Centre of chirality

Nitrogen chirality may also be produced by the action of an achiral peroxyacid on a Schiff base containing a chiral amine (75JOC3878). In this case the oxaziridine contains a configurationally known centre of chirality relative to this, absolute configurations of the centres of chirality at nitrogen and carbon, and thus the complete absolute configuration of the molecule, can be determined (see Section 5.08.2.2). 

The new centre of chirality generated by hemiacetal ring closure is called the anomeric centre. The two stereoisomers are referred to as anomers, designated a or (i according to the configurational relationship between the anomeric centre and a specified anomeric reference atom. 

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. 

When isotopic substitution creates a centre of chirality, configuration is defined as for other types of substitution (see 2-Carb-16.1 to 2-Carb-16.4). 

Bohman and Allenmark resolved a series of sulphoxide derivatives of unsaturated malonic acids of the general structure 228. The classical method of resolution via formation of diastereoisomeric salts with cinchonine and quinine has also been used by Kapovits and coworkers " to resolve sulphoxides 229, 230, 231 and 232 which are precursors of chiral sulphuranes. Miko/ajczyk and his coworkers achieved optical resolution of sulphoxide 233 by utilizing the phosphonic acid moiety for salt formation with quinine. The racemic sulphinylacetic acid 234, which has a second centre of chirality on the a-carbon atom, was resolved into pure diastereoisomers by Holmberg. Racemic 2-hydroxy- and 4-hydroxyphenyl alkyl sulphoxides were separated via the diastereoisomeric 2- or 4-(tetra-0-acetyl-D-glucopyranosyloxy)phenyl alkyl sulphoxides 235. The optically active sulphoxides were recovered from the isolated diastereoisomers 235 by deacetylation with base and cleavage of the acetal. Racemic 1,3-dithian-l-oxide 236... 

Diastereoisomers Stereoisomers that are not enantiomers. Diastereoisomers are compounds that always contain at least two centres of chirality. 

Having identified 6 centres of chirality, i.e. 4 due to the side-chains and 2 due to the atropisomerism, a total of 64 optical isomers are possible within each of the rotamers Gl, G2, G3. However, this number is reduced to 16 configurations because of identities which can be visualized as follows The Fischer projections of the iotrolan molecule are split at the bridge into two halves to give the... 

Since the benzofurane furan ring is a common structural motif in natural products it s not surprising that the intramolecular Heck reaction has also been employed towards that goal. Such variants that introduce a new centre of chirality into the product are of particular importance. 

The formation of chromane derivatives has also been realised in the palladium catalyzed intramolecular nucleophilic substitution of allyl carbonates (Tsuji-Trost reaction). In most cases the reaction is accompanied by the formation of a new centre of chirality. Using Trost s chiral ligand the ring closure was carried out in an enantioselective manner. The asymmetric allylation of the phenol derivative shown in 4.20. was achieved both in good yield and with excellent selectivity.23... 

As mentioned previously, the partially reduced forms of five membered heteroaromatic systems might act as olefins in insertion reactions. This behaviour is characteristic particularly of dihydrofuranes. The olefin insertion and the following / hydride elimination should in principle lead to a trisubstituted olefin, which is rarely observed, however. Typical products of this reaction are 2-aryl-2,3-dihydrofuranes. A characteristic example of such a reaction is presented in 6.54. The coupling of 4-iodoanisole and dihydrofurane led to the formation of the chiral 2-anisyl-2,3-dihydrofurane in excellent yield.83 The shift of the double bond, which leads to the creation of a new centre of chirality in the molecule, opens up the way for enantioselective transformations. Both intermolecular and intramolecular variants of the asymmetric Heck reaction have been studied extensively.84... 

The configuration of a group of consecutive asymmetric carbon atoms (such as >CHOH) containing one to four centres of chirality is designated by one of the configurational prefixes shown in Table 5.1. 

The rules as they apply to compounds with centres of chirality may be summarised as follows ... 

RS)- and SR)- In a one-centre compound RS) means the racemate, equivalent to ( ). In a compound with two or more centres of chirality, RS and SR are used to define the relative configurations of the centres in racemic diastereomers, e.g., ( )-threitol = 2RS,3RS)-1,2,3,4-butanetetrol, erythritol = (2RS,3S / )-l,2,3,4-butanetetrol. Priority is given to RS)-for the lowest-numbered centre. (CAS uses R and 5 together with the ( )- identifier to show that a racemate is meant.)... 

The platinum(IV), rhodium(III), and iridium(III) sepulchrates, dinitrosarcophaginates, and diaminosarcophaginates have been synthesized in high yields (45-65% for Pt(IV), 40% for Ir(III), and 90-100% for Rh(III)) starting from their tris-ethylenediaminates [94, 156, 157]. The rhodium (III) and iridium(III) complexes were prepared in a similar manner to that for cobalt (III) complexes, except of the elevated temperatures (Rh, 60°C Ir, 90°C) required for the quoted yields. Moreover, if chiral [Rh(en)3] cation was used initially, clathrochelate complexes were obtained in ca 100% chemical and chiral yields, despite the seven centres of chirality [157]. 

Figure 2. Chemical structure of (R)/(S)-dimethindene. The asterisk denotes the centre of chirality.

Some metallodendrimers with one or more stereogenic centers have been prepared without control of the chirality. Vogtle and Balzani [74] have tried several strategies to prepare dendrimers in which a ruthenium cation is the core of the final compound. In these compounds, the only centre of chirality is that of the metal, but as it was not controlled racemic mixtures were obtained. Controlling the stereochemistry of the starting complex would have allowed the authors to prepare a optically pure metallodendrimer. Denti, Campagna, Balzani, and their co-workers have studied polymetallic dendrimers based on bipyridine and 2,3- 7s -(2-pyridyl)pyrazine (2,3-... 

Several chiral pseudo-tetrahedral complexes of titanium have been reported this year. They have titanium as the sole centre of chirality or possess additional chiral... 

Carbonyl-stabilized phosphorus ylides are less nucleophilic and hence do not react with (phosphine)gold(I) halides, but their gold(I) complexes can be generated from precursors such as [Au(acac)(PPh3)] or [AuCl(SC4Hg)] by reaction with phosphonium salts and ylides, respectively, and again both mono- and bis(yhde) complexes have been obtained (equation 48 and Scheme 8)206 207. The ylide carbon atoms in these complexes are centres of chirality, but no stereospecificity was observed in the coordination process and racemic mixtures are formed throughout. 

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