Asymmetric centre

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. 

Conceptually the most simple syntheses of complex molecules involve the joining of structural units in which all functional groups and all asymmetric centres are preformed. This technique can usually only be applied to compounds in which these units are connected by —C—X— bonds rather than C—C. It is illustrated here by the standard syntheses of oligonucleotides, peptides, and polydentate macrocyclic ligands. 

Recent syntheses of steroids apply efficient strategies in which open-chain or monocyclic educts with appropiate side-chains are stereoselectively cyclized in one step to a tri- or tetracyclic steroid precursor. These procedures mimic the biochemical synthesis scheme where acyclic, achiral squalene is first oxidized to a 2,3-epoxide containing one chiral carbon atom and then enzymatically cyclized to lanostetol with no less than seven asymmetric centres (W.S. Johnson, 1%8, 1976 E.E. van Tamden, 1968). 

Non-enzymatic cyclizations of educts containing chiral centres can lead to products with additional "asymmetric centres. The underlying effect is called "asymmetric induction . Its systematic exploration in steroid syntheses started when G. Saucy discovered in 1971 that a chiral carbon atom in a cyclic educt induces a stereoselective Torgov condensation several carbon atoms away (M. Rosenberger, 1971, 1972). 

Proton-catalyzed olefin cyclizations of open-chain educts may give tri- or tetracyclic products but low yields are typical (E.E. van Tamelen, 1968, 1977 see p. 91). More useful are cyclizations of monocyclic educts with appropriate side-chains. The chiral centre to which the chain is attached may direct the steric course of the cyclization, and several asymmetric centres may be formed stereoselectively since the cyclizations usually lead to traas-fused rings. 

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... 

As Z-lelobanidine II also yields Z-lelobanine on oxidation the difference between the I and II forms must be stereochemical and lie in one of the side-chains, in spite of the quantitative identity of their specific rotations. The four asymmetric centres might have the following individual directional effects I. I. d. d. and d. 1. d. 1. in the two forms, but the total effect might be identical. 

The last six items in the table relate to ruban and rubanol. Ruban has one asymmetric centre (carbon atom 8 of formula III, p. 443). Ruban-... 

Use of deoxy- in combination with an established trivial name (see Charts I and II) is straightforward if the formal deoxygenation does not affect the configuration at any asymmetric centre. However if deoxy removes a centre of chirality, the resulting names contain stereochemical redundancy. In such cases, systematic names are preferred, especially for the naming of derivatives. 

The systematic name consists of the prefix deoxy- , preceded by the locant and followed by the stem name with such configurational prefixes as necessary to describe the configuration(s) at the asymmetric centres present in the deoxy compound. Configurational prefixes are cited in order commencing at the end farthest from C-l. Deoxy is regarded as a detachable prefix, i.e. it is placed in alphabetical order with any substituent prefixes. 

Note 2. In the last four examples, new asymmetric centres have been introduced at the carbonyl carbon atom of the aldehyde or ketone that has reacted with the saccharide. When known, the stereochemistry at such a new centre is indicated by use of the appropriate R or S symbol ([13], Section E) placed in parentheses, immediately before the locants of the relevant prefix. 

Summary The chiral tetrasilane 2,3-diphenyltetrasilane 1 is formed by stepwise cleavage of Si-aryl bonds in appropriate aryltetrasilanes with HC1 under pressure and subsequent reduction with LiAlH4. Further reaction of 1 with HC1 or HBr affords the corresponding dihalo-derivatives. The diastereomers appearing because of the presence of 2 asymmetric centres can easily be distinguished by NMR-experiments. 

Figure LI. Some of the products that can form from an alkene, carbon momoxide, hydrogen and methanol. The asterisks represent asymmetric centres in chiral molecules...

The CD spectra of polymers of a series of homologue chiral terminal acetylenes48 shows a marked dependence on the distance of the asymmetric centre from the triple bond. The relation between the two facts is however unclear, even because the UV spectrum results from the superposition of several bands, owing to the extended conjugation. 

The tetracyclic alcohol 179 is produced by the action of boron trifluoride etherate or tin(IV) chloride on the oxirane 178 (equation 85)95. A similar cyclization of the oxirane 180 yields DL-<5-amyrin (181) (equation 86)96. In the SnCLt-catalysed ring-closure of the tetraene 182 to the all-fraws-tetracycle 183 (equation 87) seven asymmetric centres are created, yet only two of sixty-four possible racemates are formed97. It has been proposed that multiple ring-closures of this kind form the basis of the biosynthesis of steroids and tetra-and pentacyclic triterpenoids, the Stork-Eschenmoser hypothesis 98,99. Such biomimetic polyene cyclizations, e.g. the formation of lanosterol from squalene (equation 88), have been reviewed69,70. 

The number of spatial isomers increases with increasing number of asymmetric carbon centres with the addition of each new asymmetric centre, the number of isomers doubles. The trihydroxy glutaric acid molecule has three identical asymmetric carbon atoms. 

In sugars the designations D or L refer to the configuration of the bottom asymmetric centre. 

Hi) A priority sequence is determined for substituents at the asymmetric centre. 

The prefix (RS) is used to denote a racemic modification. For example, (RS)-Sec butyl chloride. The symbols R and S are applied to compounds whose absolute stereochemistry has been determined. However, while applying the nomenclature to projection formulae of compounds containing several asymmetric centres Cahn, Ingold. Pielog procedures are supplemented by the following conversion rule. 

By carrying out chemical reactions without disturbing the asymmetric centre... 

In these conversions it is a rule that the bonds about the asymmetric centre remain intact. 

Reactions which involve displacements, at asymmetric centres... 

The asymmetric reagent combines with the molecule and a new asymmetric centre is created. Diastereomers are always first formed which have different energy reserves and so they are formed with different rates. 

The formation of the diastereomer is possible only if the compound to be resolved has a chemically active group which can react with the active resolving agent. Then the bonds of the asymmetric centre must remain intact during the reaction so that the possibility of racemisation is reduced to the minimum. 

The character of ORD curves depends on the structure, configuration and conformation of optically active substances and also on the nature of choromophores present and their position relative to asymmetric centre. In many cases the curves depend on solvent and temperature. This is why spectropolarimetry becomes an important physico-chemical method (for investigating organic compounds). By introducing an optically active radial into organic compounds that do not possess optical activity, it is possible to extend the range of investigation by spectropolarimetric methods. 

We should also be familiar with the meaning of the term conformational asymmetry. We know that different conformations of the same compound have different symmetry and different statistical contribution (i.e., their percentage content is different). Therefore, the total effect on the polarization of light depends on the arrangement of atoms in different conformations and also on the statistical contribution of each conformation. This is called conformational asymmetry. The compound CH3-CH2-CH (CH3)C1 has conformational asymmetry because two identical atoms (c) are situated at the asymmetric centre. This compound has three staggered conformations. 

Figure 9.1 Enantiomers of glyceraldehyde. Enantiomers are mirror images of each other and are chemically similar but will cause a beam of plane polarized light to rotate in opposite directions. Glyceraldehyde has only one asymmetric centre ( ) and the designation of D or L is determined by the orientation of the O and OH groups about this carbon atom. For carbohydrates with more than one asymmetric carbon atom, the prefix d or l refers only to the configuration about the highest numbered asymmetric carbon atom, although in such enantiomers the configuration about all the asymmetric centres will also be reversed.

Figure 9.2 Dihydroxyacetone and enantiomers of erythrulose. The triose, dihydroxyacetone, is the parent compound of the ketoses because it has the lowest number of carbon atoms but it does not contain an asymmetric centre. Therefore the d and l series of the ketoses are built up from the two enantiomers of the tetrose, erythrulose, which has one asymmetric carbon atom ( ).

The presence of an asymmetric carbon atom confers the property of optical activity on the molecule, enabling it to cause the rotation of a beam of plane polarized light in either a clockwise or an anticlockwise direction. Thus all naturally occurring carbohydrates containing asymmetric carbon atoms are optically active and can be designated (+) for clockwise (dextro) rotation or (-) for anticlockwise (laevo) rotation. The designation of d or l to glyceraldehyde, the simplest monosaccharide, which has only one asymmetric centre, refers to... 

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