Meso compounds, and

Problem 5.30 For the following compounds, draw projection formulas for all stereoisomers and point out their R,S specifications, optical activity (where present), and meso compounds (a) 1,2,3,4-tetrahydroxybutane, (b) l-chloro-2,3-dibromobutane, (c) 2,4-diiodopentane, (d) 2,3,4-tribromohexane, (e) 2,3,4-tribromopentane. 

Q Identify pairs of enantiomers, diastereomers, and meso compounds, and explain how they differ in their physical and chemical properties. 

Problem 4.12 Draw stereochemical formulas for all the possible stereoisomers of the following compounds. Label pairs of enantiomers, and meso compounds. Tell which isomers, if separated from all other stereoisomers, will be optically active. Pick out several examples of diastereomers. 

Even though the Paal-Knorr pyrrole synthesis has been around for 120 years, its precise mechanism was the subject of debate. In 1991, V. Amarnath et al. investigated the intermediates of the reaction and determined the most likely mechanistic pathway. The formation of pyrroles was studied on various racemic and meso-3,4-diethyl-2,5-hexanediones. The authors found that the rate of cyclization was different for the racemic and meso compounds and the racemic isomers reacted considerably faster than the meso isomers. There were two crucial observations 1) the stereoisomers did not interconvert under the reaction conditions and 2) there was no primary kinetic isotope effect for the hydrogen atoms at the C3 and C4 positions. These observations led to the conclusion that the cyclization of the hemiaminal intermediate is the rate-determining (slow) step. 

In working the following problems, it is important to draw the isomers in pairs of mirror images and in an orderly fashion. It will be easiest in this way to identify enantiomers, diastereomers and meso compounds since their definitions involve mirror image relationships. Before working with these examples, be sure you are thoroughly familiar with the definitions in Table 7.2 and the examples explained in sections 7.4. 

Constitutional isomers were discussed in Sections 11.6 and 12.3 and geometric isomers, a class of diastereomers, in Section 12.5. The discussion of stereoisomers is completed in this chapter with a discussion of enantiomers and two other classes of diastereomers - optically active and meso compounds. The relationship between different types of isomerism is shown in Fig. 17-1. 

P. Chautemps " obtained diepoxides from 1,4-pentadien-3-one (87) as a mixture of DL and meso compounds (88 89 as 13 7). Reduction of the carbonyl group in DL-diepoxide (88) followed by alkaline hydrolysis gave DL-arabinitol. Analogously, from meso diepoxide 89 a mixture of DL-ribitol and DL-xylitol was obtained. 

Catalytic enantioselective reductive desymmetrisation of achiral and meso-compounds, in particular, transformations of imides to lactams or lactones, anhydrides to lactones, and ring-opening of oxacycfic rings 13CC10666. 

Write structural formulas for all of the stereoisomers of 1,3-dimethylcyclopentane. Label PRACTICE PROBLEM 5.28 pairs of enantiomers and meso compounds if they exist. 

Prochiral and meso-compounds have been widely transformed into chiral products through transesterification reactions as shown with the acylation of diols (Fig. 14,1 and II). In transesterification, the prochiral or meso-substrate can also be a dicarboxylic acid diester. Common to such compounds is a plane of symmetry... 

Most of the physical properties of enantiomers are the same. A major exception is their interaction with plane-polarized light One enantiomer will rotate the polarization plane clockwise (dextrorotatory), the other counterclockwise (levorotatory). This phenomenon is called optical activity. The extent of the rotation is measmed in degrees and is expressed by the specific rotation, [a]. Racemates and meso compounds show zero rotation. The enantiomer excess or optical purity of an unequal mixture of enantiomers is given by... 

Configurations of Optically Active Tartaric Acids and Meso Compounds... 

For recent extensive reviews on biotransformations with lipases, see Kazlauskas and Bom-scheuer [77], Johnson [78], Rubin and Dennis [79], Itoh et al. [80], and Boland et al. [81]. The most widespread and frequently used biocatalytic reaction involving chiral compounds is kinetic resolution of racemates. Other biocatalytic stereoselective methods, although less frequently used, are asymmetrization of prochiral and meso compounds. These will be briefly discussed in Secs. C and D, respectively. 

In contrast to a conventional kinetic resolution of a racemate, asymmetrization of prochiral and meso compounds can give 100% theoretical yield. Still, the ratio of biocatalyzed asymmetrizations vs. kinetic resolutions has been reported to be only 1 4 [112]. Similar to a dynamic kinetic resolution, the enantiomeric purity of the product of an asymmetrization reaction remains constant and is independent of the extent of conversion. The enantiomeric excess of the product (e.e.p) is given by... 

For a recent review on enzymatic asymmetrization of prochiral and meso compounds, see Schoffers et al. [113]. Enzymatic asymmetrization of prochiral compounds was discussed by Ogston in 1948 [114]. He then rationalized the enzyme s ability to discriminate between enantiotopic groups on a prostereogenic substrate molecule based on the three-point combination model. 

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