Stereocenters stereogenic

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. 

In this chapter we describe the unified generation of stereoisomers including conform-ers of a molecular structure [102,103,105]. This method has the potential to generate stereoisomers that cannot be described in terms of stereocenters, stereogenic double bonds or single bond rotations. Fundamentals such as the concept of a (partial) orientation function are discussed, and mathematical tools such as Radon partitions and binary Grassmann-Plucker relations are used to construct tests for abstract orientation functions. Some simple examples are treated in detail. 

Fig. 4.1. Three chiral compounds whose stereoisomerism cannot be described in terms of stereocenters, stereogenic double bonds, or rotatable single bonds.

Usnic acid contains a single stereocenter (stereogenic center, or chiral center) and, therefore, has the possibility of existing as an enantiomeric pair of stereoisomers. Generally, in a given lichen, only one of fhe stereoisomers R or S) is present. Usnic acid has a very high specific rotation ( 460°) which makes it an ideal candidate for optical rotation measurements at the microscale level. 

In 1996, the lUPAC recommended that a tetrahedral carbon bearing four different groups be called a chirality center. Many other names in common use include chiral center, stereocenter, stereogenic center, and asymmetric center. For the remainder of our discussion, we will use the lUPAC-recommended term. Below are several examples of chiraUty centers ... 

The most common, although not the only, cause of chirality in an organic molecule is the presence of a carbon atom bonded to four different groups—for example, the central carbon atom in lactic acid. Such carbons are now referred to as chirality centers, although other terms such as stereocenter asymmetric center, and stereogenic center have also been used formerly. Note that chirality is a property of the entire molecule, whereas a chirality center is the cause of chirality. 

PGF2a- The cyclopentane ring of the Corey lactone (9) is the host of four contiguous stereogenic centers. Retrosynthetic simplification of 9 provides 10, a construct which is more complex than 9 Nevertheless, intermediate 10 possesses structural features that satisfy the requirement for the iodolactonization transform. The iodolactone in 10 constitutes the retron for the iodolactonization transform.11 Cleavage of the indicated bonds in 10 sacrifices two of the five stereocenters and provides unsaturated carboxylic acid... 

While the chemistry of alkyl and allylic sulfoxide anions is similar to that of phosphine oxides, phosphinates and sulfone stabilized anions (Sections 1.5.2.2.1 -2), the situation is further complicated by the additional stereogenic center at sulfur. Therefore in all cases, asymmetric induction may arise from the stereocenter at sulfur. 

The alkylation of an enolate creates a new stereogenic center when the a-substituents are nonidentical. In enantioselective synthesis, it is necessary to control the direction of approach and thus the configuration of the new stereocenter. 

In the discussion of the stereochemistry of aldol and Mukaiyama reactions, the most important factors in determining the syn or anti diastereoselectivity were identified as the nature of the TS (cyclic, open, or chelated) and the configuration (E or Z) of the enolate. If either the aldehyde or enolate is chiral, an additional factor enters the picture. The aldehyde or enolate then has two nonidentical faces and the stereochemical outcome will depend on facial selectivity. In principle, this applies to any stereocenter in the molecule, but the strongest and most studied effects are those of a- and (3-substituents. If the aldehyde is chiral, particularly when the stereogenic center is adjacent to the carbonyl group, the competition between the two diastereotopic faces of the carbonyl group determines the stereochemical outcome of the reaction. 

Stereochemical Control by the Enolate or Enolate Equivalent. The facial selectivity of aldol addition reactions can also be controlled by stereogenic centers in the nucleophile. A stereocenter can be located at any of the adjacent positions on an enolate or enolate equivalent. The configuration of the substituent can influence the direction of approach of the aldehyde. 

Owing to the concerted mechanism, chirality at C(3) [or C(4)] leads to enantiospecific formation of new stereogenic centers formed at C(l) [or C(6)].203 These relationships are illustrated in the example below. Both the configuration of the new stereocenter and the new double bond are those expected on the basis of a chairlike TS. Since there are two stereogenic centers, the double bond and the asymmetric carbon, there are four possible stereoisomers of the product. Only two are formed. The Zs-double bond isomer has the 5-con figuration at C(4) and the Z-isomer has the -configuration. These are the products expected for a chair TS. The stereochemistry of the new double bond is determined by the relative stability of the two chair TSs. TS B is less favorable than A because of the axial placement of the larger phenyl substituent. 

A tetrahedral atom with four different groups attached to it is a stereocenter (chiral center, stereogenic center)... 

Another alkyl-bridged PHOX (18, Fig. 29.6) was recently synthesized [17], and used to hydrogenate a series of substituted methylstilbenes in 75-95% ee, and /1-melhylcinnamic esters in 80-99% ee. The hydrogenation results suggest that the selectivity of these catalysts is mainly derived from the substitution at the stereogenic center on the oxazoline ring, with the other stereocenter having a relatively minor effect on the ee-value. 

The essence of asymmetric synthesis is producing a new stereogenic center in such a manner that the product consists of stereoisomers in unequal amount. In most cases, this can be achieved by the formation of a new sp3 stereocenter. There is also another type of asymmetric reaction in which the employed substrates contain either a stereogenic unit or a pro-stereogenic unit apart from the functional group, and asymmetric synthesis occurs even though the nature of the reaction is not directly related to the newly formed sp3 stereocenter. The Wittig reaction is invoked for the asymmetric synthesis of such molecules.47... 

Different types of the reagents (see Fig. 8-4) have been applied in asymmetric Wittig-type reactions. Because no new sp3 stereocenter is formed in a Witting-type reaction, a substrate containing a stereogenic or pro-stereogenic unit apart from the carbonyl group is usually required to induce an asymmetric process. 

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