Stereochemistry meso compounds

For dihydrodiols derived from substituted benzenes, the key to their significance lies in the availability of two adjacent chiral centers with an established absolute stereochemistry. The dihydrodiol from benzene is, of conrse, the meso compound, although enantiomers produced by subsequent reaction with a chiral reagent are readily separated. There are useful reviews containing nnmerous applications (Carless 1992 Ribbons et al. 1989), many of which involve, in addition, the nse of di-flnoro-, di-chloro-, or di-bromobenzene-2,3-dihydrodiols. 

The catalytic enantioselective desymmetrization of meso compounds is a powerful tool for the construction of enantiomerically enriched functionalized products." Meso cyclic allylic diol derivatives are challenging substrates for the asymmetric allylic substitution reaction owing to the potential competition of several reaction pathways. In particular, S 2 and 5n2 substitutions can occur, and both with either retention or inversion of the stereochemistry. In the... 

A-10. Write the final product of the following reaction sequence, clearly showing its stereochemistry. Is the product achiral, a meso compound, optically active, or a racemic mixture ... 

Such conventional kinetic resolution reported above often provide an effective route to access to the enantiomerically pure/enriched compounds. However, the limitation of such process is that the resolution of two enantiomers will provide a maximum 50% yield of the enantiomerically pure materials. Such limitation can be overcome in several ways. Among these ways are the use of meso compounds or prochiral substrates,33 inversion of the stereochemistry (stereoinversion) of the unwanted enantiomer (the remaining unreacted substrate),34 racemization and recycling of the unwanted enantiomer and dynamic kinetic resolution (DKR).21... 

The stereochemistry of the product of a reaction will be influenced by the structures of the reagent and substrate and the mechanisms by which they react. For example, the hydroxylation of but-2-ene by osmium tetroxide and water yields a racemate whilst bromination of the same compound with bromine produces a meso compound (Figure 10.5). Flowever, a stereoselective reaction is most likely to occur when steric hindrance at the reaction centre restricts the approach of the reagent to one direction (Figure 10.6). Furthermore, the action of both enzyme and non-enzyme catalysts may also be used to introduce specific stereochemical centres into a molecule. 

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... 

Compounds that contain stereogenic centres but are themselves achiral are called meso compounds. This means that there is a plane of symmetry with R stereochemistry on one side and S stereochemistry on the other. 

This structure has two chiral centres, so how will we know which diastereoisomer we have The answer was simple the stereochemistry has to be tram because Feist s acid is chiral it can be resolved (see later in this chapter) into two enantiomers, Now, the cis diacid would have a plane of symmetry, and so would be achiral—it would be a meso compound, The trans acid on the other hand is chiral— it has only an axis of symmetry. If you do not see this, try superimposing it on its mirror image. You will find that you cannot. 

Epoxidation by use of m-chloroperoxybenzoic acid (RCO3H) is a syn addition of oxygen to a double bond. The original bond stereochemistry is retained, and the product is a meso compound. 

For example, addition of bromine to cw-2-butene gives equal concentrations of (2 ,3 )-2,3-dibromobutane and (25,35)-2,3-dibromobutane (a racemic mix), whereas addition of bromine to rrans-2-butene gives the meso compound (2S,3R)-2,3-dibromobutane. Since the stereochemistry of the reactant is determining product stereochemistry, this reaction is stereospecific, and whatever mechanism is proposed must account for that. 

The stereochemistry of a solvolysis reaction can be affected if the substrate has a substituent that can donate a pair of electrons to the developing carbocation center. For example, treatment of ( )-t/zreo-3-bromo-2-butanol (19) with HBr gave only the racemic 2,3-dibromobutane (20). There was none of the meso compound that would have been expected if the reaction involved protonation, loss of water, and formation of a free carbocation intermediate. Similarly, reaction of ( )-eri/tizra-3-bromo-2-butanol with HBr gave only meso-2,3-dibromobutane. The reaction of 19 seems best explained by nucleophilic participation of the bromine on the adjacent atom in concert with departure of the water. The result is a bridged intermediate (21) that is the same bromonium ion expected from the electrophilic addition of Br2 to cis-2-butene (Figure 8.13). Back-side attack by bromide ion on either carbon atom involved in the three-membered bromonium ring is equally likely, so a racemic mixture results. 

In the specific case of meso compounds, the gelling properties of chiral and achiral diastereomers can be generalized. Many gelators have a C2 symmetrical structure and contain two chiral units with the same stereochemistry (Schemes 3 and 4). It is not clear whether such C2 symmetrical structures... 

Recall that a racemic substance is an equal mixture of two enantiomers of a chiral molecule. A meso compound, on the other hand, has a symmetry element such as a mirror plane or an inversion center that prevents it from being chiral. Analysis of product stereochemistry thus played a crucial role in establishing these mechanisms. 

Both paths to (1), via either (S)-(25) or via (R)-(25), result in the formation of tropinone (18) as an intermediate which is a meso compound and not a suitable object for further study of these questions. However, the isolation of (26) from Datura stramonium has been reported recently [51]. An analysis of the stereochemistry of (26) isolated from such a plant and the demonstration that (26) serves as an intermediate in the formation of (1) would go a long way to putting many of these conjectures on to firmer grounds. Unfortunately no further details from this work have been published. 

What is the stereochemistry of the product Is it one enantiomer, a pair of enantiomers, the meso compound, or a mixture of aU three stereoisomers ... 

Syn hydroxylation of ds-2-butene gives meso-2,3-butanediol because the meso compound is achiral, the product is optically inactive. Syn hydroxylation of trans-2-butene gives racemic 2,3-butanediol. Because the diol is formed as a racemic mixture, the product of the oxidation of the frans-alkene is also optically inactive. Thus, the osmium tetroxide oxidation of an alkene is stereospecific the stereochemistry of the product depends on the stereochemistry of the starting alkene. 

Notice that the stereochemistry of the product of the osmium tetroxide oxidation of lrans-2-butene is opposite that formed on the addition of bromine to lra s-2-butene. Osmium tetroxide oxidation gives the glycol as a pair of enantiomers forming a racemic mixture. Addition of bromine to trcms-2-butene gives the dibromoalkane as a meso compound. A similar difference is observed between the stereochemical outcomes of these reactions with ds-2-butene. The difference in outcomes occurs because bromination of an alkene involves anti addition, whereas oxidation by osmiinn tetroxide involves syn addition. 

The stereochemical results of these reactions have been carefully studied." If a chiral acetate is employed, then the product is found to have retention of stereochemistry. Hence, the cis acetoxy ester 9.98 gives the cis product 9.100, while the trans acetoxy ester 9.101 gives the trans product 9.103 (Scheme 9.32)." Retention is a result of two inversions - inversion during formation of the t -allyl complexes, 9.99 and 9.102, and a second inversion during attack by the nucleophile. In the case of the acetoxy esters 9.98 and 9.101, a curious observation can be made when the substrate is non-racemic. It is found that the product is racemic. This is not a general observation. It occurs here, because the intermediate V-allyl complexes, 9.99 and 9.102, each have a plane of symmetry - they are meso compounds and the nucleophile is equally likely to attack either terminus. Systems without the symmetrical intermediate do not show racemization (Scheme 9.33). 

Neither rw-l,2-dimethylcyclopropane nor c -l,2-dimethylcyclohexane can be resolved, because the cyclopropane is a meso compound (Chapter 4, p. 168), and the cyclohexane flips into its mirror image. However, both 1,2-dimethyl-cyclopropane and /rawi-l,2-dimethylcyclohexane can be resolved they are not superimposable on their mirror images. The point is that in a practical sense the planar cyclopropanes and nonplanar cyclohexanes behave in the same way. This finding has important consequences. In deciding questions of stereochemistry, we can treat the decidedly nonplanar 1,2-dimethylcyclohexanes as if they were planar. Indeed, all cyclohexanes can be treated as planar for the purposes of stereochemical analysis, because the planar forms represent the average positions of ring atoms in the rapid chair-chair interconversions. 

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