Optical activity/isomers

In complexes of chelates there are a number of types of isomerism which may occur. In a tris(ethylenediamine) octahedral complex two optically active isomers occur (often denoted A and A). 

Many of the physical properties are not affected by the optical composition, with the important exception of the melting poiat of the crystalline acid, which is estimated to be 52.7—52.8°C for either optically pure isomer, whereas the reported melting poiat of the racemic mixture ranges from 17 to 33°C (6). The boiling poiat of anhydrous lactic acid has been reported by several authors it was primarily obtained duriag fractionation of lactic acid from its self-esterification product, the dimer lactoyUactic acid [26811-96-1]. The difference between the boiling poiats of racemic and optically active isomers of lactic acid is probably very small (6). The uv spectra of lactic acid and dilactide [95-96-5] which is the cycHc anhydride from two lactic acid molecules, as expected show no chromophores at wavelengths above 250 nm, and lactic acid and dilactide have extinction coefficients of 28 and 111 at 215 nm and 225 nm, respectively (9,10). The iafrared spectra of lactic acid and its derivatives have been extensively studied and a summary is available (6). 

Many cycHc ketones occur in natural oils. Jasmone [488-10-8] (3-meth5l-2-(2-pentyl)-2-cyclopenten-l-one) (4) is an odoriferous component of the oil obtained from jasmine flowers. /-Menthone [14073-97-3] (5) is the most frequently occurring of four optically active isomers, and is a colorless Hquid with a minty odor obtained from Mentha species of plants. Muscone [541-91-3] (6) and civetone [542-46-1] (7) are expensive animal products. 

The asterisk signifies an asymmetric carbon. AH of the amino acids, except glycine, have two optically active isomers designated D- or L-. Isoleucine and threonine also have centers of asymmetry at their P-carbon atoms (1,10). Protein amino acids are of the L-a-form (1,10) as illustrated in Table 1. 

Faolini and Divizia have succeeded in partially resolving inactive linalol into its optically active isomers hut only to the extent of optical rotations of + 1° 70 and — 1° 60 respectively. Linalol was converted into its acid phthalate, and an alcoholic solution of this compound was treated with the equivalent quantity of strychnine. By fractional crystallisation the laevo-rotatory salt, yielding dextro-rotatory linalol, separates first, leaving the more soluble dextro-rotatory strychnine salt, which yielded laevo-rotatory linalol in the mother liquor. 

Citronellal which was originally termed citronellone, is one of the characteristic constituents of citronella oil, in which it occurs to a considerable extent. It can be prepared by shaking the essential oil with a hot solution of sodium bisulphite, and decomposing the resulting bisulphite compound by means of sodium carbonate and distilling the citronellal in a current of steam. Citronellal is optically active, and it is probable that the specimens isolated from natural sources are mixtures of the two optically active isomers, so that the actual theoretical rotation is not known with certainty. 

Molecules that contain two or more asymmetric centers exist in more than two stereoisomeric forms. Some are pairs of optically active isomers others may be symmetric and therefore optically inactive. An example is tartaric acid with two asymmetric central carbon atoms. It has three isomers, two of which are optically active and one inactive. 

FIGURE 16.3 The optically active isomers of [Co(en)3]3+(top) and their directions of rotation of polarized light. 

Chiral stationary phases for the separation of enantiomers (optically active isomers) are becoming increasingly important. Among the first types to be synthesized were chiral amino acids ionically or covalently bound to amino-propyl silica and named Pirkle phases after their originator. The ionic form is susceptable to hydrolysis and can be used only in normal phase HPLC whereas the more stable covalent type can be used in reverse phase separations but is less stereoselective. Polymeric phases based on chiral peptides such as bovine serum albumin or a -acid glycoproteins bonded to... 

In general, the maximum number of optically active isomers is given by 2n where n represents the number of asymmetric carbon atoms. Thus for a compound where n = 1, as in lactic acid, there would be two stereoisomers, one the dextro and the other the laevo. For a compound with two asymmetric carbon atoms, there would be 22 = 4 stereoisomers. But if the two asymmetric carbon atoms carry exactly identical groups, as in tartaric acid, the number would be fewer than four and we know that it exists in three forms, the d the 1 and the meso. 

Tetracoordinate sulfur compounds containing a lone pair of electrons at sulfur possess a more or less distorted trigonal-bipyramidal structure, in common with the vast majority of other pentacoordinated molecules of the main group elements (189,191,199). A common name, sulfurane, is generally accepted for this type of compound. In principle, sulfuranes are chiral. However, both the number of optically active isomers and their optical stability depend on the nature of substituents bonded to the central sulfur atom, the apicophilicity of the substituents, and the energy required for permutational isomerization processes. In this context it is interesting to note that acyclic sulfuranes with four different ligands should exist in 20 isomeric forms. 

Molecules which contain a chiral cobalt as well as an asymmetric nitrogen exist in four possible optical isomeric forms. These are represented for Co(sar)(hbg) +, hbg = NH2C( = NH)NHC(=NH)NH2 in Fig. 7.12. All four optically-active isomers have been isolated and characterized by cd, nmr and vis/uv absorption spectroscopy. The kinetics of... 

Optically active icco-nucleosides of type 1.1 of 2-nitromidazole were prepared from tartaric acid ester by conversion to the dioxolan followed by reduction and acylation to give 1154, which was converted to 1155. Its coupling and deacylation gave 1156 (94MIP1). All the optically active isomers showed a radiation-sensitization effect equivalent to that of the racemate. 

A metal-nucleotide complex that exhibits low rates of ligand exchange as a result of substituting higher oxidation state metal ions with ionic radii nearly equal to the naturally bound metal ion. Such compounds can be prepared with chromium(III), cobalt(III), and rhodi-um(III) in place of magnesium or calcium ion. Because these exchange-inert complexes can be resolved into their various optically active isomers, they have proven to be powerful mechanistic probes, particularly for kinases, NTPases, and nucleotidyl transferases. In the case of Cr(III) coordination complexes with the two phosphates of ATP or ADP, the second phosphate becomes chiral, and the screw sense must be specified to describe the three-dimensional configuration of atoms. 

Three methods are known, all due to Pasteur, by which an inactive mixture can be separated into its optically active isomers —... 

Alanine, aspartic acid, glutamic acid, tyrosine, leucine, phenylalanine and also a-amino-n-caproic acid and a-aminobutyric acid have in this way been separated by Fischer into their optically active isomers. To these must be added ornithine which was synthesised by Sorensen in 1903, and separated into d- and 1-ornithine in 1905. 

Systematic investigations of twofold additions of malonates to C70 revealed that the second addition takes place at one of the five a-bonds of the unfunctionalized pole [17, 26], With achiral, C2v-symmerical malonate addends, three constitutionally isomeric bisadducts are formed An achiral one (C2v-symmetrical 1), and two chiral ones (C2-symmetrical 2 and 3), which are obtained as pairs of enantiomers with an inherently chiral addition pattern (Figure 13.5). Twofold addition of chiral malonates leads to the formation of five optically active isomers, two constitutionally isomeric pairs of C2-symmetrical diastereomers and a third constitutional C2-symmetrical isomer (Figure 13.5). Twofold additions of azides to C70 lead to diazabis[70]homo-fullerenes, which served as starting material for the synthesis of bis-(aza[70]-fullerenyl) (Cg9N)2 (Chapter 12) [27]. As further bisadditions, addition reaction to C70 [2+2]cycloaddition of electron-rich bis(diethylamino)ethyne and 1-alkylthio-2-(diethylamino)ethynes [28] and the addition of transition metal fragments have been reported [29-32],... 

Diasteroisomers, also known as geometric isomers, have different relative orientations of their metal-ligand bonds. Enantiomers are stereoisomers whose molecules are nonsuperposable mirror images of each other. Enantiomers have identical chemical and physical properties except for their ability to rotate the plane of polarized light by equal amounts but in opposite directions. A solution of equal parts of an optically active isomer and its enantiomer is known as a racemic solution and has a net rotation of zero. 

Each 5ec-butyl group has a chiral C that can be R or S. Since all four groups are equivalent, the order of writing the designation is immaterial, RRRS is the same as RRSR. The possibilities are RRRR, RRRS, RRSS, SSRR, SSSR, and SSSS. RRRR and SSSS, and RRRS and SSSR, are enantiomeric pairs. These are the four optically active isomers. The mirror image of RRSS is SSRR. These are identical and therefore the isomer is meso. This isomer is a rare example of a compound which is achiral because it has only an improper axis of symmetry—it has no plane or center of symmetry. 

Isomerism due to Asymmetric Cobalt Atoms.—Werner established his formulas for the cobalt-ammines by proving the fact suggested by his theory that certain of the cobalt atoms in the ammines were centres of asymmetry, and therefore optical activity should be possible. Having established this for some of the simple cobalt-ammines, he then showed that in many of the polynuclear compounds optical activity exists. Thus he prepared optically active isomers of tetraethylenedianaino-... 

As in the case of the mononuclear cobalt salts, the configuration of the compounds suggests that some should exist in optically active forms. Several cobalt-ammines of the polynuclear series have been resolved into their optically active isomers. 

Two series of complex iridium salts corresponding to the hexammino-and diacido-iridium salts have been prepared containing ethylene-diamine. These have the same general characteristics as the ammino-salts. Both series have been resolved into optically active isomers, the dinitrito-series by means of d- and 1-bromo-camphor sulphonic acid, and the triethylenediamino-series by d- and 1-nitro-camphor. Attempts have been made to prepare etliylenediamino-derivatives containing tetravalent iridium. Thus, sodium hexaehloro-iridate, [IrCl6]Na2, reacts with ethylenediamine, but the products are syrupy and cannot be purified. Derivatives of trivalent iridium, however, have been prepared. These are crystalline and fairly easily purified. The... 

Racemization may occur in molecules in which structural changes, such as those due to resonance, enolization, substitution or elimination of groups, temporarily destroy the asymmetry needed to maintain the optical activity. Also, Walclen inversion of half of an optically active isomer can yield a racemate without the destruction of the center of asymmetry this phenomenon is observed in the reaction of n-butanol-2 with HCI(.)4. 

A Messina, AM Girelli, M Flieger, M Sinibaldi, P Sedmera, L Cvak. Direct resolution of optically active isomers on chiral packings containing ergoline skeletons. 5. Enantioseparation of amino acid derivatives. Anal Chem 68 1191-1196, 1996. 

It has been observed that the formation of the olefin and carbon monoxide, 45, is ten times more important than the formation of the bicyclic hydrocarbon and carbon monoxide, 46, at 80° and 80 mm. pressure even at 3130 A. The formation of the strained bicyclic hydrocarbon is evidently not a favorable reaction although this may not be the only consideration. In the case of camphor it should be interesting to find out if an optically active isomer of the ketone on photolysis will give rise to an optically active trimethyl bicyclo [2.1.1] hexane (XXVI). A concerted reaction, analogous to the formation of cyclobutane from cyclopentanone, may lead to only an optically active product. 

The anticancer activity of our racemic p-lactams has prompted us to devise method for the preparation of the optically active analogues. It is known that an optically active isomer of a racemic compound has better and much selective biological... 

The maximum number of optically active isomers that a single compound can have is related to the number of its chiral centers by the following formula ... 

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