Cahn-Ingold-Prelog nomenclature

Notice that in the previous examples, the atom of the higher atomic weight is given the higher priority (Br = 79.1 versus Cl = 15.5, and I = 126.0 versus N = 14.0). These assignments are based on the priority rules of Cahn-Ingold-Prelog nomenclature. 

The d, l nomenclature is chosen in this chapter in order to unmistakably distinguish homochiral stereoisomers. According to the specifications of the Cahn-Ingold-Prelog nomenclature L-l would equal (S)-l, while L-2 equals (J )-2. 

FIGURE 2.11 Change of chirality on derivatization according to the Cahn-Ingold-Prelog nomenclature. 

CE CGE CHOL CIP CPG CTAB CZE dA, dG, dC DBU DEAE DMF DMT DNP DOPE DOTMA EDTA EM EOF ESI-MS Fmoc FPE ICAM-1 Capillary electrophoresis Capillary gel electrophoresis Cholesterol Cahn-Ingold-Prelog nomenclature system for absolute configuration Controlled pore glass Hexadecyltrimethylammonium bromide Capillary zone electrophoresis Deoxyadenosine, deoxyguanosine, deoxycytosine l,8-Diazabicyc o[5.4.0]undec-7-en Diethylaminoethyl- Dimethylformamide Bis(4-methoxyphenyl)phenylmethyl-, (syn. Dimethoxytrityl-) 2,4-DinitrophenyI- Dioleylphosphatidylethanolamine N-[l-(2,3-dioleyloxy)propyl]-N, N, N-trimethylammonium chloride Ethylenediamine tetra-acetic acid Electrophoretic mobility Electro-osmotic flow Electrospray ionization mass spectrometry 9-Fluorenylmethoxycarbonyl-Fluid phase endocytosis Intracellular Adhesion Molecule-1... 

The isolated 2-hydroxy-5-methylhexanoic acid belongs to the L-series with S-configuration, as indicated by the positive ORD plain curve (41). It follows that (-) humulone would be the L-form. However, because in the Cahn-Ingold-Prelog nomenclature (37) the sequence of the substituents at the chiral centre in humulone differs from that in the hydroxycarboxylic acid, (-) humulone possesses the R-configuration. This result confirms the conclusion reached from the chiroptical studies. 

FIGURE 1.1 Chemical and stereochemical nature of amino acids. Substituents in (a) and (b) are on opposite sides of the plane N-Ca-C, the bold bond being above the plane. Interchange of any two substituents in (a) changes the configuration. For the Cahn-Ingold-Prelog system of nomenclature, the order of preference NH2 > COOH > R2 relative to H is anticlockwise in (a) = (S) and clockwise in (c) = (R). 

Scheme 2.1 Kinetic resolution of glycidyl butyrate catalyzed by porcine pancreatic lipase, PPL The preferential conversion of (S)-glycidyl butyrate into (R)-glycidol and butyric acid results from the stereochemical nomenclature rules under the Cahn, Ingold, Prelog convention, the configuration around the chiral centre is not affected.

In response to this nomenclature dilemma, the Cahn-Ingold-Prelog (IUPAC, International Union of Pure and Applied Chemistry) system of nomenclature was developed and is now the standard mediod to specify the relative configuration of chiral centers in molecules. Each chiral center will have two possible mirror-image configurations, which are designated as eidter R or S. 

Deprotonation of a-alkylated acetic acid esters (e.g., the propionic acid ester of Figure 10.13) with LDA at —78°C selectively yields the -enolatcs.Thc quotation marks indicate that this application of the term is based on an extension of the E/Z-nomenclature here, the Cahn-Ingold-Prelog priority of the O I, i1 substituent is considered to be higher than the priority of the OR group. The deprotonation of the ester shown in Figure 10.13 occurs via the strain-free transition state A. The alternative transition state B is destabilized by a 1,3-diaxial interaction. 

To deal with this problem, we use the E-Z system of nomenclature (pun intended) for cis-trans isomers, which is patterned after the Cahn-Ingold-Prelog convention for asymmetric carbon atoms (Section 5-3). It assigns a unique configuration of either E or Z to any double bond capable of geometric isomerism. 

Cahn-Ingold-Prelog system of nomenclature (Section 5.6) The system of designating a stereogenic center as either Rot S according to the arrangement of the four groups attached to the center. 

---