Isomer, optical

When several different ligand gronps are attached to a central P atom, various isomers are possible. Pyramidal phosphines, Pabc, and tetrahedral compounds, Pabcd, can exist as mirror plane-related isomers which show optical activity (3.31). In the latter case the isomerism is anal-ogons to that based on the asymmetric tetrahedral carbon atom, long established in carbon chemistry (3.31). [Pg.64]

Five- and six-coordinated phosphorns componnds containing different ligands (Pabcde and Pabcdef) can show positional isomerism withont necessarily involving optical activity (Chapter 13.2). [Pg.64]

Among the earliest examples of asymmetric P compounds to be resolved into optically active forms were simple phosphines, phosphine oxides and phosphonium salts such as (3.32). [Pg.64]

In a presentation by Ciurczak [75], it was observed that pure d- and Z-amino acids gave identical NIR spectra, while racemic crystals generated quite different spectra. A 1986 paper outlines work later completed [Pg.78]

Mustillo and Ciurczak [78] presented a paper discussing the spectral effect of optically active solvents on enantiomers. This information was later used to screen for polar modifiers in normal phase chromatographic systems where racemic mixtures were involved [79]. [Pg.79]

A phase change takes place when one enantiomer is converted to its optical isomer. As illustrated in Figure 9, when the chiral center is a tetra-substituted carbon atom, the conversion of one enantiomer to the other is equivalent to the exchange of two electron pairs. This transformation is therefore phase inverting. [Pg.346]

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. [Pg.47]

When the asymmetric carbon atoms in a chiral compound are part of a ring, the isomerism is more complex than in acyclic compounds. A cyclic compound which has two different asymmetric carbons with different sets of substituent groups attached has a total of 2 = 4 optical isomers an enantiometric pair of cis isomers and an enantiometric pair of trans isomers. However, when the two asymmetric centers have the same set of substituent groups attached, the cis isomer is a meso compound and only the trans isomer is chiral. (See Fig. 1.15.)... [Pg.47]

Chiral separations are concerned with separating molecules that can exist as nonsupetimposable mirror images. Examples of these types of molecules, called enantiomers or optical isomers are illustrated in Figure 1. Although chirahty is often associated with compounds containing a tetrahedral carbon with four different substituents, other atoms, such as phosphoms or sulfur, may also be chiral. In addition, molecules containing a center of asymmetry, such as hexahehcene, tetrasubstituted adamantanes, and substituted aHenes or molecules with hindered rotation, such as some 2,2 disubstituted binaphthyls, may also be chiral. Compounds exhibiting a center of asymmetry are called atropisomers. An extensive review of stereochemistry may be found under Pharmaceuticals, Chiral. [Pg.59]

Although scientists have known since the time of Louis Pasteur (1) that optical isomers can behave differentiy in a chiral environment (eg, in the presence of polarized light), it has only been since about 1980 that there has been a growing awareness of the implications arising from the fact that many dmgs are chiral and that living systems constitute chiral environments. Hence, the optical isomers of chiral dmgs may exhibit different bioactivities and/or biotoxicities. [Pg.59]

Traditionally, chiral separations have been considered among the most difficult of all separations. Conventional separation techniques, such as distillation, Hquid—Hquid extraction, or even some forms of chromatography, are usually based on differences in analyte solubiUties or vapor pressures. However, in an achiral environment, enantiomers or optical isomers have identical physical and chemical properties. The general approach, then, is to create a "chiral environment" to achieve the desired chiral separation and requires chiral analyte—chiral selector interactions with more specificity than is obtainable with conventional techniques. [Pg.60]

Because they are similar, the aLkanolamines often can be used interchangeably. However, cost/perfomiance considerations generally dictate a best choice for specific appHcations. AMPD is manufactured in very low volumes for use as a reagent in certain medical diagnostic tests, although some is used in certain cosmetic products. 2-Ainino-1-butanol is used primarily as a taw material for the synthesis of ethambutol [74-55-5] an antituberculosis dmg. The first step in the synthesis of this dmg is the resolution of AB into its optical isomers because only (i)-2-amino-l-butanol, [5856-62-2] is utilized in this synthesis. [Pg.19]

When additional substituents ate bonded to other ahcycHc carbons, geometric isomers result. Table 2 fists primary (1°), secondary (2°), and tertiary (3°) amine derivatives of cyclohexane and includes CAS Registry Numbers for cis and trans isomers of the 2-, 3-, and 4-methylcyclohexylamines in addition to identification of the isomer mixtures usually sold commercially. For the 1,2- and 1,3-isomers, the racemic mixture of optical isomers is specified ultimate identification by CAS Registry Number is fisted for the (+) and (—) enantiomers of /n t-2-methylcyclohexylamine. The 1,4-isomer has a plane of symmetry and hence no chiral centers and no stereoisomers. The methylcyclohexylamine geometric isomers have different physical properties and are interconvertible by dehydrogenation—hydrogenation through the imine. [Pg.206]

Table 3 fists cycloaliphatic diamines. Specific registry numbers are assigned to the optical isomers of /n t-l,2-cyclohexanediamine the cis isomer is achiral at ambient temperatures because of rapid interconversion of ring conformers. Commercial products ate most often marketed as geometric isomer mixtures, though large differences in symmetry may lead to such wide variations in physical properties that separations by classical unit operations are practicable, as in Du Font s fractional crystallisation of /n t-l,4-cyclohexanediamine (mp 72°C) from the low melting (5°C) cis—trans mixture. [Pg.206]

J. Jaques, A. CoUet, and S. WiUen, Enantiomers, Racemate, and Resolutions,]o m Wiley Sons, Inc., New York, 1981 The Chemical Society of Japan, eds., Kikan Kagaku Sosetsu (No. 6, Resolution of Optical Isomers), Gakkai Shuppan Senta, Tokyo, Japan, 1989 G. C. Barrett ia Ref. 1, Chapt. 10, pp. 338—353 S. Otsuka and T. Mukaiyama, Progress of ylsymmetric Synthesis and Optical Resolution (ia Japanese), Kagaku Dojia, Kyoto, Japan, 1982. [Pg.298]

N. Oi ia Chemical Society of Japan, eds., Kagaku Sosetsu (No. 6, Resolution of Optical Isomers), Gakkai Shuppan Senta, Tokyo, Japan, 1989,... [Pg.298]

When ofloxacin was first introduced it was made available as the racemate. Later the optical isomers were prepared and it was found that the (3)-enantiomer, DR 3355 (6b), was substantially more active (8—128-fold) than the (R)-isomer against a broad range of bacteria (47—50). This chiral preference is not unique to ofloxacin and has been demonstrated in other quinolones as well (51,52). This significant finding has already had an impact on the design of new quinolone antibacterials still in development (53). [Pg.454]

The steric bulk of the three iodine atoms in the 2,4,6-triiodoben2ene system and the amide nature of the 1,3,5-substituents yield rotational isomers of the 5-A/-acyl-substituted 2,4,6-triiodoisophthalamides. Rotational motion in the bonds connecting the side chains and the aromatic ring is restricted. These compounds also exhibit stereoisomerism when chiral carbon atoms are present on side chains. (R,5)-3-Amino-l,2-propanediol is incorporated in the synthesis of iohexol (11) and ioversol (12) and an (3)-2-hydroxypropanoyl group is used in the synthesis of iopamidol (10). Consequendy, the resulting products contain a mixture of stereoisomers, ie, meso-isomers, or an optical isomer. [Pg.466]

Infrared spectroscopy has broad appHcations for sensitive molecular speciation. Infrared frequencies depend on the masses of the atoms iavolved ia the various vibrational motions, and on the force constants and geometry of the bonds connecting them band shapes are determined by the rotational stmcture and hence by the molecular symmetry and moments of iaertia. The rovibrational spectmm of a gas thus provides direct molecular stmctural information, resulting ia very high specificity. The vibrational spectmm of any molecule is unique, except for those of optical isomers. Every molecule, except homonuclear diatomics such as O2, N2, and the halogens, has at least one vibrational absorption ia the iafrared. Several texts treat iafrared iastmmentation and techniques (22,36—38) and thek appHcations (39—42). [Pg.314]

Chiral separations have become of significant importance because the optical isomer of an active component can be considered an impurity. Optical isomers can have potentially different therapeutic or toxicological activities. The pharmaceutical Hterature is trying to address the issues pertaining to these compounds (155). Frequendy separations can be accompHshed by glc, hplc, or ce. For example, separation of R(+) and 5 (—) pindolol was accompHshed on a reversed-phase ceUulose-based chiral column with duorescence emission (156). The limits of detection were 1.2 ng/mL of R(+) and 4.3 ng/mL of 3 (—) pindolol in semm, and 21 and 76 ng/mL in urine, respectively. [Pg.251]

Immobilization. The abiUty of cyclodextrins to form inclusion complexes selectively with a wide variety of guest molecules or ions is well known (1,2) (see INCLUSION COMPOUNDS). Cyclodextrins immobilized on appropriate supports are used in high performance Hquid chromatography (hplc) to separate optical isomers. Immobilization of cyclodextrin on a soHd support offers several advantages over use as a mobile-phase modifier. For example, as a mobile-phase additive, P-cyclodextrin has a relatively low solubiUty. The cost of y- or a-cyclodextrin is high. Furthermore, when employed in thin-layer chromatography (tic) and hplc, cyclodextrin mobile phases usually produce relatively poor efficiencies. [Pg.97]

Addition to cis- and /n t-2-butene theiefoie yields different optical isomers (10,11). The failure of chlorine to attack isobutylene is attributed to the high degree of steric hindrance to approach by the anion. The reaction intermediate stabilizes itself by the loss of a proton, resulting in a very rapid reaction even at ambient temperature (12). [Pg.363]

Chira.lHydrogena.tion, Biological reactions are stereoselective, and numerous dmgs must be pure optical isomers. Metal complex catalysts have been found that give very high yields of chiral products, and some have industrial appHcation (17,18). The hydrogenation of the methyl ester of acetamidocinnamic acid has been carried out to give a precusor of L-dopa, ie, 3,4-dihydroxyphenylalanine, a dmg used in the treatment of Parkinson s disease. [Pg.165]

Chiral Chromatography. Chiral chromatography is used for the analysis of enantiomers, most useful for separations of pharmaceuticals and biochemical compounds (see Biopolymers, analytical techniques). There are several types of chiral stationary phases those that use attractive interactions, metal ligands, inclusion complexes, and protein complexes. The separation of optical isomers has important ramifications, especially in biochemistry and pharmaceutical chemistry, where one form of a compound may be bioactive and the other inactive, inhibitory, or toxic. [Pg.110]

Shikonin [517-89-5] (Cl 75535) occurs as an acetyl derivative in the Japanese shikone, Uthospermum eTythrorhi n another member of the Boraginaceae family. It is the (R)-optical isomer of alkannin (66). Tissue cultures of E. eythrorhi n are used in Japan to manufacture shikonin mainly for cosmetic use (67). Both alkannin and shikonin are mordant dyes producing violet to gray colors on fabrics. In Japan, shikonin was used to dye fabrics a color known as Tokyo Violet. Shikalkin [54952-43-1] the racemate (11), has been synthesized (68). [Pg.398]

The synthesis of dextromethorphan is an outgrowth of early efforts to synthesize the morphine skeleton. /V-Methy1morphinan(40) was synthesized in 1946 (58,59). The 3-hydroxyl and the 3-methoxy analogues were prepared by the same method. Whereas the natural alkaloids of opium are optically active, ie, only one optical isomer can be isolated, synthetic routes to the morphine skeleton provide racemic mixtures, ie, both optical isomers, which can be separated, tested, and compared pharmacologically. In the case of 3-methoxy-/V-methylmorphinan, the levorotatory isomer levorphanol [77-07-6] (levorphan) was found to possess both analgesic and antitussive activity whereas the dextrorotatory isomer, dextromethorphan (39), possessed only antitussive activity. Dextromethorphan, unlike most narcotics, does not depress ciUary activity, secretion of respiratory tract fluid, or respiration. [Pg.523]

Synthetic pyrethroids is one of the group of modern insecticides of cyclopropancai bonic acid derivate. The pyrethroids prepai ation is the racemic mixture of optical isomers or mixture of cis- or tran.s-isomers. [Pg.130]

Mostly applicable to activities, e.g. reverse flow, chemical reaction. Can also be applied to substances, e.g. poison instead of antidote, D instead of L optical isomers... [Pg.399]

Crystallization continues to be the most widely used method of separating or resolving enantiomers (optical resolutions). The manufacture of chemicals and pharmaceuticals as purified optical isomers, or enantiomers, has taken on a pivotal importance in the pharmaceutical, agricultural and fine chemicals industries over the past 15-20 years. Crystallization has been and continues to be the most widely used method of separating or resolving enantiomers (optical resolutions), and is particularly well suited to separations at large scale in manufacturing processes (Jacques etal., 1981 Roth etai, 1988 Wood, 1997 Cains, 1999). [Pg.5]

See also in sourсe #XX -- [ Pg.915 , Pg.919 , Pg.1125 ]

See also in sourсe #XX -- [ Pg.257 ]

See also in sourсe #XX -- [ Pg.198 ]

See also in sourсe #XX -- [ Pg.83 ]

See also in sourсe #XX -- [ Pg.257 ]

See also in sourсe #XX -- [ Pg.338 , Pg.339 ]

See also in sourсe #XX -- [ Pg.5 , Pg.84 ]

See also in sourсe #XX -- [ Pg.61 ]

See also in sourсe #XX -- [ Pg.116 ]

See also in sourсe #XX -- [ Pg.273 ]

See also in sourсe #XX -- [ Pg.69 ]

See also in sourсe #XX -- [ Pg.461 ]

See also in sourсe #XX -- [ Pg.73 ]

See also in sourсe #XX -- [ Pg.8 , Pg.9 ]

See also in sourсe #XX -- [ Pg.231 ]

See also in sourсe #XX -- [ Pg.338 , Pg.339 ]

See also in sourсe #XX -- [ Pg.37 ]

See also in sourсe #XX -- [ Pg.83 ]

See also in sourсe #XX -- [ Pg.144 , Pg.144 , Pg.237 ]

See also in sourсe #XX -- [ Pg.33 , Pg.34 , Pg.34 , Pg.112 , Pg.281 ]

See also in sourсe #XX -- [ Pg.187 ]

See also in sourсe #XX -- [ Pg.7 ]

See also in sourсe #XX -- [ Pg.262 ]

See also in sourсe #XX -- [ Pg.297 ]

See also in sourсe #XX -- [ Pg.302 , Pg.310 ]

See also in sourсe #XX -- [ Pg.454 , Pg.539 , Pg.2143 ]

See also in sourсe #XX -- [ Pg.297 ]

See also in sourсe #XX -- [ Pg.47 ]

See also in sourсe #XX -- [ Pg.196 ]

See also in sourсe #XX -- [ Pg.201 , Pg.201 ]

See also in sourсe #XX -- [ Pg.137 , Pg.151 , Pg.228 , Pg.234 ]

See also in sourсe #XX -- [ Pg.25 ]

See also in sourсe #XX -- [ Pg.115 ]

See also in sourсe #XX -- [ Pg.291 ]

See also in sourсe #XX -- [ Pg.21 ]

See also in sourсe #XX -- [ Pg.85 ]

See also in sourсe #XX -- [ Pg.915 , Pg.919 , Pg.1125 ]

See also in sourсe #XX -- [ Pg.882 , Pg.946 ]

See also in sourсe #XX -- [ Pg.5 , Pg.84 ]

See also in sourсe #XX -- [ Pg.27 , Pg.378 ]

See also in sourсe #XX -- [ Pg.95 , Pg.549 , Pg.552 ]

See also in sourсe #XX -- [ Pg.11 , Pg.467 , Pg.467 , Pg.468 , Pg.745 , Pg.747 , Pg.747 ]

See also in sourсe #XX -- [ Pg.3 , Pg.4 , Pg.23 , Pg.198 , Pg.256 , Pg.261 ]

See also in sourсe #XX -- [ Pg.110 ]

See also in sourсe #XX -- [ Pg.137 , Pg.151 , Pg.228 , Pg.234 ]

See also in sourсe #XX -- [ Pg.768 ]

See also in sourсe #XX -- [ Pg.56 ]

See also in sourсe #XX -- [ Pg.461 ]

See also in sourсe #XX -- [ Pg.3 , Pg.4 , Pg.23 , Pg.198 , Pg.256 , Pg.261 ]

See also in sourсe #XX -- [ Pg.294 , Pg.311 ]

See also in sourсe #XX -- [ Pg.118 ]

See also in sourсe #XX -- [ Pg.315 ]

See also in sourсe #XX -- [ Pg.147 , Pg.150 ]

See also in sourсe #XX -- [ Pg.111 ]

See also in sourсe #XX -- [ Pg.181 ]

See also in sourсe #XX -- [ Pg.55 ]

See also in sourсe #XX -- [ Pg.1067 ]

See also in sourсe #XX -- [ Pg.965 , Pg.1032 ]

See also in sourсe #XX -- [ Pg.493 , Pg.494 ]

See also in sourсe #XX -- [ Pg.254 , Pg.783 , Pg.804 ]

See also in sourсe #XX -- [ Pg.675 ]

See also in sourсe #XX -- [ Pg.11 , Pg.467 , Pg.467 , Pg.745 , Pg.747 , Pg.747 ]

See also in sourсe #XX -- [ Pg.10 ]

See also in sourсe #XX -- [ Pg.10 ]

See also in sourсe #XX -- [ Pg.214 ]

See also in sourсe #XX -- [ Pg.9 ]

See also in sourсe #XX -- [ Pg.468 , Pg.468 , Pg.469 , Pg.748 , Pg.749 ]

See also in sourсe #XX -- [ Pg.26 , Pg.33 ]

See also in sourсe #XX -- [ Pg.980 , Pg.981 , Pg.982 , Pg.1046 ]

See also in sourсe #XX -- [ Pg.30 ]

See also in sourсe #XX -- [ Pg.383 , Pg.865 , Pg.866 ]

See also in sourсe #XX -- [ Pg.143 ]

See also in sourсe #XX -- [ Pg.150 ]

See also in sourсe #XX -- [ Pg.198 ]

See also in sourсe #XX -- [ Pg.305 ]

See also in sourсe #XX -- [ Pg.173 ]

See also in sourсe #XX -- [ Pg.407 , Pg.929 ]

See also in sourсe #XX -- [ Pg.7 , Pg.1142 ]

See also in sourсe #XX -- [ Pg.63 ]

Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...