Stereogenic carbon atom

Although the ion pairs of a-substituted benzyl anions and the corresponding cations are chiral species, which, in addition, often bear a pyramidal and hence stereogenic carbon atom, in most cases rapid racemization of the alkali and alkaline earth metal derivatives occurs in solution... 

The strategy described here explains the different possibilities of enzymatic ammonolysis and aminolysis reaction for resolution of esters or preparation of enantiomerically pure amides, which are important synthons in organic chemistry. This methodology has been also applied for the synthesis of pyrrolidinol derivatives that can be prepared via enzymatic ammonolysis of a polyfunctional ester, such as ethyl ( )-4-chloro-3-hydroxybutanoate [30]. In addition, it is possible in the resolution of chiral axe instead of a stereogenic carbon atom. An interesting enzymatic aminolysis of this class of reaction has been recently reported by Aoyagi et al. [31[. The side chain of binaphthyl moiety plays an important role in the enantiodis-crimination of the process (Scheme 7.14). 

A molecule that contains just one chiral carbon atom (defined as a carbon atom connected to four different groups also called an asymmetric or stereogenic carbon atom) is always chiral, and hence optically active. As seen in Figure 4.1, such a molecule cannot have a plane of symmetry, whatever the identity of W, X, Y, and Z, as long as they are all different. However, the presence of a chiral carbon is neither a necessary nor a sufficient condition for optical activity, since optical activity may be present in molecules with no chiral atom and since some molecules with two or more chiral carbon atoms are superimposable on their mirror images, and hence inactive. Examples of such compounds will be discussed subsequently. 

Stereoinversion Stereoinversion can be achieved either using a chemoenzymatic approach or a purely biocatalytic method. As an example of the former case, deracemization of secondary alcohols via enzymatic hydrolysis of their acetates may be mentioned. Thus, after the first step, kinetic resolution of a racemate, the enantiomeric alcohol resulting from hydrolysis of the fast reacting enantiomer of the substrate is chemically transformed into an activated ester, for example, by mesylation. The mixture of both esters is then subjected to basic hydrolysis. Each hydrolysis proceeds with different stereochemistry - the acetate is hydrolyzed with retention of configuration due to the attack of the hydroxy anion on the carbonyl carbon, and the mesylate - with inversion as a result of the attack of the hydroxy anion on the stereogenic carbon atom. As a result, a single enantiomer of the secondary alcohol is obtained (Scheme 5.12) [8, 50a]. 

In the compounds discussed above, the sulfur atom was attached directly to the stereogenic carbon atom. However, some structures that have remote sulfur... 

Enantioselective deprotonation can also be successfully extended to 4,4-disubstituted cyclohexanones. 4-Methyl-4-phenylcyclohexanone (3) gives, upon reaction with various chiral lithium amides in THF under internal quenching with chlorotrimethylsilane, the silyl enol ether 4 having a quaternary stereogenic carbon atom. Not surprisingly, enantioselectivities are lower than in the case of 4-tm-butylcyclohexanone. Oxidation of 4 with palladium acetate furnishes the a./i-unsaturated ketone 5 whose ee value can be determined by HPLC using the chiral column Chiralcel OJ (Diacel Chemical Industries, Ltd.)59c... 

Pastor and Togni pointed out that the central chirality and the planar chirality in the ferrocenylphosphine ligand 2 are cooperative for stereoselection (the concept of internal cooperativity of chirality) [16,23,24]. As Table 8B1.7 shows, the change of chirality of the stereogenic carbon atom from R to, S results in the formation of the other trans-oxazoline enantiomer with moderate enantiomeric excess. 

A wide range of fluorinated compounds are applied as pharmaceuticals and agrochemicals. Several stereoselective methods are used for synthesis of optically active molecules bearing a C-F bond at the stereogenic carbon atom [72, 73]. These are mainly based on diastereoselective fluorination of chiral molecules or enantioselective alkylation of fluoroorganic compounds. Asymmetric introduction of a fluorine... 

Know the meaning of stereogenic carbon atom, stereogenic center, R-S convention, priority order, E-Z convention, Fischer projection. 

In each case the stereogenic carbon atom is marked with an asterisk. 

It should also be mentioned that atactic polymers may be formed from monodeuteropropylene and pentadeuteropropylene, similarly to the case of the polymerisation of non-deuterated propylene. However, atactic structures with respect to one stereogenic tertiary carbon atom, with the other possibly arranged to form tactic polymer sequences, are not known. Also, ditactic polymers, which might be characterised by the appearance of two distinct series of tacticity as regards both stereogenic tertiary carbon atoms, i.e. the isotactic with respect to one stereogenic carbon atom and syndiotactic with respect to the other one, and vice versa, are not known. 

As with fulgides, the photochromism of diarylethenes is also bas bir-electrocyclization. Therefore the helical conformation of the open for transferred to two newly formed stereogenic carbon atoms of the cyclohexa moiety by UV irradiation (Scheme 5). 

A diastereoselective photochromism of a bisbenzothienylethene was reported by Yokoyama et al. By employing A1,3-strain around the double bond of one of the benzothiophene of D-9, the conformation of the stereogenic carbon atom with regard to the hexafluorocyclopentene ring was fixed. As a result, the second benzothiophene lay closer to the medium-sized substituent so that it would generate one diastereomer predominantly by photoirradiation. Indeed, the diastereosel-ectivity of photocyclization of D-9 in toluene at room temperature was 88% de at 85% conversion [41]. 

An azobenzene-modified oligonucleotide 5 -AAAXAAAA-3 [15, X is the residue having an azobenzene moiety in the side chain (Fig. 4A)] was prepared and further separated into two diastereomers (15a and 15b) based on the chirality of the stereogenic carbon atom of 15 by re versed-phase HPLC [53]. The melting temperature (Tm) of the duplex of each diastereomer of 15 with its complementary oligonucleotide counterpart (5 -TTTTTTTT-3 ) was photoregulated by the trans-... 

Although sulfur forms a variety of structures differing in the number of ligands and valency, the overwhelming majority of stereoselective C-S bond forming reactions are limited to those which afford chiral carbon compounds with the sulfenyl, sulfinyl and sulfonyl moieties bonded to the stereogenic carbon atom. In accord with this, discussion of stereoselective C—S bond formation will be divided into three corresponding subsections. Procedures with stereoselective C—S bond formation where the sulfur is in a less common form are also presented. 

The thermal rearrangement of allylic xanthates to allylic dithiocarbonates is a well-known reaction which results in the creation of a new stereogenic carbon center when the starting xanthate contains an unsymmetrically substituted C-C double bond. Moreover, in the case of chiral xanthates this rearrangement is accompanied by the transfer of chirality from the stereogenic carbon atom in the substrate to the newly created stcrcogcnic center in the product. 

A-Sulfinyl amides function as dienophiles in the Diels Alder reaction and with suitably substituted dienes yield 3,6-dihydrothiazine 1-oxides containing two stereogenic carbon atoms bonded to the sulfinyl sulfur and nitrogen93 recent studies have shown that the reaction occurs in a stepwise fashion via dipolar intermediates94. 

A molecule that contains just one chiral (stereogenic) carbon atom (defined as a carbon atom connected to four different groups also called an asymmetric carbon atom) is always chiral, and hence optically active. As seen in Fig. 4.1, such a... 

Compounds with a Stereogenic Carbon Atom. If there is only one such atom, the molecule must be optically active. This is so no matter how slight the differences are among the four groups. For example, optical activity is present in... 

The tetrahedron incorporating the stereogenic carbon atom should be placed on the paper with its edge vertical at the back and the edge in front horizontal (Figure 2.3a). 

A stereogenic carbon atom (chiral center, chiral atom, asymmetric atom) is bound to four unlike groups and thus generates chirality. Note that a molecule may possess a molecular chirality without having a stereogenic center. 

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