Conformational isomers cyclic compounds

More evidence for the existence of several conformational isomers, at least in liquid and gaseous substances comes from infrared and also Raman spectra. For example each conformer has its own I.R. spectrum, but the peak positions are often different. Thus the C-F bond in equatorial fluorocyclohexane absorbs at 1062 Cm-1, the axial C-F bonds absorbes at 1129 Cm . So the study of infrared spectrum tells, which conformation a molecule has. Not only this, it also helps to tell what percentage of each conformation is present in a mixture and since there is relationship between configuration and conformation in cyclic compounds the configuration can also be frequently determined. 

The same stereochemical principles apply to both acyclic and cyclic compounds. With simple cyclic compounds that have little or no conformational mobility, it can even be easier to foUow what is going on. Let us first look at cyclopropane-1,2-dicarboxylic acid. These compounds were considered in Section 3.4.3 as examples of geometric isomers, and cis and trans isomers were recognized. 

Cyclic sulphites have been prepared from 3a,5-dihydroxy-5a-cholestan-6-one and its 3/),5/l-isomer. ° The compounds exist with the six-membered sulphite ring in a boat conformation. Reduction of the 6-oxo-group in the 3 6,5)8-compound... 

The isomer that is the meso compound is the one with a plane of symmetry when an acyclic compound is drawn in its eclipsed conformation (B), or when a cyclic compound is drawn with a planar ring (D and F). 

Enolates derived from cyclic compounds such as cyclohexane carboxylic acid or cyclohexane carboxalde-hyde generate enolates that are unique. These enolates have an exocyclic double bond that can exist as ( ) and (Z) isomers. The facial and orientational bias in alkylation and condensation reactions of such enolates is influenced by the conformation of the ring it is attached to. Alkylidene cyclohexane enolates show a preference for equatorial attack, just as cyclohexanone derivatives do (sec. 4.7.C,D). 

Cyclic molecules of small numbers of ring-atoms are rigid and compact that they can serve as model-compounds for plastic crystals. As the number of ring-atoms increases they develop different conformational isomers. This series of compounds is thus suited to study the interrelation between conformational and orientational dynamic disorder. 

Stereoisomers are compounds that have the same chemical formula and connectivity but a different arrangement of the bonds in space. In cycloalkanes, they are similar to cis-trans isomers in that they cannot interconvert without breaking a bond. It is easiest to show the stereoisomers of a cyclic compound by drawing the ring flat and not worrying about conformations. 

The real test of single compound MI/FTIR was to differentiate between p-menthane-based conformational isomers Most of the acyclic and cyclic terpenoids... 

For the establishment of anchimeric assistance in tm zs-substituted cyclic compounds, the cis isomer contains essentially the same c-inductive effects and may, from a cursory inspection, seem to be a reasonable model. There are some problems with this model, however (1) Field effects are difficult to determine, and they probably affect many rate comparisons (2) the conformation for reaction may differ from the ground-state-preferred conformation, and hence estimates based on the wrong conformation may lead to errors and (3) hydrogen participation (or facile elimination) in the cis model may introduce errors. We shall now discuss this method and its potential pitfalls with the use of examples. 

Molecules of cyclobutane, cyclopentane, and cyclohexane are nonplanar. Besides determining configuration at each carbon atom in these compounds, conformational aspects of the whole molecule should also be considered. Figure 2.15 shows the most stable conformations of rings in these molecules. Of course, such molecules can also exist in any intermediary conformations between those shown. In monosubstituted rings of cyclobutane, cyclopentane, and cyclohexane there is no asymmetric carbon atom. In disubstituted isomers of these cyclic compounds, optical activity can appear even if the substituents are identical. The cis forms will not be optically active in any case since they exist in meso form. On the other hand, the trans forms will be optically active (they occur as two... 

In open-chain compounds, the molecule can usually adopt that conformation in which H and X are anti periplanar. However, in cyclic systems this is not always the case. There are nine stereoisomers of 1,2,3,4,5,6-hexachlorocy-clohexane seven meso forms and a dl pair (see p. 161). Four of the meso compounds and the dl pair (all that were then known) were subjected to elimination of HCl. Only one of these (1) has no Cl trans to an H. Of the other isomers, the fastest elimination rate was about three times as fast as the... 

The structures of the salts (131)168 and (132),159 a chelated ylide,160 and the insecticide coroxon161 have been determined. The structures of the cis- and trans-isomers of the cyclic phosphonamide (133) differed in that the cw-isomer has a half-chair conformation whereas the trans-isomer has an envelope conformation. Both compounds had hydrogen bonds of approximately 269 pm.162 The phosphadiazane (134)... 

The cavitands are essentially synthesized from their resorc[4]arene precursors which are readily obtained by resorcinol condensation with aldehydes. The main feature comes from the different configurations that are expected for this tetrameric species and the relative thermodynamical stability of each isomer, which has been widely investigated by several authors. In addition, the conformational mobility of the resorc[4]arene molecules will depend on substitution at the upper and lower rims [28, 36, 40, 41]. The first attempt to synthesize a phosphorus bridged cavitand was to treat resorc[4]arene la (1, R=CH3) with phenylphosphonic dichloride or phenylphosphonothioic dichloride. Only inseparable isomer mixtures were obtained and isolation of the desired cavitands was not possible [42]. The first isolated phosphorylated resorcinol-based cavitand was described in 1992 by Markovsky et al., who prepared compound D from la and four equivalents of o-phenylenechlorophos-phate in the presence of triethylamine [43, 44]. For this compound, a tautomeric temperature and solvent dependent equilibrium exists between the spirophosphorane structure and the cyclic phosphate form (Scheme 4). 

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