Optical-/stereo-isomerism

Before 1940 optically active compounds could only be obtained in stereo-isomerically pure form by isolation from natural sources, by resolution of racemic mixtures, or by a few laboratory controlled enzymic reactions. Many of the chemical reactions described in this book lead to products which contain chiral centres, axes, or planes, but in which the isolated material is the optically inactive (racemic) form. This is a direct consequence of the fact that the reactants, reagents, or solvents are achiral or are themselves racemic. The following selection of reactions drawn from the text illustrate this statement they may be cross-referenced to the relevant discussion sections, namely (a) Section 5.4.1, p. 519, (b) Section 5.4.3, p.542, (c) Section 5.11.7, p.687, (d) Section 8.1.3, p. 1133, e) Section 5.2.4, p. 504 and (/) Section 5.4.2, p. 531. 

Asymmetric Carbon.—Now van t Hoff and LeBel found that all optically active compounds contained at least one such carbon atom. They ascribed the existence of two optically active forms to the presence in the compound of this uns3anmetrically related or asymmetric carbon atom. The asymmetry of the compounds, in that one form is dextrorotatory the other levo-rotatory, is due to this asymmetric arrangement of the molecule in space. We emphasized the fact that our structural formulas as we have been using them are simply plane representations of relationships, and indicate nothing as to the arrangement in space of the atoms or groups in a molecule. The theory of van t Hoff and LeBel considers the molecule as it is arranged in space. The isomerism so explained is known as stereo-isomerism or space isomerism. 

Stereo-isomerism.—In addition, however, to the two structurally isomeric acids which we have been considering, one being an alpha-hydroxy acid, the other a eto-hydroxy acid, there are known two other acids of the same composition both of which prove to be a-hydroxy propionic acid. Of these three alpha-2iC ds> two are optically active one being dextro- and the other levo-rotditoYy. The third is optically inactive but resolvable into its optical components. An examination of the formula of a-hydroxy propionic acid shows that it contains an asymmetric carbon atom. 

Stereo Isomerism of Malic Acid.—On examination of the formula of malic acid it will be seen that one of the carbons is asymmetric i.e.j it has united to it four different elements or groups, viz., (—H), (—OH), (—COOH), and (—CH2—COOH). We should, therefore, expect to find that malic acid is optically active and that it exists in the three forms of dextroj levo, and inactive. This is in accordance with the facts. The formulas for the three stereo-isomeric forms of malic acid may be written as follows, corresponding exactly to those for lactic acid. 

This sugar is also an aldo-pentose and is stereo-isomeric with arabinose. It is known as wood sugar because it is obtained by the hydrolysis of wood gum, i.e.f of the pentosans present in this gum. It is crystalline and melts at 140°- 60°. It is optically active, being dextrorotatory. Its osazone melts at 160°. By reduction it yields a penta-hydroxy alcohol and by oxidation it yields tri-hydroxy glutaric acid. 

As each of these menthenes will yield isomeric mentha di-enes the number of isomers possible in this group is still larger. That is, one men-thane yields six menthenes and these a larger number, viz., fourteen, mentha-di-enes. Furthermore, stereo-isomerism with accompanying optical activity due to the presence of asymmetric carbons, increases the number of possible isomers. It will not be necessary to dwell further upon the isomerism of the terpenes it being necessary simply to explain the fact of the existence of structural isomers and of stereo-isomers with optical activity. The system of nomenclature of the isomers will not be considered. Reference to larger books will be necessary to make this plain. 

Structural-, positional-, stereo-isomerism (optical and geometric) in aliphatic hydrocarbon systems. 

Condensation of the unstable, highly functionalised aldehydo-ester (162) with 2-hydroxytryptamine gave a mixture of three stereo-isomeric, tetracyclic oxindole esters (163). The major isomer, when heated with polyphosphoric acid, underwent cyclisation, hydrolysis, decarboxylation, and further cyclization, to give the hexacyclic ketone (164), reduction of which gave (+)-19-hydroxyaspidofractinine (Scheme 29), identical (except in optical rotation) with an authentic sample prepared from minovincine. Kuehne s versatile biomimetic synthesis has been further... 

A large number of stereo- and optical isomers are possible. Iso- refers to stereo-isomerism about Cj and Neo- to stereoisomerism about C2. 

Each carbon atom on the backbone chain may contain four substitutes different from each other, two of which are the rest parts of the chain at both sides with different chain lengths. These carbon atoms exhibit chiral asymmetry, giving rise to the optical activity labeled with R for the left-handed direction and with S for the right-handed direction. Irregularities of chain sequences in optical activities are called optical- or stereo-isomerism. A central mirror exists at the middle of the... 

The Optical Properties ol Carbohydrates.— All carbohydrates contain one or more as3Tnmetric carbon atoms in which the carbon is united to four different kinds of components. This type of configuration endows the compound with (i) optical activity and (ii) optical or stereo-isomerism. By optical activity is meant the power possessed by the compound, both in crystalline form and in solution, of rotating in one or other direction a beam of plane-polarised light sent through it. 

Optical Activity Stereo-chemistry.—The study of active amyl alcohol and other substances that rotate the plane of polarized light, has led to the discovery of a class of isomerism which is not explained by the structure theory of organic compounds as it has been developed up to this point. Three amyl... 

Stereo (or geometrical) isomerism is related to different orientations of the side-groups or main-chain bonds (while the sequence and the nature of all bond connections remain the same). The two main typ>es of stereoisomerism are optical isomerism (tarticity) and cis-tram isomerism. The optical isomerism is also related to macromolecular chirality (enan-tiomerism, see Figure 8 and Sertion 1.02.1.8). 

Isomerism. Distinguish between the following types of isomerism (a) stereo and structural (b) geometrical and optical. 

The isomers that differ with respect to the relative orientation of bonds are known as stereo isomers or space isomers. There are two forms of stereoisomerism geometrical isomerism, also known as diastereoisomerism, and Optical isomerism, also known as mirror image isomerism. 

When the humulinic acids are formed either from humulone or from the isohumulones, the ratio of (+) cis (-) trans is 7 3 (35). The percentage of cis humulinic acid decreases as a function of time to ca. 10% of the mixture. Conversion of cis to trans humulinic acid may occur via proton abstraction or via enolizatlon at both chiral centres. This means that the four stereo-isomers may be interconverted. When only one chiral carbon atom is epimerized, isomerization is the result. There is no loss of optical activity when the epimerization occurs at C-4, while inversion at C-5 causes racemization. Isomerization at both centres will be accompanied by racemization. Humulone must be converted first to the isohumulones, which further may be deacylated to the humulinic acids (36). In principle, this reaction can start from both epimeric isohumulones, but cis isohumulone is hydrolyzed much faster than trans isohumulone. In fact, hydrolysis of trans isohumulone is only observed in conditions, whereby trans isohumulone is epimerized to cis isohumulone. This phenomenon does not exclude direct hydrolysis, but the fraction of trans isohumulone, that is converted via the cis epimer, will probably be much higher. Up to pH 12 hydrolysis is slower than epimerization, while above pH 12 the formation of humulinic acids increases much faster than epimerization. As a consequence, it is possible to obtain a rather high conversion rate of trans to cis isohumulone without appreciable degradation. 

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