Conformations axial

We first examine the case of piperidine. The two conformations, axial and equatorial are shown below along with an enumeration of the major stabilizing interactions, if we only consider vicinal bonds. 

Now, we are prepared to compare the relative stabilities of cis and trans- 1,2-difluorocyclohexane. The cis conformation, axial-equatorial (ae), is shown below along with the dominant a—a interactions. 

MW/ED study38. A comparison between the experimental results and MM3 calculations is provided in Table 2 and reveals good agreement between the two methods. The latest gas-phase NMR study of this system has clearly demonstrated the presence of two conformers [axial and equatorial with respect to the H(N)] in accord with early IR and Raman studies but in disagreement with a later IR work. MM3 results are consistent with the two-minima representation of the system, the equatorial one calculated to be more stable by... 

Some theoretical works report the geometric features of penams skeleton, considering the strain of the bicyclic system and the bridgehead nitrogen pyramidality (see Section 2.03.3.1). For instance, the two possible conformations (axial and equatorial with respect to the C-3 carboxyl group) of the penicillins thiazolidine ring have been localized as two minima by ab initio calculations employing several theoretical methods (Hartree-Fock (HF), density functional... 

Epoxide openings are not alone in always giving diaxial products. We can give the general guideline that, for any reaction on a six-membered ring that is not already in the chair conformation, axial attack is preferred. You will see in later chapters that this is true for cyclohexenes, which also have the half-chair conformation described in the next section. Cyclohexanones, on the other hand, already have a chair conformation, and so can be attacked axially or equatorially. 

A monomeric Py is not possible, bnt the dimeric Pm" is known from the solvated Na4(en)6Pi4. EnPy is a Eu compound with a polymeric Py stmcture of two-dimensional pnckered layers resnlting from a network of six-membered rings (chair conformation axial and eqnatorial linkage). For a description of Py in CnyPy, see Section 6.5.6. 

Chemical shifts from six-membered rings are often easy to understand. Most six-membered rings are found to be in the chair conformation axial resonances are commonly found 0.4ppm upheld relative to their equatorial counterparts. This difference is ascribed to variations... 

The arrangement of substituents differs e. g., in a-D-idopyranose. Here, all the substituents are in axial positions in the " Ci-conformation (axial HO-groups at 1, 2, 3, 4), except for the CH2OH-group, which is equatorial. However, the Cq-conformation is thermodynamically more favorable despite the fact that the CH20H-group is axial (cf. Table 4.5) ... 

Cycloalkanes C H2 hydrocarbons n = 3, 4 Cyclopropane, cyclobutane (strained bond angles) n = 6 Cyclohexane (most stable, chair conformation) axial (less stable) equatorial (more stable) substituent positions (4-2, 3)... 

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