Stereochemistry asymmetric carbons

The presence of asymmetric carbon atoms also confers optical activity on the compound. When a beam of plane-polarized hght is passed through a solution of an optical isomer, it will be rotated either to the right, dextrorotatory (+) or to the left, levorotatory (—). The direction of rotation is independent of the stereochemistry of the sugar, so it may be designated d(—), d(+), l(—), or l(+). For example, the naturally occurring form of fructose is the d(—) isomer. 

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. 

J. Jacques, C. Gros and S. Brourcier, in Stereochemistry, Vol. 4, Absolute Configurations of 6000 Selected Compounds with One Asymmetric Carbon Atom (Ed. H. B. Kagen), Georg Thieme, Stuttgart, 1977. 

The dissection of a molecular model into those components that are deemed to be essential for the understanding of the stereochemistry of the whole may be termed factorization (9). The first and most important step toward this goal was taken by van t Hoff and Le Bel when they introduced the concept of the asymmetric carbon atom (10a, 1 la) and discussed the achiral stereoisomerism of the olefins (10b,lib). We need such factorization not only for the enumeration and description of possible stereoisomers, important as these objectives are, but also, as we have seen, for the understanding of stereoselective reactions. More subtle differences also giving rise to differences in reactivity with chiral reagents, but referable to products of a different factorization, will be taken up in Sect. IX. 

There s a whole area of chemistry dealing with the spatial configurations of organic molecules called stereochemistry. To get into this area, you have to have molecules that have an asymmetrical carbon atom. That s one that has four dissimilar atoms or groups attached to it. PP has that condition on a repeating basis—the methyl groups on every ocher backbone carbon. Such a polymer can be stereoregular or stereospecific. 

A few examples are chosen in order to illustrate the potentialities of this remarkable methodology. In Reaction (6.6) the sequence is initiated by the removal of the PhSe group and the formation of a carbamoyl radical. It is worth mentioning that the stereochemical outcome of these cascade reactions is controlled by the stereochemistry of the oxygen-bearing asymmetric carbon in 29. Indeed, Reactions (6.6) and (6.7) show clearly the stereochemical control. On the other hand, Reactions (6.7) and (6.8) illustrate the role of R which is carried as a terminal group in the acetylenic moiety. For R = Ph the last step is the hydrogen abstraction, whereas for R = SnBus, the last step is the ejection of BusSn radical (cf. Scheme 6.7). 

SCHEME 16. The observed stereochemistry of the reported reactions by the use of (a) chiral ketones and (b) Grignard reagents in Grignard reactions. Asymmetric carbon atoms are denoted by i or 5... 

Pyrrolizidine derivatives with at least one substituent, and particularly the pyrrolizidine alkaloid components, have one or more asymmetric carbon atoms. The stereochemistry of pyrrolizidine was clarified for the most part in the course of investigation of the naturally occurring pyrrolizidine alcohols. Here, the problems of relative and absolute configuration and of stereoisomeric transformations will be considered. 

Oxidative Addition of Alkyl Halides to Palladium(0). The stereochemistry of the oxidative addition (31) of alkyl halides to the transition metals of group VIII can provide information as to which of the many possible mechanisms are operative. The addition of alkyl halides to d8-iridium complexes has been reported to proceed with retention (32), inversion (33), and racemization (34, 35) via a free radical mechanism at the asymmetric carbon center. The kinetics of this reaction are consistent with nucleophilic displacement by iridium on carbon (36). Oxi-... 

The 3-(acetylsulfanyl)alkanoic acids 7 are obtained as a mixture of stereoisomers that are not separated in subsequent synthetic steps. In most cases, these sulfanylalkanoyl compounds are obtained as a mixture of four stereoisomers. The stereochemistry of one derivative [8, R1 = Bzl R2= iBu R3= Me) has been determined using HPLC and NMR data.1141 The ratio of the isomers is 3.5 3.5 1 1 for 8a/8b/8c/8d, respectively. This indicates that one set of enantiomers of the precursor 7, which is obtained by addition of thiolacetic acid to 6, is formed in a greater proportion than the other set of enantiomers. Therefore, it was concluded that isomers 8a and 8b are obtained from the major set of enantiomers of 7, while 8c and 8d are formed from the minor set of enantiomers. The NMR spectra of the separated isomers, 8a/8b/8c/8d, was compared to the spectrum of Phe-Ala. This allowed determination of the relative configuration of the benzyl group at the C5 asymmetric carbon of the 8 related to the adjacent Ala residue. The configuration at C5 of all four stereoisomers is shown in Scheme 2. The configuration at C6 has not been determined. 

Optically active aldehydes can be obtained by asymmetric hydroformylation of olefinic substrates when at least one asymmetric carbon atom is formed either by addition of a formyl group or of a hydrogen atom to an unsaturated carbon atom (Scheme 1, reactions (1) and (2)). In the case of trisubstituted olefins, two new asymmetric carbon atoms can form due to the cis stereochemistry of the reaction10), in the absence of isomerization, the formation of only one epimer is expected. 

This is a property of the arrangement in space of the atoms in a compound, i.e. their stereochemistry. A carbon atom with four different groups attached to it by single bonds is said to be asymmetric. This was previously described under isomerism in Chapter 2. 

For nucleophilic substitution at tetrahedral carbon, the relationships between stereochemistry and mechanism are well understood. Direct displacement on a carbon turns the molecule inside out (inversion of configuration) if the site of reaction is an asymmetric carbon, a molecule in the D series will be converted to one in the L series (or vice versa). Substitution by the dissociation mechanism at an asymmetric carbon... 

Summary Fischer Projections andTheir Use 201 Diastereomers 201 Summary Types of Isomers 203 5-12 Stereochemistry of Molecules withTwo or More Asymmetric Carbons 204 5-13 Meso Compounds 205 5-14 Absolute and Relative Configuration 207 5-15 Physical Properties of Diastereomers 208 5-16 Resolution of Enantiomers 209 EssentialTerms 213 Study Problems 215... 

We have been using dashed lines and wedges to indicate perspective in drawing the stereochemistry of asymmetric carbon atoms. When we draw molecules with several asymmetric carbons, perspective drawings become time-consuming and cumbersome. In addition, the complicated drawings make it difficult to see the similarities and differences in groups of stereoisomers. 

At the turn of the twentieth century, Emil Fischer was studying the stereochemistry of sugars (Chapter 23), which contain as many as seven asymmetric carbon atoms. To draw these structures in perspective would have been difficult, and to pick out minor stereochemical differences in the drawings would have been nearly impossible. Fischer developed a symbolic way of drawing asymmetric carbon atoms, allowing them to be drawn rapidly. The Fischer projection also facilitates comparison of stereoisomers, holding them in their most symmetric conformation and emphasizing any differences in stereochemistry. 

Interchanging any two groups on an asymmetric carbon (for example, those on the horizontal line) inverts its stereochemistry. 

Stereochemistry of Molecules with Two or More Asymmetric Carbons... 

Most of the common sugars are diastereomers of glucose. All these diastereomers have different physical properties. For example, glucose and galactose are diastere-omeric sugars that differ only in the stereochemistry of one asymmetric carbon atom, C4. 

Use Fischer projections to represent the stereochemistry of compounds with one or more asymmetric carbon atoms. 

For more than one asymmetric carbon atom, the Fischer projection represents a totally eclipsed conformation.This is not the most stable conformation, but it s usually the most symmetric conformation, which is most helpful for comparing stereochemistry. 

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