Optical enantiomers

Different optical enantiomers of amino acids also have different properties. L-asparagine, for example, tastes bitter while D-asparagine tastes sweet (see Figure 8.3). L-Phenylalanine is a constituent of the artificial sweetener aspartame (Figure 8.3). When one uses D-phenylalanine the same compound tastes bitter. These examples clearly demonstrate the importance of the use of homochiral compounds. 

FIGURE 8. Comparison of the optical enantiomers of MDMA for the acute (3 hours) and long-term (7 days) effects on the serotonergic system... 

D.R. Thakker, H. Akagi, M. Koreeda, D.M. Jerina, and A.H. Conney. Tumorigenicity of the optical enantiomers of the diastereomeric benzo(a)pyrene 7,8-dio1-9,10-epoxides in newborn mice Exceptional activity of... 

In moving to other classes of JH analogs, major departures from the farnesane skeleton have been reported in the form of phenyl ethers (22, Z3,2 ), cyclohexenes such as juvabione (25), and small peptides (26) as an extreme case of completely selective action on one family of bugs. The latter compounds are most remarkable for the pronounced differential activity of their optical enantiomers, in which one antipode is several thousand times more active biologically than the other (2 7). In connection with the peptides, it should be noted that there is no formal proof that these compounds exert their action as true mimics of juvenile hormones at the target tissue level. One may well ask whether these peptides act directly on the corpora allata glands as allatotropins. 

Selenomethionine forms optical enantiomers and a chiral column is needed to separate the two forms [160-162], As the presence of salts and organic matter can cause retention times to vary, it is essential that spiking is used to verify them. Also, it should be noted that retention time alone does not allow peaks to be unequivocally assigned. It has been found that the removal of lipid material prior to protein extraction dramatically improves the reproducibility of the chromatographic system and the lifetime of the columns. 

The earlier literature on quebrachamine has been dealt with in previous volumes, and its occurrence in some new sources is recorded in Table I. Most important from the biogenetic point of view is the existence in nature of both optical enantiomers. The common levorotatory form corresponds in absolute configuration to aspidospermine (II, vide infra), while the dextrorotatory form found in Stemmadenia donnell-smithii (8) is related to (+ )-pyrifolidine (XLVI, 9, 10) and to (— )-tabersonine (XCII, 7). 

Ten alkaloids have been isolated from A. populifolium, of which the greater part belong to the aspidofractinine group (48). They include (+ )-0-methyldeacetylaspidofiline [CXXXIX, optical enantiomer of that derived from aspidofiline (Section III, E)], its 16-methoxy analog, CXLI-A, and their respective A7a-formyl derivatives, CXLI-B and CXLI-C. The structures of all four compounds could be established by comparison of their IR-, UV-, NMR-, and especially mass spectra with... 

A variety of mixed (P—P), (N—P) chelates containing acac-, C03, C20and 2,3,2-tet have been prepared (Table 37) some have been resolved into optical enantiomers by conventional or chromatographic means (Table 37), and lH, 31P, CD and electronic spectra have been reported. Metathesis of /rans-[Co(Cl)2(P—P)2](C104) with NaX in methanol leads to other trans-[Co(X)2(P—P)2] complexes (X = Nf, NCO, NCS, CN-) however N02 addition to trans-[Co(Cl)2(dppv)2](C104) results in the isolation of the unsaturated 14-electron complex [Co(NO)2(dppv)] and oxidized ligand.423... 

The polarimeter is commonly used in organic and analytical chemistry as an aid in identification of optically active compounds (especially natural products) and in estimation of their purity and freedom from contamination by their optical enantiomers. The polarimeter has occasional application to chemical kinetics as a means of foUowing the course of a chemical reaction in which opticaUy active species are involved. Since the rotation a is a linear function of concentration, the polarimeter can be used in studying the acid-catalyzed hydrolysis of an optically active ester, acetal, glycocide, etc. 

Chirasil- Val is a unique chiral phase used in capillary columns for the separation of optical enantiomers. Most amino acids and many drugs containing polar functional groups may be resolved. 

The importance of the work term is shown by the observation of chiral recognition in some electron-transfer reactions. Differences in the rate of reaction of different optical enantiomers of the same complex, with a common chiral complex, are attributed to differences in the preequilibrium constant Xjp rather than electronic effects. ... 

Bis (3-phenylimino-l-phenyl-l-butanonato) copper (II) crystallizes as green platelets (from 95% ethanol) which melt at 180 to 182°. The compound has been partially resolved into optical enantiomers by means of a chromatographic technique. ... 

Since the optical enantiomers racemize rapidly in solution, all operations should be carried out as quickly as possible using iced solutions and iced apparatus. 

Although Werner considered both enantiomers of potassium tris-(oxalato)chromate(III) to be monohydrates, Charonnat and Dele-pine, on the basis of isomorphism with the corresponding iridium salt, proposed that they are dihydrates. The (+) enantiomer has been described as bluish red in the dry, pure condition. - The optical enantiomers are less soluble in water than the racemic mixture. Because of the high rate of racemization in water, recrystallization has been reported to be accompanied by a considerable loss of optical rotation. For the same reason, the optical rotation values reported by different workers are not directly comparable. ... 

Sulfoxides possess a non-planar pyramidal configuration consequently, chiral sulfoxides can exist as optical enantiomers (see Chapter 5, p. 63). This is illustrated by the structures (33a) and (33b) showing the optical isomers of ethyl methyl sulfoxide (Figure 2). 

The nucleophilic character ofdialkyl sulfides is illustrated by their nucleophilic addition reaction with alkyl halides to form the corresponding sulfonium salts (35) (Scheme 13). Asymmetric sulfonium salts (36) have a tetrahedral configuration therefore, like the analogous chiral saturated carbon compounds, they can be resolved into optical enantiomers (see Chapter 6, p. 81). They are, however, generally less optically stable than sulfoxides, but in sulfonium salts the unshared electron pair can hold its configuration at ordinary temperatures, unlike nitrogen in quaternary ammonium salts, enabling their resolution to be achieved. 

Ethyl(methyl)carboxymethylsulfonium bromide (4) was resolved into optical enantiomers by Pope and Peachey (1900), and since that time a large number of optically pure sulfonium salts have been obtained by resolution of racemic mixtures or by stereospecific syntheses. Chiral sulfonium salts can suffer stereomutation at sulfur by three major mechanisms (Scheme 2) (i) pyramidal inversion, (ii) reversible dissociation into the sulfide... 

Replacement of the molecule by its optical enantiomer changes the sign of (6.5) so that in the term in the dispersion energy (6.6) that depends on electric-magnetic coupling changes sign when one molecule is replaced,... 

HPLC. " Different perfluorinated carboxylic acids have been examined as the ion-pairing agents (pH 2.5. 5) in the analyses of enriched yeast and vegetable extracts Alkylsulfonic acids have been successfully used in speciation analysis carried out on nuts, onion leaves and yeast " " Owing to their different biological activities, optical enantiomers of Se-amino acids were analyzed by several chiral separation techniques - ... 

Figure 16.11 Chemical structure of the three most important representatives of the Cayl (L-type) channel organic antagonists DHPs (nifedipine), phenylalkylamines (verapamil), and benzothiazepines (diltiazem). The DHP derivative. Bay K8644, can be either agonist or antagonist depending on the optical enantiomer.

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