Stereogenic center chirality without

Although four is the maximum possible number of isomers when the compound has two stereogenic centers (chiral compounds without a chiral carbon, or with one chiral carbon and another type of stereogenic center, also follow the rules described... 

On the other hand, acyclic diastereomers are not necessarily compounds without any cyclic structural element, but the fragment connecting the stereogenic centers is acyclic. For example, a bicyclo[2.2.1]heptane derivative with a chiral side-chain attached to a nonbridgehead carbon belongs to this second category of acyclic diastereomers. 

In other cases, the oxidation reaction may not be asymmetric, but stereogenic centers within the substrate are preserved in the product allowing for an asymmetric reaction. An example of this type of reaction is provided by ozonolysis, which is discussed in Chapter 11. The use of ozone also overcomes one of the major problems that has been associated with oxidations at scale—the use of toxic, heavy metals their separation from the reaction product and waste disposal. However, there are still some useful reactions that use metals without chiral ligands and provide stereodifferentiation. An example is provided by the manganese oxide oxidation of ferrocenyl amino alcohols (Scheme 9.2).14... 

AUyl Sulfoxide Rearrangements of Compounds Without a Stereogenic Center at C-l (Chirality Transfer from Sulfur)... 

Chiral allylic sulfoxides without a stereogenic center at C-l were obtained by the following methods ... 

Chirality Transfer from Selenium Without an Additional Stereogenic Center... 

Enantiomeric cyclic amides and imides, esters, ketones, 8- and 7-lactones, and alkyl-substituted indenes can be resolved on CTA-I without derivatization. Chiral alcohols can also be resolved without deiivatiza-tion, but the best results are obtained when they are converted to esters. In general, para-nitrobenzoic esters are recommended if the hydroxyl group is directly attached to the stereogenic center benzoate or para-biomobenzoate... 

The aminofiinction is then introduced by a selective organocatalyzed a-addition of this aldehyde to an azodicarboxylate moiety. In this way, the quaternary stereogenic center of an a-aminoaldehyde is built up catalytically without the need of any chiral auxiliary. 

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. 

Chemoinformatics is characterized by the use of large amounts of information. Specific ways to represent the molecules, and to organize and analyze the data have been and continue to be developed. There are different ways to represent the molecules, and they can be classified according to the information that they encode. The most basic level corresponds to representations that depend on or are associated with one dimensional (ID) representation, such as molecular weight. The next level corresponds to the 2D representations associated with the connectivity of the molecules without the consideration of the stereochemistry. In 3D methods the incorporation of stereochemistry conveys not only the specification of the chirality of stereogenic centers but also the possible conformation or conformations. In this regard, the research fields dedicated to conformational analysis have an important impact in reactivity prediction, molecular design, and stability. Examples of ID, 2D, and... 

Kiyooka et al. have reported that stoichiometric use of chiral oxazaborolidines (e.g. (S)-47), derived from sulfonamides of a-amino acids and borane, is highly effective in enantioselective aldol reactions of ketene TMS acetals such as 48 and 49 (Scheme 10.39) [117]. The use of TMS enolate 49 achieves highly enantioselective synthesis of dithiolane aldols, which can be readily converted into acetate aldols without epimerization. The chiral borane 47-promoted aldol reaction proceeds with high levels of reagent-control (Scheme 10.40) [118] - the absolute configuration of a newly formed stereogenic center depends on that of the promoter used and not that of the substrate. 

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-... 

I.4.C. Chiral Molecules without a Stereogenic Center (Molecules Containing a Chiral Axis)... 

A few classes of organic molecules have a chiral axis although they do not have a stereogenic center.34 The mirror image of such a molecule is not superimposable so it is possible to have enantiomers without the... 

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