain ends. 

Pentacene ethers 37-39 (Fig. 3.8) were prepared in an attempt to reduce the HOMO energy level relative to functionalized pentacene 29 [52]. These cyclic ether compounds were surprisingly stable, soluble and easily crystallized. Saturated diox-ane compound 37 has an oxidation potential of 0.580 V vs SCE (similar to that of unfunctionalized pentacene), but the axial hydrogen atoms on the dioxane ring prevent -stacking interactions in the crystal. Derivatives 38 and 39 were synthesized to overcome this structural issue. Although the change of oxidation potential... 

The cis-selectivity of these reactions can be rationalised by invoking a pseudo-chair transition structure. Transition structure A leading to the trans product suffers from a 1,3-diaxial interaction between the alkyl substituent and an axial hydrogen atom on the ring. This unfavourable interaction is... 

Take another look at the chair conformation on p. 458. All six carbon atoms are identical, but there are two types of protons—one type stick either vertically up or down and are called axial hydrogen atoms the other sort stick out sideways and are called equatorial hydrogen atoms. 

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