Configuration asymmetric

Fischer Projections Stereoisomerism in Cyclic Compounds Methods of Determining Absolute Configuration Asymmetric Synthesis... 

The friction force measurements were performed in two different configurations asymmetric, where only the fiat substrate surface was coated by PLL-g-PEG and the colloidal probe of the LPM remains bare, and... 

In polymers made of dis-symmetric monomers, such as, for example, poly(propylene), the stmcture may be irregular and constitutional isomerism can occur as shown in figure C2.1.1(a ). The succession of the relative configurations of the asymmetric centres can also vary between stretches of the chain. Configuration isomerism is characterized by the succession of dyads which are named either meso, if the two asymmetric centres have the same relative configurations, or racemo if the configurations differ (figure C2.1.1(b )). A polymer is called isotactic if it contains only one type of dyad and syndiotactic if the dyad sequence strictly alternates between the meso and racemo fonns. 

Figure C2.1.1. (a) Constitutional isomerism of poly (propylene). The upper chain has a regular constitution. The lower one contains a constitutional defect, (b) Configurational isomerism of poly(propylene). Depending on tire relative configurations of tire asymmetric carbons of two successive monomer units, tire corresponding dyad is eitlier meso or racemo.

A simpler representation of molecules containing asymmetric carbon atoms is the Fischer projection, which is shown here for the same lactic acid configurations. A Fischer projection involves... 

Structures [VIII] and [IX] are not equivalent they would not superimpose if the extended chains were overlaid. The difference has to do with the stereochemical configuration at the asymmetric carbon atom. Note that the asymmetry is more accurately described as pseudoasymmetry, since two sections of chain are bonded to these centers. Except near chain ends, which we ignore for high polymers, these chains provide local symmetry in the neighborhood of the carbon under consideration. The designations D and L or R and S are used to distinguish these structures, even though true asymmetry is absent. 

For most hydrardic pressure-driven processes (eg, reverse osmosis), dense membranes in hoUow-fiber configuration can be employed only if the internal diameters of the fibers are kept within the order of magnitude of the fiber-wall thickness. The asymmetric hoUow fiber has to have a high elastic modulus to prevent catastrophic coUapse of the filament. The yield-stress CJy of the fiber material, operating under hydrardic pressure, can be related to the fiber coUapse pressure to yield a more reaUstic estimate of plastic coUapse ... 

Any of the four monomer residues can be arranged in a polymer chain in either head-to-head, head-to-tail, or tail-to-tail configurations. Each of the two head-to-tail vinyl forms can exist as syndiotactic or isotactic stmctures because of the presence of an asymmetric carbon atom (marked with an asterisk) in the monomer unit. Of course, the random mix of syndiotactic and isotactic, ie, atactic stmctures also exists. Of these possible stmctures, only... 

Fig. 12. Structures of the mycotrienins, mycotrienols, trienomycins, and ansatrienins where represents configuration of the asymmetric center.

These reactions can be cataly2ed by bases, eg, pyridine, or by Lewis acids, eg, 2inc chloride. In the case of asymmetric alcohols, steric control, ie, inversion, racemi2ation, or retention of configuration at the reaction site, can be achieved by the choice of reaction conditions (173,174). Some alcohols dehydrate to olefins when treated with thionyl chloride and pyridine. 

In the esterification of organic acids with alcohols, it has been shown that in most cases under acid catalysis, the union is between acyl and alkoxy groups. Acid hydrolysis of acetoxysuccinic acid gives malic acid with retention of configuration at the asymmetric carbon atom (11) ... 

Individual compound names are derived from parent names in the usual way by specifying the degree of hydrogenation (with -ene, -yne, hydro- and dehydro-) e.g. 84 and 85) and the substituents (with appropriate prefixes and suffixes). However, there are other ways in which parent names can be modified. Changes in stereochemistry can be indicated by use of the prefix ent- (meaning a reversal in configuration of all asymmetric centres) or by... 

If a molecule contains several asymmetric C atoms, then the diastereomers show diastereotopic shifts. Clionasterol (28a) and sitosterol (28b) for example, are two steroids that differ only in the absolute configuration at one carbon atom, C-24 Differing shifts of C nuclei close to this asymmetric C atom in 28a and b identify the two diastereomers including the absolute configuration of C-24 in both. The absolute configurations of carboxylic acids in pyrrolizidine ester alkaloids are also reflected in diastereotopic H and C shifts which is used in solving problem 54. 

The presence of asymmetric C atoms in a molecule may, of course, be indicated by diastereotopic shifts and absolute configurations may, as already shown, be determined empirically by comparison of diastereotopic shifts However, enantiomers are not differentiated in the NMR spectrum. The spectrum gives no indication as to whether a chiral compound exists in a racemic form or as a pure enantiomer. 

There are a number of important kinds of stereogenic centers besides asymmetric carbon atoms. One example is furnished by sulfoxides with nonidentical substituents on sulfur. Sulfoxides are pyramidal and maintain dieir configuration at room temperature. Unsymmetrical sulfoxides are therefore chiral and exist as enantiomers. Sulfonium salts with three nonidentical ligands are also chiral as a result of their pyramidal shape. Some examples of chiral derivatives of sulfur are given in Scheme 2.1. 

Although unsynunetrically substituted amines are chiral, the configuration is not stable because of rapid inversion at nitrogen. The activation energy for pyramidal inversion at phosphorus is much higher than at nitrogen, and many optically active phosphines have been prepared. The barrier to inversion is usually in the range of 30-3S kcal/mol so that enantiomerically pure phosphines are stable at room temperature but racemize by inversion at elevated tempeiatuies. Asymmetrically substituted tetracoordinate phosphorus compounds such as phosphonium salts and phosphine oxides are also chiral. Scheme 2.1 includes some examples of chiral phosphorus compounds. 

The addition of methylmagnesium iodide to 2-phenylpropanal is stereoselective in producing twice as much syn-3-phenyl-2-butanol as the anti isomer (entry 5). The stereoselective formation of a particular configuration at a new stereogenic center in a reaction of a chiral reactant is called asymmetric induction. This particular case is one in which the stereochemistry can be predicted on the basis of an empirical correlation called Cram s rule. The structural and mechanistic basis of Cramls rule will be discussed in Chapter 3. 

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