Optical activity cause

Optical Activity Caused by Restricted Rotation of Other Types. Substituted paracyclophanes may be optically active and 25, for example, has been resolved. In this case, chirality results because the benzene ring cannot rotate in such a way that the carboxyl group goes through the alicyclic ring. Many chiral layered cyclophanes (e.g., 26) have been prepared. ... 

Optical activity caused by restricted rotation of other types. Substituted paracyclo-phanes may be optically active and 20, for example, has been resolved.57 In this case chirality... 

Many of the interesting properties of liquid crystals are a result of chirality or handedness, which is manifest in optical properties by optical activity. For isotropic materials or anisotropic materials viewed along their optic axes, optical activity causes the plane of polarization of propagating light to be rotated by an angle 0. This can be expressed in terms of a difference between refractive indices for left ( ) and right (n ) circularly polarized light ... 

In certain crystals, e.g. in quartz, there is chirality in the crystal structure. Molecular chirality is possible in compounds which have no chiral carbon atoms and yet possess non-superimposable mirror image structures. Restricted rotation about the C=C = C bonds in an allene abC = C = Cba causes chirality and the existence of two optically active forms (i)... 

Specific Rotation. Optical rotation is caused by individual molecules of the optically active compound. The amount of rotation depends upon how many molecules the light beam encounters in passing through the tube. When allowances are made for the length of the tube that contains the sample and the sample concentration, it is found that the amount of rotation, as well as its direction, is a characteristic of each individual optically active compound. 

A synthesis of optically active citroneUal uses myrcene (7), which is produced from P-piaene. Reaction of diethylamine with myrcene gives A/,A/-diethylgeranyl- and nerylamines. Treatment of the aHyUc amines with a homogeneous chiral rhodium catalyst causes isomerization and also induces asymmetry to give the chiral enamines, which can be readily hydrolyzed to (+)-citroneUal (151). 

Chirazymes. These are commercially available enzymes e.g. lipases, esterases, that can be used for the preparation of a variety of optically active carboxylic acids, alcohols and amines. They can cause regio and stereospecific hydrolysis and do not require cofactors. Some can be used also for esterification or transesterification in neat organic solvents. The proteases, amidases and oxidases are obtained from bacteria or fungi, whereas esterases are from pig liver and thermophilic bacteria. For preparative work the enzymes are covalently bound to a carrier and do not therefore contaminate the reaction products. Chirazymes are available form Roche Molecular Biochemicals and are used without further purification. 

Canadine is bitter and in small doses causes drowsiness and depression. In large doses it gives rise to transient excitement succeeded by depression and paralysis of the central nervous system. Its injection is followed by violent peristalsis with diarrhoea. It is said to have no effect on the blood pressure. The pharmacological action of canadine a- and -meihochlorides was examined by Laidlaw, who found both to have the curare-like action common to ammonium bases, the -isomeride being the more active the relative activities of the four optically active forms are given as h da. ip dp = 1 Q 2 28. 

A recent report describes the conversion of A-formyl- and N-acetyl-L-leucine into optically active azlactones with dicyclohexyl-carbodiimide (DCC) [Eq. (29)]. Other cyclization reagents, e.g. acetic anhydride, POCI3, SOCI2, and polyphosphoric acid, cause racemiza-tion. These azlactones react with optically active amino acid esters to give esters of dipeptides with retention of activity. 

Today, however, GC-GC coupling is seldom used to determine pesticides in environmental samples (2), although comprehensive MDGC has been applied to determine pesticides in more complex samples, such as human serum (19). On the other-hand, new trends in the pesticide market, which is now moving towards the production of optically active enantiomers and away from racemic mixtures, may make this area suitable for GC-GC application. The coupling of non-chiral columns to chiral columns appears to be a suitable solution to the separation problems that such a trend might cause. 

A polarimeter. The sample lube contains an optically active compound. The analyzing filter has been tuned clockwise to restore the light field and measure the rotation caused by the sample. 

Stereochemical constraints in cyclic sulfones and sulfoxides impart increased weight to strain and conformational factors in the generation of carbanions and their stability, causing distinct differences between the behavior of cyclic and open-chain systems233, due primarily to the prevention of extensive rotation about the C —S bond, which is the major way that achiral carbanions racemize. Study of the a-H/D exchange rate fce and the racemization rate ka may provide information concerning the acidity-stereochemical relationships in optically active cyclic sulfone and sulfoxide systems. 

Optically active 2,2,2-trifluorophenylethanol, when used as NMR solvent, causes enantiomeric spectral dissimilarities for chiral episulphoxides the relative field positions of non-equivalent NMR resonances are analyzed with respect to the absolute configuration of the solvated compounds220. 

Sulfoxides (R1—SO—R2), which are tricoordinate sulfur compounds, are chiral when R1 and R2 are different, and a-sulfmyl carbanions derived from optically active sulfoxides are known to retain the chirality. Therefore, these chiral carbanions usually give products which are rich in one diastereomer upon treatment with some prochiral reagents. Thus, optically active sulfoxides have been used as versatile reagents for asymmetric syntheses of many naturally occurring products116, since optically active a-sulfinyl carbanions can cause asymmetric induction in the C—C bond formation due to their close vicinity. In the following four subsections various reactions of a-sulfinyl carbanions are described (A) alkylation and acylation, (B) addition to unsaturated bonds such as C=0, C=N or C= N, (C) nucleophilic addition to a, /5-unsaturated sulfoxides, and (D) reactions of allylic sulfoxides. 

Optically active peracids such as percamphoric acid have been used to oxidize selectively one sulphoxide enantiomer in a racemic mixture. These reactions involve the use of 0.5 molar equivalents of the peracid in either ether47 or chloroform48 as solvent. The presence of nitro groups causes the oxidant to be consumed without oxidation of the sulphoxide functionality. This method is usually used to obtain an optically active sulphoxide by recovery of the unreacted material after oxidation. 

When three, five, or any odd number of cumulative double bonds exist, orbital overlap causes the four groups to occupy one plane and cis-trans isomerism is observed. When four, six, or any even number of cumulative double bonds exist, the situation is analogous to that in the allenes and optical activity is possible. Compound 20 has been resolved. ... 

Therefore, another analogous reaction was studied with a more reactive olefin, viz. methyl acrylate, which reacts with (+)-methylneophylphenyltin deuteride (86) at room temperature and yields after 18 h again an optically inactive adduct which is reduced with lithium aluminum hydride to give racemic isotopically labeled (55) 44). After 18h in the presence of AIBN at room temperature, (86) only loses 30% of its optical activity in benzene. The fact that the obtained adduct is optically inactive might be due to the nucleophilicity of methyl acrylate, which might be important enough to cause the racemization of (56). 

Most of the physical properties (e.g., boiling and melting point, density, refractive index, etc.) of two enantiomers are identical. Importantly, however, the two enantiomers interact differently with polarized light. When plane polarized light interacts with a sample of chiral molecules, there is a measurable net rotation of the plane of polarization. Such molecules are said to be optically active. If the chiral compound causes the plane of polarization to rotate in a clockwise (positive) direction as viewed by an observer facing the beam, the compound is said to be dextrorotatory. An anticlockwise (negative) rotation is caused by a levorotatory compound. Dextrorotatory chiral compounds are often given the label d or ( + ) while levorotatory compounds are denoted by l or (—). 

Stannane 37 lost 50% of its optical activity when allowed to stand as a 0.2 M solution in benzene for 17 days. Addition of A1BN to the solution at 80 °C caused complete racem-ization after 30 min. With added hydroquinone, the benzene solution at 80 °C showed no decrease in rotation after 2 h. It was thus concluded that racemization proceeds by homolysis. 

An electric dipole operator, of importance in electronic (visible and uv) and in vibrational spectroscopy (infrared) has the same symmetry properties as Ta. Magnetic dipoles, of importance in rotational (microwave), nmr (radio frequency) and epr (microwave) spectroscopies, have an operator with symmetry properties of Ra. Raman (visible) spectra relate to polarizability and the operator has the same symmetry properties as terms such as x2, xy, etc. In the study of optically active species, that cause helical movement of charge density, the important symmetry property of a helix to note, is that it corresponds to simultaneous translation and rotation. Optically active molecules must therefore have a symmetry such that Ta and Ra (a = x, y, z) transform as the same i.r. It only occurs for molecules with an alternating or improper rotation axis, Sn. 

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