Chiral compounds nomenclature

The molecular helices and propellers discussed above contain no center of chirality, and the P and M nomenclature is thus the only way of describing their absolute configuration. This nomenclature, however, is also applicable to some series of chiral compounds which display several centers of chirality. As will be discussed in Section 6, the presence in a molecule of two or more centers of chirality usually implies the existence of several stereoisomers, but steric reasons may reduce down to two the possible number of stereoisomeric forms. Thus, 2,3-epoxycyclohexanone contains two asymmetric carbon atoms, but for steric reasons only two stereoisomers, namely the (2S 3S)-(—)- and the (2/ 3/J)-( + )-enantiomer, exist the former is depicted in diagram XL [49]. 

Inositol is a deceptively simple molecule. On closer study, a number of sophisticated stereochemical, prochiral, chiral, and conformational issues associated with inositols and their derivatives become evident. Inositols, in particular myo-inositol, play a central role in cellular metabolism. An array of complicated molecules that incorporate the inositol moiety are found in nature. Structural heterogeneity of inositol derivatives is compounded by the presence of stereo- and regioisomers of the inositol unit. Because of the large number of isomeric inositols and their derivatives present in nature, a detailed understanding of the structural, stereochemical, and nomenclature issues involving inositol and its derivatives is essential to investigate biological aspects. A discussion of the stereochemical, conformational, prochiral, chiral, and nomenclature issues associated with inositols and the structural variety of insoitol derivatives is presented in this chapter. 

It is often possible for a chiral compound to have several different isomers. Isocitrate has four possible isomers, but only one of the four is produced by this reaction. (We shall not discuss nomenclature of the isomers of isocitrate here. See Question 28 at the end of this chapter for a question about the other isomers.) Aconitase, the enzyme that catalyzes the conversion of citrate to isocitrate, can select one end of the citrate molecule in preference to the other. 

The priority rules we defined in Chapter 5 for describing the configuration of geometric isomers also apply to R,S configurational nomenclature for chiral compounds. 

The Naming of Chiral Drugs. The nomenclature of chiral compounds is difficult and sometimes confusing. We have already introduced the prefixes R and S, (- -) and (—), and D and L. We have already seen many common names for drugs that have an implied chirality, as in methamphet-amine and ephedrine. The hyphenated prefixes R/S, D/L, and + / — render the alphabetic indexing of pharmaceutics problematic. One approach to this problem that has been used to help with this issue is to select drug names where the chirality has been imbedded into the common chemical name. One example that we have discussed is esomeprazole,... 

Systems of Nomenclature for Chiral Compounds The two enantiomers of a chiral drug or natural product are best identified on the basis of their absolute configuration [(/ /5)-system] or their optical rotation... 

Isopropyl group (Section 2 13) The group (CH3)2CH— Isotactic polymer (Section 7 15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain Isotopic cluster (Section 13 22) In mass spectrometry a group of peaks that differ in m/z because they incorporate differ ent isotopes of their component elements lUPAC nomenclature (Section 2 11) The most widely used method of naming organic compounds It uses a set of rules proposed and periodically revised by the International Union of Pure and Applied Chemistry... 

The nomenclature for biaryl, allene, or cyclohexane-type compounds follows a similar rule. Viewed along the axis, the nearer pair of ligands receives the first two positions in the order of preference, and the farther ligands take the third and fourth position. The nomination follows a set of rules similar to those applied in the central chiral system. In this nomination, the end from which the molecule is viewed makes no difference. From whichever end it is viewed, the positions remain the same. Thus, compound 7a has an ( -configuration irrespective of which end it is viewed from. 

Given the importance of stereochemistry in reactions between biomolecules (see below), biochemists must name and represent the structure of each biomolecule so that its stereochemistry is unambiguous. For compounds with more than one chiral center, the most useful system of nomenclature is the RS system. In this system, each group attached to a chiral carbon is assigned a priority. The priorities of some common substituents are... 

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