Groups, equatorial

In six-membered rings containing heteroatoms, the basic principles are the same that is, there are chair, twist, and boat forms, axial and equatorial groups, and so on. The conformational equilibrium for tetrahydropyridines, for example has been studied. In certain compounds, a number of new factors enter the picture. We deal with only two of these. ... 

We go all the way around the ring like this, until we have drawn aU of the equatorial groups ... 

FIGURE 14.8 The Berry pseudorotation that leads to interchange of axial and equatorial groups. The letters a, e, A, etc., indicate only positions because all five groups are identical. 

The enhanced reactivity of the axial 5-hydroxyl group over that of the equatorial group on C-3 in methyl quinate towards acid chlorides in pyridine has been attributed147 to intramolecular hydrogenbonding of the former group to the syn-axially disposed oxygen atom on C-l. 

Comparative Fcyclohexane derivatives (and steroids) are more strongly retained than axial derivatives... 

In chromic acid oxidation of isomeric cyclohexanols, it is usually found that axial hydroxyl groups react more rapidly than equatorial groups. For example, trans-4-t-butylcyclohexanol is less reactive (by a factor of 3.2) than the cis isomer. An even larger difference is noted with cis- and trans-3,3,5- trimethylcyclohexanol. The trans alcohol is more than 35 times more reactive than the cis. Are these data compatible with the mechanism given on p. 748 What additional detail do these data provide about the reaction mechanism Explain. 

In a first set of experiments [52], the methyl ester of cholic acid 66a (Structures 8) was reacted with diphenylketene (59a) in the presence and in the absence of catalysts [pyridine (50), concave pyridine 3r]. But in all experiments, only the equatorial group in 3-position could be acylated (66b). No functionalization of the axial OH-groups in 7- and 12-position was found (66c, 66d). 

When using chair conformers, the better way to determine whether substituents are cis or trans is to look at he axial rather than the equatorial groups. If one axial bond is up and the other is down, the isomer is irons if X)th axial bonds are up (or down), the geometric isomer is cis. 

One type of permutational rearrangement, known as pseudorotation, can be visualized as in Eq. 12-28.668 669 The axial groups a and b move back while equatorial groups 2 and 3 move forward, still in the same equato-... 

The greatly increased steric repulsion between the axial and the bulky equatorial groups in trisilylated 1-methyltriazasilatrane (198) leads to a significant weakening of the transannular Si — N bond. The Si—N distance in 198 is the longest ever recorded in a triazasilatrane (2.775 A)410. It makes the atom sufficiently basic to react with CFsSOsMe, forming the cationic species 199 in which the silicon atom is tetracoordinated... 

---