Asymmetric molecules isomerism

Note that a single molecule may correspond to many different, but equivalent, SMILES strings. For example, for a given asymmetric molecule, starting from a different asymmetric atom will lead to a different, but equally valid, SMILES string. These various SMILES are called isomeric SMILES. They can be converted to a unique form called canonical SMILES (11). 

Molecules that show optical activity have no plane of symmetry. The commonest case of this is in organic compounds in which a carbon atom is linked to four different groups. An atom of this type is said to be a chiral centre. Asymmetric molecules showing optical activity can also occur in inorganic compounds. For example, an octahedral complex in which the central ion coordinates to six different ligands would be optically active. Many natur y occurring compounds show optical isomerism and usually only one isomer occurs naturally. 

Mirror-Image Isomerism appears in asymmetric molecules, of which the simplest member is a C atom with four different substituents. The C atom thus substituted is usually—but somewhat incorrectly—called the asymmetric C atom. Two substances related by this mirror-image isomerism are called enantiomers. They... 

Although lupinine is thus a comparatively simple alkaloid its detailed chemistry has been difficult to unravel owing (a) to the presence in its molecule of two asymmetric carbon atoms as asterisked in (XI), and (6) the possibility of cis-trans isomerism in certain of its proximate (ieriva-tives. Winterfeld and Holschneider have pointed out that a further complexity arises from the presence in natural Z-lupinine of a structural isomeride, aZZolupinine for which formula (XII) is suggested. They also quote Kreig s observation that by the action of sodium on a benzene solution of Z-lupinine (m.p. 68-9° [ajo — 23-52°), the latter is converted... 

Absolute configurations of the amino acids are referenced to D- and L-glyceraldehyde on the basis of chemical transformations that can convert the molecule of interest to either of these reference isomeric structures. In such reactions, the stereochemical consequences for the asymmetric centers must be understood for each reaction step. Propose a sequence of reactions that would demonstrate that l( —)-serine is stereochemically related to l( —)-glyceraldehyde. 

Citrate is isomerized to isocitrate by the enzyme aconitase (aconitate hydratase) the reaction occurs in two steps dehydration to r-aconitate, some of which remains bound to the enzyme and rehydration to isocitrate. Although citrate is a symmetric molecule, aconitase reacts with citrate asymmetrically, so that the two carbon atoms that are lost in subsequent reactions of the cycle are not those that were added from acetyl-CoA. This asymmetric behavior is due to channeling— transfer of the product of citrate synthase directly onto the active site of aconitase without entering free solution. This provides integration of citric acid cycle activity and the provision of citrate in the cytosol as a source of acetyl-CoA for fatty acid synthesis. The poison fluo-roacetate is toxic because fluoroacetyl-CoA condenses with oxaloacetate to form fluorocitrate, which inhibits aconitase, causing citrate to accumulate. 

Two asymmetric carbon atoms, the or carbon in the glutamic acid portion of the molecule and the C6 carbon in the tetrahydropteridine ring, allow four possible isomers. Since synthetic procedures would undoubtedly start with L-glutamic acid, the isomeric possibilities are reduced to the dL and 1L diastereomers. Of these, the biologically more active form is the 1L separation of the diastereomers is effected by solubility differences of the calcium salts.2... 

Optical Isomerism of (D-L-Isomerism) and Tacticity of Polymers Optical isomerism has its origin in the way different substituents occupy positions on an asymmetric carbon atom in a polymer molecule. For example, polyethylene molecule has fully saturated carbon atoms as shown in the following chemical formula ... 

Bot these C atoms provides a site for optical isomerism. Each such site can exhibit either d- or /-type isomerism which depends on whether the R group is located below the plane of carbon-carbon chain or above. The regularity or the order in which the successive asymmetric carbon sites, C, exhibit their d-or /- form leads to three different types of isomeric structure in the polymer molecule. The structures shown in figure below. 

So for a molecule with two different asymmetric carbon atoms we will have not more than four stereoisomeric forms. Similarly other cyclic molecules show a similar stereoisomerism. However, with even-numbered rings certain molecules show only geometrical isomerism as in 1-methyl-3 -chlorocyclobutane. 

Another type of isomerism is optical isomerism. These molecules are capable of rotating light to either the left or right and are said to be optically active. The presence of an asymmetric or chiral carbon (a carbon atom with four different groups attached to it) will make a compound optically active. 

The general subject of asymmetric synthesis has been reviewed extensively (1-5). The term asymmetric synthesis has been defined in more than one way (1,4) however, a useful definition is the one given by Morrison and Mosher (1) a process which converts a prochiral unit [refs. 6 and 7] into a chiral unit so that unequal amounts of stereoisomeric products result. The stereoisomeric products may be enantiomeric or they may be diastereomeric. The substrate molecule must contain either enantiotopic or diastereotopic groups or faces (8,9), since the attack of a reagent at equivalent groups or faces cannot lead to isomeric products. 

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... 

Another type of isomerism arises when a molecule contains a chiral center or is chiral as a whole. Chirality (from the Greek cheir, hand) leads to the appearance of structures that behave like image and mirror-image and that cannot be superimposed ( mirror isomers). The most frequent cause of chiral behavior is the presence of an asymmetric C atom—i.e., an atom with four different substituents. Then there are two forms (enantiomers) with different configurations. Usually, the two enantiomers of a molecule are designated as L and D forms. Clear classification of the configuration is made possible by the R/S system (see chemistry textbooks). 

Optical isomerism is the result of a dissymmetry in molecular suhstitution. The basic aspects of optical isomerism are discussed in various textbooks of organic chemistry. Optical isomers (enantiomers) may have different physiological activities from each other provided that their interaction with a receptor or some other effector structure involves the asymmetric carbon atom of the enantiomeric molecule and that the three different substituents on this carbon atom interact with the receptor. The Easson-Stedman hypothesis assumes that a three-point interaction ensures stereospecificity, since only one of the enantiomers will fit the other one is capable of a two-point attachment only, as shown in figure 1.13 for the reaction with a hypothetical planar receptor. However, it is reasonable to assume that receptor stereospecificity can also undergo a change when the receptor conformation is altered by a receptor-drug interaction. 

Ammino-derivatives op Cobalt Salts—Cobaltous Salt Ammines—Cobaltic Salt Ammines—Mononuclear Cobalt-ammines containing One Atom of Cobalt in the Molecule—Cobaltic Salts with Trivalent Cation—Cobalt-ammines Containing Divalent Cation—Cobalt-ammines containing Monovalent Cation—Cobalt-ammines consisting of Non-dissociable Complex— Cobalt-ammines containing Monovalent Anion—Cobalt Salts containing Trivalent Anion—Polynuclear Cobalt-ammines containing Two or more Cobalt Atoms in the Molecule—Cobalt-ammines of Unknown Constitution— Ionisation Metamerism—Polymerisation Isomerism—Valency Isomerism —Co-ordination Position Isomerism—Isomerism due to Asymmetric Cobalt Atoms. 

A second form of optical isomerism analogous to that shown by organic spirocyclic compounds has been demonstrated. Any molecule will be optically active if it is not superimposable on its mirror image. The two enantiomers of bisfben-zoylacetonato)beryllium are illustrated in Fig. 12.3. In order for the complex to be chiral, the chelating ligand must be unsymmetric (no/ necessarily asymmetric or chiral, itself) [Be cac ] is not chiral. 

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

In older literature optical isomerism of the type represented by d and l pairs was usually discussed in terms of "asymmetric carbon atoms" or "asymmetric centers." Now the terms chiral (pronounced ki-ral) molecules, chiral centers, and chirality (Greek "handedness") are preferred. 

In 1894, Fischer wrote "It will probably be possible to obtain all riiem-bers of the sugar group by a combination of the cyanohydrin reaction with the reduction of lactones, as soon as we have succeeded in finding the two optically active forms of glyceraldchyde. All observations agree with the isomerisms foreseen by Van t Hoff, above all the disappearance of isomers if the molecule becomes constitutionally symmetric. This includes the transformation of different stereoisomers into one and the same substance if one of several asymmetric centers is abolished." An example of this is... 

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