Stereochemical terms

The 1996 lUPAC recommen dations for stereochemical terms can be viewed at http //WWW chem qmw ac uk/iupac/stereo... 

One of the most actively investigated aspects of the biohydrolysis of carboxylic acid esters is enantioselectivity (for a definition of the various stereochemical terms used here, see [7], particularly its Sect. 1.5) for two reasons, one practical (preparation of pure enantiomers for various applications) and one fundamental (investigations on the structure and function of hydrolases). The synthetic and preparative aspects of enantioselective biocatalysis by hydrolases have been extensively investigated for biotechnology applications but are of only secondary interest in our context (e.g., [16-18], see Sect. 7.3.5). In contrast, the fundamental aspects of enantioselectivity in particular and of structure-metabolism relationships in general are central to our approach and are illustrated here with a number of selected examples. 

There exists a large literature on enzyme polymorphism and species adaptation (91), but none of it can as yet be interpreted in stereochemical terms. The best-studied species is Drosophila melanogaster, in which the frequency of the two dominant alleles of alcohol dehydrogenase varies with latitude in several continents one of these alleles has a consistently lower Michaelis constant for alcohols than the other (92). The two enzymes have been found to differ by the single substitution of a lysine for... 

This stereochemical term (short for the Latin word rectus) is the designation for a stereoheterotopic face of a trigonal atom whose ligands appear in a clockwise sense in the order of the Cahn-Ingold-Prelog scheme when viewed from that side of the face. 

Definitions It is important to define precisely the stereochemical terms that will be employed in this discussion. The term racemization has often been used loosely by chemists to describe any situation in which a mixture of enantiomers or diastereomers is produced as a result of an amide-bond-forming reaction, without regard to the ratio of stereoisomers formed. For the purposes of this discussion though, the term racemization will be used to describe the situation leading toward the formation of an exact 1 1 mixture of stereoisomers. Racemization, therefore, is a process that occurs to a collection of molecules, and can happen to a single residue or to one residue in a peptide sequence (Scheme 1). This is a macroscopic event, as the result is detected subsequent to the amide bond formation. 

Explain briefly (in one or two short sentences) the meaning of the following basic stereochemical terms. 

Explain clearly and succinctly the following stereochemical terms. 

Unsaturated acids are named using the name of the corresponding alkene, with the final -e replaced by -oic acid. The carbon chain is numbered starting with the carboxyl carbon, and a number gives the location of the double bond. The stereochemical terms cis and trails (and Z and E) are used as they are with other alkenes. Cycloalkanes with —COOH substituents are generally named as cycloalkanecarboxylic acids. 

Like ft and S, these stereochemical terms are merely labels they are of no consequence chemically. 

What is happening in stereochemical terms in this sequence of reactions What is the other product from the crystallization from hexane The product is one enantiomer of a phosphine oxide. If you wanted the other enantiomer, what would you do ... 

In Section 4.3 we mentioned two important stereochemical terms, enantiomers and enantiotopic nuclei. Enantiomers are structures related as the left hand is related to the right nonsuperimposable mirror images. Any chiral (dissymmetric) molecule can exist in two (and only two) enantiomeric forms. For example, chiral alcohol 10-11 has two enantiomeric configurations, labeled R and S.10 Enantiotopic nuclei are those related by a plane of symmetry. The methylene hydrogens of benzyl alcohol (10-12) are enantiotopic and are labeled pro-R and pro-S10 ... 

These CAS rules, which had been in use since the beginning of the 9th Collective Index period (1972), have now been thoroughly revised to give a simplified and more intuitive description. The need for a single expression to describe the total stereochemistry of a molecule has been eliminated. Stereochemical terms are now placed within the parts of a chemical name to which the stereochemical information applies. The following diagram shows the now superseded 9C1 descriptors alongside the revised equivalents, which are closer to now current CAS practice. 

The terminology used to describe stereochemical relationships is often a maze of capital D s and L s, lowercase d s and I s, mixed in with R) s, (S) s, (+) s, and (-) s. This chapter is designed to give the reader a quick overview of stereochemical terms and concepts and to lay a foundation for the more technical discussions to come, It is not meant to be an in-depth treatment of this topic and the interested reader is urged to consult one of the many fine texts on the subject. 

The introductory section of this book covers the early history of stereochemistry and defines a number of stereochemical terms. These contributions are designed to provide the reader with a background for the concepts discussed in the rest of the work. 

The two compounds look much the same in stereochemical terms and these are the very similar results of a simple hydrogenation. 

Making sure you understand some of these stereochemical terms and concepts. 

E isomer A stereochemical term used to indicate that the two groups with the highest ranking under the CIP system are diametrically positioned across a double bond. If they are on the same side, then the system is designated Z. E and Z are abbreviations of entgegen and zusammen, meaning opposite and together respectively. 

Meso A stereochemical term meaning that there is a plane of symmetry passing through the molecule, and hence it is not optically active. 

Threo isomer A stereochemical term used to describe the configuration of a C,lbxC,lbv system. See also erythreo isomer. 

Trans A stereochemical term used to indicate that the two groups under consideration are attached one at each end of a double bond and are on opposite sides of the double bond, rather than on the same side as in the cis isomer. To characterise more fully a system in which there are more than two different groups, then the CIP rules need to be invoked, and the E/Z notation used instead. 

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