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What are Epimers?

Epimer in stereochemistry explicitly states one of a pair of stereoisomers.  At the stereogenic center, two isomers present in the molecules differ, while the remaining remains identical. A molecule may contain various stereocenters leading to several stereocenters.

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Stereoisomers are the isomeric molecules that possess the same constitution and the molecular formula, but they vary in three-dimensional orientations of their atoms that are in the space.

Stereoisomers illustration that is given below explains the L and D configurations of the glucose. Here, glucose is referred on the basis of the last chiral carbon atom.

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Epimers of glucose involve some formations such as starch, glycogen, glucose, polysaccharides, and oligosaccharides. Because of the presence of carbon in glucose molecules, it can exhibit stereoisomerism, which is enantiomers and diastereomers.

Examples of Epimers

The stereoisomers β-D-mannopyranose and β-D-glucopyranose are epimers examples because they differ only in the C-2 position of stereochemistry. The hydroxyl group in the β-D-glucopyranose is equatorial (in the "plane" of the ring), whereas, in β-D-mannopyranose, the C-2 hydroxyl group is axial (up from the "plane" of the ring). These two molecules are epimers, but because they are not the mirror images of each other, they are not enantiomers. (Enantiomers contain the same name, but differ in L and D classification). Also, they are not sugar anomers since it is not the anomeric carbon that is involved in the stereochemistry. Likewise, β-D-galactopyranose and β-D-glucopyranose are epimers that differ at the position C-4, with the latter being axial and the former being equatorial.

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Considering the case, the difference is the -OH groups on C-1, the anomeric carbon, like in the case of β-D-glucopyranose and α-D-glucopyranose, the molecules are both anomers and epimers (as indicated by the designations of α and β).

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Other compounds that are closely related include lipoxin and epilipoxin, epi-inositol, and inositol.

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Enantiomers Definition

Enantiomers are also called Optical isomers, which are two isomers that are relevant to each other by reflection. Let us look at the enantiomers definition and examples. To define enantiomers, they are non - superimposable and consist of similar physical properties expected in a manner they interact with various optical isomers that are related to the other compounds. Hence different optical isomers can have the variant biological effects.

Enantiomers Examples

Sedative thalidomide is one of the enantiomers examples, which was sold around the world in a number of countries from 1957 until 1961. When it was found to occur birth defects, it was withdrawn from the market. One enantiomer caused the desirable sedative effects, while the other caused birth defects, presenting equal quantities unavoidably.

The structure of the two enantiomeric forms (R right, S left) of mecoprop is represented below.

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The herbicide mecoprop, a racemic mixture, containing the (R) - (+) - enantiomer ("Mecoprop-P" and "Duplosan KV") possessing the herbicidal activity.

Another example is antidepressant drugs, citalopram, and escitalopram. Escitalopram [(S)-citalopram] is a pure enantiomer, and citalopram is a racemate [1:1 mixture of (S)-citalopram and (R)-citalopram]. Typically, the dosages for escitalopram are 1/2 of those for citalopram.

The enantiomers of citalopram are represented below. The bottom is (S)-citalopram, and the top is (R)-citalopram.

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Diastereomers are otherwise called diastereoisomers, a type of stereoisomer. These are defined as the non-mirror image non-identical stereoisomers. Hence, they occur when two or more compound stereoisomers have different configurations at either one or more (but not all) of the equivalent or related stereocenters, and they are not the mirror images of each other. When two of the diastereoisomers differ from each other at only one stereocenter, they are known to be epimers. Each of the stereocenters gives rise to two different configurations and thereby typically increases the stereoisomers count by a factor of two.

Diastereomers vary from enantiomers in that the latter are stereoisomers pairs that differ in all stereocenters, and therefore they are the mirror images of one another. The enantiomers compound having more than one stereocenter are also the diastereomers of other stereoisomers of the same compound that are not their mirror image (i.e., excluding the opposing enantiomer). Diastereomers also have different physical properties (unlike most aspects of enantiomers) and different chemical reactivity often.

Diastereomerism can occur at a double bond also, where the trans relative vs. cis positions of substituents produce two non-superposable isomers. Also, many conformational isomers are diastereomers examples.

Diastereoselectivity is the preferred formation of either one or more than one diastereomer over the other in an organic reaction.

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To illustrate the formation of an enantiomeric pair, consider the example given below, where mesotartaric acid forms a diastereomeric pair with the dextro tartaric acids and Levo.

1. Dextrotartaric Acid

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2. Levotartaric Acid

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Doxorubicin and Epirubicin are also the epimers that are used in drugs.

Applications of Diastereomers

As discussed previously, two diastereomers will not have chemical properties as identical. This knowledge is harnessed in chiral synthesis to separate an enantiomers mixture. This is the principle behind the chiral resolution. They are separated by chromatography or recrystallization after preparing the diastereomers.

FAQ (Frequently Asked Questions)

1. Explain the Epimer and Give an Example?

Monosaccharides such as glucose, fructose, galactose, and mannose can show different isomerism. For suppose, fructose and glucose are the functional isomers of each other as glucose contains the aldehyde group, whereas fructose contains a ketonic group in a molecule. Also, they can show stereoisomerism because of the presence of chiral carbon atoms.

Those stereoisomers which are different in its configuration at only one chiral carbon atom are called Epimers.

For example, galactose and glucose are Epimers of each other because they differ only in the position of a hydroxyl group at C4(chiral carbon atom).

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2. Explain Anomers, Enantiomers, and Epimers?

Epimer is a stereoisomer where the configuration is different in any one of the stereogenic centres in a carbohydrate molecule. All the other stereocenters in the molecules are the same in each.

Anomer is also an epimer where the configuration is different at the hemiacetal/ketal/acetal carbon in the cyclic form. The cyclic form is required because the configuration difference does not arise if the carbonyl group (aldehyde/ketone) does not form hemiacetal or acetal.

Epimers or anomers are also diastereoisomers.

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Enantiomers are simply the pair of optically active molecules, where one is the mirror image to another. They are also non-superimposable, which means they don't match if one enantiomer is placed over another.

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