Chair conformations comparing stability

Problem 9.16 (a) Draw the possible chair conformational structures for the following pairs of dimethylcy-dohexanes (i) cis- and trans-1,2-, (ii) cis- and frans-1,3- (iii) cis- and trans-lA-. (b) Compare the stabilities of he more stable conformers for each pair of geometric isomers, (c) Determine which of the isomers of limethylcyclohexane are chiral. <... 

Problem 9.7 Compare stabilities, of. the possible chair conformations of (a) cis ... 

Let s compare the stability of chair conformations once again, this time for compounds that bear more than one substituent. Consider the following example ... 

Every compound with a cyclohexane ring has two chair conformers thus, both the cis isomer and the trans isomer of a disubstituted cyclohexane have two chair conformers. Let s compare the structures of the two chair conformers of cw-l,4-dimethylcyclohexane to see if we can predict any difference in their stabilities. 

Note that the C2, C3 and the C5, C6 sigma bonds no longer retain their parallel orientation in the twist conformation. If the ring system is twisted too far, another boat conformation results. Compare the nonbonded (van der Waals repulsion) interactions and the torsional strain present in the boat, twist, and chair conformations of cyclohexane. Is it apparent why the relative order of thermodynamic stabilities is chair > twist > boat ... 

In its primary structure, the AGU are existing in the conformation c (chair conformation). The valence angles between the AGU are favoring a helical conformation, formed by 6-8 AGU, as the energetically most suitable state. The normal state in solution is that of a disturbed helix. An ideal stable helix conformation is formed and stabilized when hydrophobic molecules (iodine, aliphatic alcohols and acids) are allowed to penetrate into the molecular channel. The formation of such inclusion complexes is a typical property of a. and may be compared best with the inclusion behavior of - cyclodextrin. Insoluble complexes with organic solvents are used to precipitate amylose from starch solutions during fractionation. 

The substitution of a heteroatom for an a-sulfoxy methylene group substantially increases the preference for an axial orientation of the sulfoxide oxygen320, despite the smaller space requirement of the sulfur with its lone pairs, compared to that of a methylene group321, at least in the case of 1,3-dithiolane oxides. The substituting heteroatom, therefore, should decrease the conformation stability (i.e. lower the barrier to chair-chair interconversion). 

Transannular cation radicals with the intramolecular sulfur-sulfur bond of the 2ct-1ct type generated from medium-ring disulfides like 1,5-dithiacyclooctane are an exception in terms of their stability, although they are not resistant to water (Musker 1980). ESR and resonance Raman spectroscopy studies revealed the existence of the 1,5-dithia-cyclooctane cation radical, with substantial bonding between the sulfur atoms (T. Brown et al. 1981 Tamaoki et al. 1989). Computations confirmed this statement and pointed out that the chair-boat conformer has the lowest energy as compared to other possible conformers (Stowasser et al. 1999). 

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