Chiral centers naming

As far as biosynthesis is concerned, it is clear that lyaloside (23) and pauridianthoside (26) readily derive from strictosidine (17). These three molecules display identical stereochemistry at the three chiral centers, namely, a,a,(3 for the hydrogen atoms at the 15, 20, 21 positions, respectively. They are closely related to a number of glucoalkaloids identified in Pertusadina euryncha (= Adina rubescens), e.g., rubenine (31) and desoxycordifoline (34, R = H) (49), and in Adina cordifolia, e.g., cordifoline (34, R = OH) (50). Palinine (35),... 

A remarkable feature of this reaction is that it creates three chiral centers. Two of the chiral centers, namely those at the two ring junctions, are established by the Diels-Alder reaction. The third, namely the endo position of the ester group, is also established by the Diels-Alder reaction. Without the chiral auxiliary 8-phenylmenthyl group, two of the eight possible stereoisomers would be produced, namely the pair of enantiomers shown. Although both enantiomers of the bicyclic products were formed in Corey s scheme, they were formed in the ratio of 97 3 and the desired enantiomer could be separated in pure form. In subsequent steps, the 8-phenylmenthyl ester was hydrolyzed and the pure enantiomer was converted to the so-called Corey lactone and then to enantiometically pure prostaglandin... 

As shown for the aldotetroses an aldose belongs to the d or the l series accord mg to the configuration of the chirality center farthest removed from the aldehyde func tion Individual names such as erythrose and threose specify the particular arrangement of chirality centers within the molecule relative to each other Optical activities cannot be determined directly from the d and l prefixes As if furns ouf bofh d eryfhrose and D fhreose are levorofafory buf d glyceraldehyde is dexfrorofafory... 

Another name for glucitol obtained by reduction of d glucose is sorbitol it is used as a sweetener especially in special diets required to be low in sugar Reduction of D fructose yields a mixture of glucitol and mannitol corresponding to the two possi ble configurations at the newly generated chirality center at C 2... 

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... 

Chirality and Optical Activity. A compound is chiral (the term dissymmetric was formerly used) if it is not superimposable on its mirror image. A chiral compound does not have a plane of symmetry. Each chiral compound possesses one (or more) of three types of chiral element, namely, a chiral center, a chiral axis, or a chiral plane. 

Asymmetric center (Section 7.2) Obsolete name for a chirality center. 

Aldotetroses are four-carbon sugars with two chirality centers and an aldehyde carbonyl group. Thus, there are 22 = 4 possible stereoisomeric aldotetroses, or two d,l pairs of enantiomers named erythrose and threose. 

Steps 6-8 of Figure 29.5 Reduction and Dehydration The ketone carbonyl group in acetoacetyl ACP is next reduced to the alcohol /S-hydroxybutyry] ACP by yS-keto thioester reductase and NADPH, a reducing coenzyme closely related to NADH. R Stereochemistry results at the newly formed chirality center in the /3-hydroxy thioester product. (Note that the systematic name of a butyryl group is biitanoyl.)... 

For many open-chain compounds, prefixes are used that are derived from the names of the corresponding sugars and that describe the whole system rather than each chiral center separately. Two such common prefixes are erythro- and threo-, which are applied to systems containing two stereogenic carbons when two of the groups are the same and the third is different. The erythro pair has the identical Y Y Y Y... 

There is another point of nomenclature that must be discussed, namely where a chiral center is involved. Taking the simple case of 2-butanol, CH3CH(OH)CH2CH3, we can explain the point as follows (Scheme 1). Two configurations are possible at the chiral center. In both the R and the S series, three conformations are possible. The +sc form in the R series and the -sc form in the S series are enantiomers and their free energies must be the same under achiral conditions. However, the - sc form in the R and that in the S series differ in free energies. Therefore, it is not sufficient to call a conformation -sc if a chiral center is involved. In this case we may have to call such conformations - sc(R) and — sc(S) to distinguish them. 

Threonine (Thr or T) ((2S,31( )-2-amino-3-hydroxybutanoic acid) has an aliphatic hydroxyl side chain and is classified as a polar, uncharged amino acid with the formula HOOCCH(NH2)CHOHCH3. Together with Ser and Tyr, Thr is one of the three proteinogenic amino acids bearing an alcohol group. Thr can be seen as a hydroxylated version of Val. With two chiral centers, Thr can exist in four possible stereoisomers, or two possible diastereomers of L-Thr. However, the name L-Thr is used for one single enantiomer, (2S, 3if)-2-amino-3-hydroxybutanoic acid. The second diastereomer (2S,3S), which is rarely present in nature, is called L- //o-Thr. 

Whereas racemization is the complete loss of optical activity with time, epimerization is the reversible interconversion of diastereoisomers to an equilibrium mixture which is not necessarily optically inactive. Diastereoisomers arise from the combination of the two chiral centers in 9, namely the metal centered, R and S, and the resolved (S) optically active ligand center. The diastereoisomers (RS) and (SS) differ in their properties. 

Taxol is the trademark name given to the complex small molecule, pachtaxel (Figure 7.1). The molecular complexity of paclitaxel is evident in the tetracyclic nucleus and 11 chiral centers. Throughout this text, I will use the brand name Taxol synonymously with its generic name, paclitaxel. 

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... 

Another naming system for stereoisomers, the D and L system, is described in Chapter 3. A molecule with a single chiral center (glyceraldehydes, for example) can be named unambiguously by either system. 

Diastereoisomers. Whereas compounds with one chiral center exist as an enantiomorphic pair, molecules with two or more chiral centers also exist as diastereoisomers (diastereomers). These are pairs of isomers with an opposite configuration at one or more of the chiral centers, but which are not complete mirror images of each other. An example is L-threonine which has the 2S, 3R configuration. The diastereoisomer with the 2S, 3S configuration is known as i-a//o-threonine. L-isoleucine, whose side chain is -CH(CH3) CH2CH3, has the 2S, 3R configuration. It can be called 2(S)-amino-3(R)-methyl-valeric acid but the simpler name L-isoleucine implies the correct configuration at both chiral centers. 

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