Achiral stereoisomers

In Table 1 the numbers of possibly chiral and achiral stereoisomers of various [n]chochins are compiled 64) their chiroptical properties are treated in Section 2.9. 

Structures VII and VIII are identical because rotating either one 180° in the plane of the paper makes is superposable with the other one. VII possesses a symmetry plane and is achiral. Achiral stereoisomers which have chiral centers are called meso. The meso structure is a diastereomer of either of the enantiomers. The meso structure with two chiral sites always has the (RS) configuration. 

Fig. 37. An achiral stereoisomer of 3,4,3, 4 -tetramethylspiro-(l,l )-bipyrrolidinium (p-toluenesulfonate salt) 89 possessing an S4 axis...

More elaborate molecules can also have a plane of symmetry. For example, there are only three stereoisomers of tartaric acid (2,3-dihydroxybutanedioic acid). Two of these are chiral but the third is achiral. In the achiral stereoisomer, the substituents are located with respect to each other in such a way as to generate a plane of symmetry. Compounds that contain two or more stereogenic centers but have a plane of symmetry are called meso forms. Because they are achiral, they do not rotate plane polarized light. Note that the Fischer projection structure of meio-tartaric acid reveals the plane of symmetry. 

Which of the following compounds have an achiral stereoisomer ... 

Fig. 10.2 Two achiral stereoisomers of meso-type (pseudo-asymmetric case), where the ligand p represents the mirror image of p in isolation...

Let us consider two achiral stereoisomers (2 and 3) shown in Fig. 10.2. They are also stereoisomeric to 1 and 1 shown in Fig. 10.1. By the above definitions, the relationship between 1 and 2 (or 3) is concluded to be diastereomeric, because the enantiomer 1 is different to the achiral 2. On the other hand, the relationship between the achiral 2 and 3 is also concluded to be diastereomeric, because their mirror images (2 and 3 themselves) are different to each other. It should be noted, however, that the diastereomeric relationship between 1 and 2 is different from the diastereomeric relationship between 2 and 3 in whether there exist enantiomeric relationships or not. 

Only three not four stereoisomeric 2 3 butanediols are possible These three are shown m Eigure 7 10 The (2R 3R) and (2S 3S) forms are enantiomers of each other and have equal and opposite optical rotations A third combination of chirality centers (2R 3S) however gives an achiral structure that is superimposable on its (2S 3R) minor image Because it is achiral this third stereoisomer is optically inactive We call achiral mole cules that have chnahty centers meso forms The meso form m Eigure 7 10 is known as meso 2 3 butanediol... 

FIGURE 7 10 Stereo isomeric 2 3 butanediols shown in their eclipsed con formations for convenience Stereoisomers (a) and (b) are enantiomers of each other Structure (c) is a diastereo mer of (a) and (b) and is achiral It is called meso 2 3 butanediol... 

Wnte structural formulas or make molecular models for all the compounds that are tnchloro derivatives of cyclopropane (Don t forget to include stereoisomers ) Which are chiraL Which are achiral" ... 

Compare 2 3 pentanediol and 2 4 pentanediol with respect to the number of stereoisomers possible for each constitution Which stereoisomers are chiraL Which are achiraL... 

Before leaving this biosynthetic scheme notice that PGE2 has four chirality cen ters Even though arachidomc acid is achiral only the stereoisomer shown m the equa tion IS formed Moreover it is formed as a single enantiomer The stereochemistry is controlled by the interaction of the substrate with the enzymes that act on it Enzymes offer a chiral environment m which biochemical transformations occur and enzyme catalyzed reactions almost always lead to a single stereoisomer Many more examples will be seen m this chapter... 

Enantiomers (Section 7 1) Stereoisomers that are related as an object and its nonsupenmposable mirror image Enantioselective synthesis (Section 27 4) Reaction that converts an achiral or racemic starting material to a chiral product in which one enantiomer is present in excess of the other... 

Merrifield method See solid phase peptide synthesis Meso stereoisomer (Section 7 11) An achiral molecule that has chirality centers The most common kind of meso com pound IS a molecule with two chirality centers and a plane of symmetry... 

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

The reason that the third stereoisomer is achiral is that the substituents on the two asymmetric carbons are located with respect to each other in such a way that a molecular plane of symmetry exists. Compounds that incorporate asymmetric atoms but are nevertheless achiral are called meso forms. This situation occurs whenever pairs of stereogenic centers are disposed in the molecule in such a way as to create a plane of symmetry. A... 

Since the presence of a plane of symmetry in a molecule ensures that it will be achiral, one a q)ro h to classification of stereoisomers as chiral or achiral is to examine the molecule for symmetry elements. There are other elements of symmetry in addition to planes of symmetry that ensure that a molecule will be superimposable on its mirror image. The trans,cis,cis and tmns,trans,cis stereoisomers of l,3-dibromo-rranj-2,4-dimethylcyclobutaijte are illustrative. This molecule does not possess a plane of symmetry, but the mirror images are superimposable, as illustrated below. This molecule possesses a center of symmetry. A center of symmetry is a point from which any line drawn through the molecule encouniters an identical environment in either direction fiom the center of ixnimetry. 

F. Silan, P. Jadaud, F. R. Whitheld and I. W. Wainer, Determination of low levels of the stereoisomers of leucovorin and 5-methylteti ahydrofolate in plasma using a coupled clii-ral-achiral liigh-performance liquid chromatograpliic system with post-cliiral column peak compression , 7. Chromatogr. 532 227-236 (1990). 

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