Stereocenters definition

The preparation of aldol adducts may occur with simple diastereoselectivity. A definition of the term was given in Section 11.1.3. In a slightly different formulation, simple diastereoselectivity means that a single relative configuration is established at two neighboring C atoms that become stereocenters for the following reasons (1) Both C atoms were sp2-hybridized in the reactants one was part of a nonhomotopic C=X double bond and the other was part of a nonhomotopic C=Y double bond. (2) The formation of a (7-bond between these C atoms causes them to be sp3-hybridized in the reaction product. 

The term stereocenter (stereogenic atom) is not consistently defined. The original (Mislow) definition is given here. Some sources simply define it as a synonym for an asymmetric carbon (chiral carbon) or for a chirality center. 

One source defines a meso compound as an achiral compound with stereocenters. Why is this a poor definition ... 

A stereocenter is an atom at which the interchange of two groups gives a stereoisomer. Stereocenters include both chirality centers and double-bonded carbons giving rise to cis-trans isomers. For example, the isomers of but-2-ene are achiral and they contain stereocenters (circled), so they would meet this definition. They have no chiral diastereomers, however, so they are not correctly called meso. 

In the Cope rearrangements of terminal substituted substrates, new stereogenic centers are created at positions 1 or 6. The configuration of these stereocenters is often regulated by a stereocenter which is immediately attached to the new one or by a remote stereocenter. This section discusses stereochemical aspects which concern relative asymmetric induction in Cope rearrangements. For the definition of the term relative asymmetric induction see also Section 1.6.3.1.1.5. p 3481. 

In these examples, two stereocenters are created, but only two of the four possible stereoisomers are formed. As will be explained in detail in Chapter 5, the two syn isomers are produced stereoselectively from the <7)-enolate. (See glossary, section 1.6, for definition of this term.) Obviously if the absolute configuration of either of the chiral reactants in Schemes 1.5a or 1.5b were reversed, the absolute configuration of the new stereocenters would also be reversed i.e., the enantiomers of the illustrated products would be produced in the same ratio. 

Figure 7.11. Terminology definitions for hydroboration transition structures [141] (a) The auxiliary may be either syn or anti to the alkene substituents, but anti to the substituent (R) on the nearest carbon, (b) A stereocenter attached to boron, in a staggered conformation with respect to the forming C-B bond, has substituents in anti, inside, and outside positions, (c) Definition of the Large, Medium, and Small substituents of IpcBH2.

Chapter 5 Using the Mislow and Siegel definition (/ Am. Chem. Soc. 1984, I06y 3319), I introduce the popular (but often incorrectly defined) term stereocenter and explain the differences between this term and the lUPAC terms chirality center and asymmetric carbon atom (or chiral carbon atom). The term stereocenter is much broader than the more precise term asymmetric carbon atom, and it assumes that one already knows the stereochemical properties of the molecule (to know which bonds will give rise to stereoisomers upon their interchange). Therefore, I have continued to encourage students to identify the (immediately apparent) asymmetric carbon atoms to use as tools in examining a molecule to determine its stereochemistry. 

Addition reactions of a nucleophile such as a hydride or Grignard reagent to a carbonyl adjacent to a stereocenter are stereoselective (see Section 6.9 for the definition). Many chemists have provided rationalizations for the observed products. 

Synthetically, the Diels-Alder reaction is the most important cycloaddition and arguably the most important pericyclic reaction. Because of this, we will consider several additional features of this reaction here. It is a [ 4 +, 2 ] cycloaddition, and it best illustrates a key feature of pericyclic reactions that we have yet to touch on. Since, by definition, pericyclic reactions involve a well controlled array of a toms/orbitals in the transition state, well-defined stereochemistry is a hallmark of pericyclic reactions. In general, a high degree of control of stereochemistry is associated with pericyclic reactions, and this is one of their most valuablefeatures. Eq. 15.6 illustrates this aspect of the Diels-Alder reaction. As many as four new stereocenters are created, and the control is often complete. 

Despite the lUPAC recommendation to use the term "chirality center," the terms stereocenter and stereogenic center are more commonly used. However, these terms have a broader definition a stereocenter, or stereogenic center, is defined as a location at which the interchange of two substituents will generate a stereoisomer. This definition includes chirality centers, but it also includes c/s and trans double bonds, which are not chirality centers. 

The parity coding of the structure can only be unambiguous if it is possible to reconstruct the molecule from its code. The two-valued codes like the one of Davis can easily be converted back to the configuration of a suitably numt red molecule by simply arranging the ligands in the way implied by the definition of the parity. Codes which also represent non-discriminating centers differently may have problems with structures like 32, where two stereocenters (a and b) discriminate only as a set but not individually. 

---