Diastereoisomers or diastereomers

For compounds with two stereogenic centres, four stereoisomers are possible, as there are four possible combinations of R and S. 

Example (the numbers represent the numbering of the carbon backbone) 

For a compound containing n chiral centres, the total number of stereoisomers will be 2 and the number of pairs of enantiomers will be 2n.  

---

The answers are shown in Figure 4.20. Structures (1) and (2) are enantiomeric pairs. Structures (1) and (3) and structures (2) and (3) are pairs of diastereoisomers (or diastereomers), while structure (3) is a meso compound. A meso compound is optically inactive since it possesses a plane of symmetry and is superimpos-able on its mirror image. It does, however, contain two chiral carbon atoms. This reminds us that not all compounds that contain chiral centres are optically active. 

Diastereoisomers (or diastereomers) are stereoisomers that are not mirror images of each other. This means that diastereoisomers must have a different (R or S) configuration at one of the two chiral centres. 

It is, of course, possible to have more than one centre of asymmetry in a molecule. Each centre can have either the R- or -configuration and so with two centres of asymmetry, there will be four possible stereoisomers R-R, R-S, S-R and S-S. Such isomers are known as diastereoisomers, or diastereomers. Similarly, if a third asymmetric carbon is introduced, the number of possible isomers doubles again, this time to eight. Simple mathematics reveals that, if there are n asymmetric carbon atoms in a molecule, then there are 2 stereoisomers. 

The other two isomers are the (15,25) and (IR,2R) isomers, and these two also share a mirror image relationship, have the opposite configuration at both chiral centres, and are, therefore, a pair of enantiomers. From a structure with two chiral centres, we thus have four stereoisomers that consist of two pairs of enantiomers. Stereoisomers that are not enantiomers we term diastereoisomers, or sometimes diastereomers. Thus, the (15,25) and (1R,2R) isomers are diastereoisomers of the (l/f,25) isomer. Other enantiomeric or diastere-omeric relationships between the various isomers are indicated in the figure. 

Stereoisomers can be classified according to their symmetry properties into enantiomers and diastereomers. Enantiomers are mirror images of each other and, so, are often called antipodes (Figure 3-2) each isomer has the same energy. Diastereoisomers are not, on the other hand, mirror images they do not have the same internal energy. Consequently, two stereoisomers may be either enantiomers or diastereomers they may not be both simultaneously. 

Diastereoisomer Stereoisomers with two or more chiral centers and where the molecules are not mirror images of one another, for example, d-erythrose and D-threose often contracted to diastereomer. 

Diastereoisomers. Whereas compounds with one chiral center exist as an enantiomorphic pair, molecules with two or more chiral centers also exist as diastereoisomers (diastereomers). These are pairs of isomers with an opposite configuration at one or more of the chiral centers, but which are not complete mirror images of each other. An example is L-threonine which has the 2S, 3R configuration. The diastereoisomer with the 2S, 3S configuration is known as i-a//o-threonine. L-isoleucine, whose side chain is -CH(CH3) CH2CH3, has the 2S, 3R configuration. It can be called 2(S)-amino-3(R)-methyl-valeric acid but the simpler name L-isoleucine implies the correct configuration at both chiral centers. 

Here, the authors were faced with the problem that the AD of 94 proceeded in a sluggish and non-diastereoselective manner. Almost equal amounts of diastereomers 95 and 96 were obtained. After optimization both diastereoisomers were produced in 97% combined yield and in a 64 34 ratio in favor of the desired diastereomer 96. It is interesting that the use of either (DHQ)2PHAL or (DHQD)2PHAL afforded this diastereoisomer predominantly. Attempts to increase the diastereomeric ratio by protection of the primary hydroxy group as the 4-methoxy benzoate failed. 

If we are dealing with diastereoisomers the same thing applies. Compound 20 is not chiral so the question of enantiomers doesn t arise but each diastereomer of 20, syn or anti gives a different diastereomer of 21 with inversion. 

Distinction should be made at this time between diastereoisomers and enantiomers. The former are characterized by the presence of at least two closely associated asymmetric centers in the molecular structure, either of which can epimerize. Altogether then there are two pairs of enantiomers for a total of four stereochemically unique individuals. Diastereoisomers have different physical properties and as a result discriminations, and even separations, can be done relatively easily. Enantiomers on the other hand differ in only one physical property, i.e. the direction of rotation of polarized light. Reaction of an enantiomeric racemic mixture with a third chiral species will produce a mixture of diastereomers therefore facilitating their identification or their separation. Early examples of this were the separations done by fractional crystallization of salts produced by a derivatization reaction with, for example, the alkaloid (-)-brucine. Fractional crystallization would never seem to be an effective analytical method yet it was used with some success in a forensic sciences context to confirm the presence of (L)-cocaine by a carefully contrived microcrystalline test. The physical properties... 

The diastereomers of Mg-ATP in Fig. 2 exemplify in a limited way the stereochemical problem in metal nucleotides. The two isomers shown are in rapid exchange equilibrium, however, so it is not possible to separate them and study their individual interactions with enzymes. The problem is further complicated by the fact that these diastereomers represent the stereochemical possibilities in only one of the coordination isomers. Others are possible, including the a-, / - and y-monodentate isomers and a,/ ,y-tridentate isomers, most of which exist as two or more diastereoi-somers. All of the coordination isomers and their diastereoisomers of Mg-ATP are in rapid exchange equilibrium. 

M] or [rh] Molecular rotation, defined as [a] x MW/100. Specific rotation corrected for differences in MW. The symbol [M] and the term molecular rotation are now deemed incorrect, and the term molar rotation denoted by [d ] is preferred. meso- Denotes an internally compensated diastereoisomer of a chiral compound having an even number of chiral centres, e.g., me o-tartaric acid. Formally defined as an achiral member of a set of diastereomers that also contains chiral members, mutarotation Phenomenon shown by some substances, especially sugars, in which the optical activity changes with time. A correct presentation is, e.g., [a]n ° + 20.3 -101.2 (2h)(c, 1.2 in HjO). 

When a molecule contains more than one chiral center, there are pairs of molecules not related by mirror symmetry. These molecules are called diastereomers. They differ in the relative chiralities at pairs of chiral centers (see Figure 14.3) and their chemical and physical properties are not identical. In this they differ from enantiomers, which have identical chemical and physical properties except those connected with their interaction with chiral detectors — plane-polarized light or enzymes, for example. Members of a pair of diastereoisomers that differ only in the configuration at one carbon atom are called epimers. 

Toth et fl/.w9.35o.w7.408 established the conformations of three diaste-reomers of the dodecahydropyrido[2,l-i>]quinazolin-l 1-ones 343,346, and 347 (R = H or Me). In the diastereomers 343, characterized at positions C-4a, C-1 la, and C-5a by relative configurations the predominant conformation is 445, which contains 4a-H and the lone-pair electrons of N-5 in the antiperiplanar disposition. In the a,a,a diastereoisomers 346 the conformational equilibrium is shifted toward conformer 446, in which N-5 is axial with respect to ring A, while the equatorial position is preferred for the iV-methyl group and the axial position for the N-H proton. 

---