Cyclic compounds stereoisomers

Configurations in cyclic compounds are considered in the same way as for acyclic compounds. If you cannot get an answer with the first atom, move on to the next, even though this may mean working around the ring system. Consider, for example, the stereoisomer of 3-methylcyclohexanol. 

The same stereochemical principles are going to apply to both acyclic and cyclic compounds. With simple cyclic compounds that have little or no conformational mobility, it is easier to follow what is going on. Consider a disubstituted cyclopropane system. As in the acyclic examples, there are four different configurational stereoisomers possible, comprising two pairs of enantiomers. No conformational mobility is possible here. 

Figure 21. The equilibrium between the helical interlaced system precursor of the trefoil knot and its face-to-face analogous complex leading to the face-to-face complexes. Interconversion between the two isomeric cyclic products is, of course, not possible. For the cyclic compounds, the total number of atoms x connecting two phenolic oxygen atoms is 16 if n=4 (pentakis(ethyleneoxy) fragment) or 19 if n = 5 (hexakis(ethyleneoxy) linker). Each knot is represented by the letter k accompanied by the overall number of atoms included in the cycle. The face-to-face complexes contain two monocycles (letter m), the number of atoms in each ring also being indicated. It can be noted that each knot has a face-to-face counterpart. For instance [Cu2(k-90)]2+ and [Cu2(m-45)2]2+ are constitutional isomers. They are by no means topological stereoisomers [34, 35].

The four different groups attached to a chiral carbon can be different elements, isotopes, or functional groups, and chiral centers can be present in bodi open-chain molecules or cyclic compounds. The recognition of chirality and chiral centers in molecules is an important step in determining the numbers of stereoisomers that are possible for a given compound. 

The presence of two or more substituents on a ring—any size ring—introduces the possibility of stereoisomers. The existence of stereoisomers is independent of conformations and should be analyzed first because different stereoisomers will have different conformations. It is easiest to examine the stereoisomers of cyclic compounds by considering the rings to be flat, even though they may actually exist in chair or other conformations. Once all the stereoisomers have been identified, the conformations of each can be scrutinized. 

The cis- and trans-stereoisomers of cyclic compounds that were presented previously are actually just special cases of the type of stereoisomers that we have just discussed. For example, consider the case of 1,2-dimethylcyclohexane ... 

The Julia olefin synthesis is rather like the Wittig reaction with a sulfone instead of a phosphonium salt but with one other important difference the elimination step is stereoselective and both dia-stereoisomers of the intermediate can give the same isomer of the alkene. Treatment of the sulfone 147 with a strong base gives the anion 148 (or a metal derivative) that combines with an aldehyde to give a diastereomeric mixture of adducts 149. Elimination by various methods gives, in open chain compounds, mostly -150 but, in cyclic compounds, mostly the Z-alkene.29... 

When the two substituents on the ring are both pointing up (or both pointing down), these cyclic compounds are designated ew-stereo-isomers. When one substituent is pointing up and the other down, these cyclic compounds are designated tram-stereoisomers (see Section 3.3). 

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. 

Finally, a bidentate ligand has also been employed to prepare cyclic phosphoramidate esters such as the cyclic compounds 67 [145]. The (N-3-hydroxypropyl)-amino esters could be prepared from commercially available amino acids, ultimately allowing preparation of both the Sp and Rp isomers, and the phosphoms stereochemistry was assigned based on NMR data. Small differences in activity were found between the phosphorus stereoisomers, and the most... 

Cis and trans stereoisomers of a cyclic compound can be classified as diastereomers. (6.5)... 

When a molecule has two or more stereogenic (chiral) centers, there are a maximum of 2" stereoisomers, where n = the number of chiral centers. When a molecule has two or more chiral centers, diastereomers are possible. Diastereomer is the term for two or more stereoisomers that are not superimposable and not mirror images. A diastereomer that has symmetry such that its mirror image is superimposable is called a meso compound. If there is no symmetry, cyclic molecules can have enantiomers and diastereomers. If there is symmetry in one diastereomer, cyclic compounds can have meso compounds 23, 24, 25, 26, 27, 28, 29, 30, 48, 49, 59, 60, 62,63,67,68,69, 70, 71, 75, 78, 79. 

Even isomeric pairs of achiral molecules may exist as separable stereoisomers. Examples include cis- and trans-, or (Z)- and (E)-, alkenes such as 9 and 10, and cyclic compounds such as 11 and 12. The molecules are nonsuperimposable, yet they are not mirror images that is, they are not enantiomers. Rather, they are diastereomers and have different chemical and physical properties. 

In the case of the acyclic compound, the meso compound is the stereoisomer with a plane of symmetry when drawn as an eclipsed conformer (B). For the cyclic compounds, the meso compound is the cis isomer (D and F). 

It is instructive to compare the stereoisomers of 2-bromo-3-chlorobutane with those of a cychc analog, l-bromo-2-chlorocyclobutane (Figure 5-9). In both cases, there are four stereoisomers RJt, S,S, R,S, and 5,/f. In the cyclic compound, however, the stereoisomeric relation of the first pair to the second is easily recognized One pair has cis stereochemistry, the other trans. Cis and trans isomers (Section 4-1) in cycloalkanes are in fact diastereomers. 

In Summary The presence of more than one stereocenter in a molecule gives rise to diastereomers. These are stereoisomers that are not related to each other as object and mirror image. Whereas enantiomers have opposite configurations at every respective stereocenter, two diastereomers do not. A molecule with n stereocenters may exist in as many as 2" stereoisomers. In cyclic compounds, cis and trans isomers are diastereomers. 

Diastereomers are stereoisomers that are not related to each other as object to mirror image. Cis and trans isomers of cyclic compounds are examples of diastereomers. 

As we found when we considered acyclic compounds containing two stereogenic centers, the number of stereoisomers of cyclic compounds depends on whether or not the centers are equivalent. This also true for cychc compounds. First, we U examine the isomeric cis- and 1-bromo-2-chlorocyclobutanes (Figure 8.14a). These compounds are diastereomers. The compounds in Figure 8.14 are arranged to demonstrate the mirror image relationship of the two tram enantiomers, whose configurations are IR,2R and 15,25. There are also two enantiomeric cis isomers. 

Cyclic diols give dicarbonyl compounds The reactions are faster when the hydroxyl groups are cis than when they are trans but both stereoisomers are oxidized by periodic acid... 

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