enantiomer

Examples of enantiomer in the following topics:

• Occurrence of Aldehydes and Ketones

  • When chiral compounds are found in nature they are usually enantiomerically pure, although different sources may yield different enantiomers.
  • For example, carvone is found as its levorotatory (R)-enantiomer in spearmint oil, whereas, caraway seeds contain the dextrorotatory (S)-enantiomer.

• Enantiomers and Stereoisomerism in Organic Compounds

  • Stereoisomers can be divided into enantiomers, diastereomers and meso compounds.
  • Enantiomers are nonsuperimposable mirror images of one another.
  • Diastereomers are stereoisomers that are not enantiomers.
  • The (R, R) and (S, S) isomers are enantiomers.
  • Recognize when a molecule can exist as different enantiomers or diastereomers

• Compounds Having Two or More Chiral Centers

  • Since the properties of these compounds (see below) are significantly different, they cannot be enantiomers.
  • Since these two compounds are optically active, each must have an enantiomer.
  • Some of these stereoisomers will have enantiomeric relationships, but enantiomers come in pairs, and non-enantiomeric stereoisomers will therefore be common.
  • In the example above, either of the ephedrine enantiomers has a diastereomeric relationship with either of the pseudoephedrine enantiomers.

• Resolution of Racemates

  • As noted earlier, chiral compounds synthesized from achiral starting materials and reagents are generally racemic (i.e. a 50:50 mixture of enantiomers).
  • Separation of racemates into their component enantiomers is a process called resolution.
  • Since enantiomers have identical physical properties, such as solubility and melting point, resolution is extremely difficult.
  • Reversing the first reaction then leads to the separated enantiomers plus the recovered reagent.
  • Chemical reactions of enantiomers are normally not so dramatically different, but a practical distinction is nevertheless possible.

• Optical Activity

  • The observed rotations (α) of enantiomers are opposite in direction.
  • For example, the lactic acid and carvone enantiomers discussed earlier have the following specific rotations.
  • A 50:50 mixture of enantiomers has no observable optical activity.
  • A racemate may be separated into its component enantiomers.
  • A pure enantiomer may be transformed into its racemate.

• Hydroboration

  • Stereoselectivity refers to the preferential formation in a chemical reaction of one product stereoisomer (enantiomer or diastereomer) over another, as a result of inherent reaction specificity, or the influence of chiral features in the substrate, reagent, catalyst or environment.
  • When this selectivity results in the formation of an excess of one enantiomer over the other from an achiral or racemic substrate it is sometimes called asymmetric induction.
  • Two new stereogenic centers have been created in this diastereoselective reaction, but since reaction takes place equally at both faces of the double bond, a 50:50 mixture of enantiomers is obtained (equation 1 of the second diagram below).
  • An addition of a given reagent at the re face will necessarily give the enantiomer of the same addition at the si face, provided the new substituent is not identical to one of the three original groups.
  • When used for the hydroboration of 1-phenylcyclopentene, the 1R,2S enantiomer is the sole product (equation 2 below).

• Enantioselective Aldol Reactions

  • If this substituent exerts a controlling influence, and if the diastereoselectivity of the reaction is excellent, the product should be obtained as a single enantiomer.
  • Small amounts of the syn-enantiomer, S2, and anti-diastereomer A1 are also formed.
  • Interestingly, when the large Lewis acid (C2H5)2AlCl was used to activate the aldehyde, the anti-enantiomer A1 was formed in 90% de, accompanied by S2.
  • The second diagram above shows the two procedures leading to the syn-enantiomers.
  • The third diagram above changes the display to procedures leading to the anti-enantiomers.

• Synthesis of Amino Acids

  • If pure L or D enantiomers are desired, it is necessary to resolve these racemic mixtures.
  • Of course, the same procedure could be used to obtain the (-)-enantiomer of the amino acid.
  • For example, an aminoacylase enzyme from pig kidneys cleaves an amide derivative of a natural L-amino acid much faster than it does the D-enantiomer.
  • If the racemic mixture of amides shown in the green shaded box above is treated with this enzyme, the L-enantiomer (whatever its rotation) will be rapidly converted to its free zwitterionic form, whereas the D-enantiomer will remain largely unchanged.
  • This separation of enantiomers, based on very different rates of reaction, is called kinetic resolution.

• Total Synthesis

  • One enantiomer may have medicinal properties, while the other may be entirely useless or worse, have adverse effects.
  • Thalidomide , a drug used to relieve morning sickness in pregnant women in the 1950s, was initially sold as a racemic mixture of enantiomers.
  • The R-enantiomer has no adverse effects, but the S-enantiomer is a teratogen that caused horrible birth defects during clinical testing.
  • An enantioselective reaction is one in which one enantiomer is preferred over another in a reaction.
  • Because both enantiomers of any compound have identical physical and chemical properties, there must be another influence to favor one product over the other.

• Anchimeric Assistance

  • This alcohol has two chiral centers, and therefore has four stereoisomers in the form of two pairs of enantiomers.
  • In the first diagram below, a tosylate derivative of one enantiomer of each diastereomer is drawn as a Fischer projection.
  • As a result of equal rates of product formation by acetate bonding to C2 & C3, the achiral threo intermediate yields a 50:50 (racemic) mixture of threo enantiomers: (2R,3S) from the blue arrows and (2S, 3R) from the green arrows.
  • In contrast, acetate bonding to C2 & C3 of the erythro intermediate produces the same enantiomer of the erythro product (2S,3S).
  • Tosylate Derivative of One Enantiomer of each Diastereomer Drawn as a Fischer Projection