Stereoisomers Enantiomer

Now, consider the physical properties of these stereoisomers. Enantiomers should have many of the same physical properties, such as energy and dipole moment, but diastereomers should not. Obtain the energy of each conformer and use equation (1) to calculate the composition of a large sample of each stereoisomer at 298 K. Then, obtain the dipole moment of each conformer and use equatiori (2) to calculate the dipole moment of a large sample of each stereoisomer at 298 K. Do enantiomers have the same dipole moment Do diastereomers have different dipole moments ... 

In a series of papers, the methodologies applied for amino acids were extended to peptide stereoisomer (enantiomer and diastereomer) separations [59,116,121-123],... 

Unlike other stereoisomers, enantiomers have identical physical properties and, consequently, are difficult to separate. A process by which enantiomers can be separated is called resolution. 

Figure 13. Rapid simultaneous separation of eight stereoisomers (enantiomers and diastereomers) of 2-mcthyl-3-(l -methylpropyl)oxiranc on 0.125 M nickel(ll) bis[3-(heptafluorobutanoyl)-(l / )-camphorate] in SB-30 [20 m x 0.25 mm (i.d.) glass capillary column. 90 C. 1 bar nitrogen].

Ibuprofen is a popular analgesic and anti-inflammatory drug. There are two stereoisomeric forms of ibuprofen. This drug can exist as (S)- and (/ )-stereoisomers (enantiomers). Only the (5)-form is active. The (/ )-form is completely inactive, although it is slowly converted in the body to the active (5)-form. The drug marketed under the trade names, commercially known as Advil , Anadin , Arthrofen , Brufen , Nurofen , Nuprin , Motrin etc., is a racemic mixture of (/ )- and (5)-ibuprofen. 

One of the problems attaching to the HPLC separation of peptides is the analysis of stereoisomers (enantiomers and diastereoisomers), that is, of peptides that differ only in the configuration of their amino acid residues. 

Two compounds are stereoisomers of one another if the bonding arrangement in one is identical to that in the other except in the way the atoms are oriented in space. Thus, 1 and 2, 3 and 4, 5 and 6, 7 and 8 are four pairs of stereoisomers. There are two types of stereoisomers enantiomers and diasteriomers. 

Two molecules with the same molecular formula and the same bond-to-bond connectivity that are not the same compound are called stereoisomers. Unless they are geometric isomers, stereoisomers must each contain at least one chiral center in the same location. There are two types of stereoisomers enantiomers and diastereomers. 

Stereoisomers Enantiomers and Diastereomers Figure 9. conformational changes. 

There are only two types of stereoisomers Enantiomers are stereoisomers that are mirror images. Diastereomers are stereoisomers that are not mirror images. 

There are only two kinds of stereoisomers enantiomers and diastereomers. 

Many compounds contain more than one chiral center. If a molecule contains n chiral centers, there will be a maximum of 2" stereoisomers. If there is only one chiral center, there are two stereoisomers (enantiomers), while if there are two asymmetric carbon atoms, then there will be four isomers (two pairs of mirror-image-related molecules). While two chiral centers lead, theoretically, to four stereoisomers, these stereoisomers may not all be different. Some pairs may be identical, containing mirror-image symmetry because the two chiral centers are... 

For convenience, we laid down (Sec. 4.20) the following ground rule for discussions and problems in this book unless specifically indicated otherwise, the terms stereoisomers," ""enantiomers," and ""diastereomers" will refer only to configurational isomers, including geometric isomers, and will exclude conformational isomers. The latter will be referred to as conformational isomers," con-formers, conformational enantiomers, and conformational diastereomers. ... 

It is the primary goal of this book to analyze the factors that influence stereoselectivity when one stereoisomer predominates over others. For illustrative purposes, consider the addition of a nucleophile to a carbonyl. The faces of unsymmetric carbonyls are heterotopic, either enantiotopic (if there are no stereocenters in the molecule) or diastereotopic (if there are), as shown in Figure 1.1 (see also glossary. Section 1.6). In order to achieve a predominance of one stereoisomer (enantiomer or diastereomer) over the other, the transition states resulting from attack from the heterotopic Re or Si faces must be diastereomeric. This will be the case if either the carbonyl compound or the reagent (or both) is (are) chiral. 

Because the compound has two asymmetric carbons, it has four stereoisomers. Enantiomers can be drawn for the cis isomer, and enantiomers can be drawn for the trans isomer. 

---