Meso compounds definition

The same kind of spontaneous racemization oc curs for any as 1 2 disubstituted cyclohexane in which both substituents are the same Because such compounds are chiral it is incorrect to speak of them as meso compounds which are achiral by definition Rapid chair-chair interconversion however converts them to a 1 1 mixture of enantiomers and this mix ture IS optically inactive... 

Of course, the trans isomer will give the opposite results the threo pair if the addition is syn and the erythro pair if it is anti. The threo and erythro isomers have different physical properties. In the special case where Y=W (as in the addition of Br2), the erythro pair is a meso compound. In addition to triple-bond compounds of the type ACsCA, syn addition results in a cis alkene and anti addition in a trans alkene. By the definition given on page 166 addition to triple bonds cannot be stereospecific, though it can be, and often is, stereoselective. 

Since the two reactants cis and trans butenes are stereoisomers, being diastereomers, the product from cis is a meso compound and that from the trans give a pair of enantiomers, by definition both the reactions are stereospecific. 

D. Molecular Orbital Calculations. Pharmacological Activity of Meso-ionic Compounds Meso-ionic— Definition and Delineation... 

C is correct, Meso compounds are (by definition) achiral although they do contain chiral carbons. They are not optically active (do not rotate plane-polarized light). 

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

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. 

The reactants CM-2-butene and trawr-2-butene are stereoisomers they are diastereo-mers. The product of reaction 1, (2i ,35)-2,3-dibromobutane, is a meso compound, and it is a stereoisomer of both of the products of reaction 2 (the enantiomeric 2,3-dibro-mobutanes). Thus, by definition, both reactions are stereospecific. One stereoisomeric form of the reactant (e.g., trans-2- mtene) gives one product (the meso compound), whereas the other stereoisomeric form of the reactant ( i--2-butene) gives a stereoisomeri-cally different product (the enantiomers). 

A meso compound, by definition, does not have an enantiomer. A. addition on one face of the alkene generates exactly the same compound as a syn addition on the other face of the alkene. And therefore, care must be taken not to write -h Enantiomer or + En for short ... 

The study of optical isomers has shown a similar development. First it was shown that the reduction potentials of several meso and racemic isomers were different (Elving et al., 1965 Feokstistov, 1968 Zavada et al., 1963) and later, studies have been made of the ratio of dljmeso compound isolated from electrolyses which form products capable of showing optical activity. Thus the conformation of the products from the pinacolization of ketones, the reduction of double bonds, the reduction of onium ions and the oxidation of carboxylic acids have been reported by several workers (reviewed by Feokstistov, 1968). Unfortunately, in many of these studies the electrolysis conditions were not controlled and it is therefore too early to draw definite conclusions about the stereochemistry of electrode processes and the possibilities for asymmetric syntheses. 

However, if both maleic and fumaric acid gave the dl pair or a mixture in which the dl pair predominated, the reaction would be stereoselective but not stereospecific. If more or less equal amounts of dl and meso forms were produced in each case, the reaction would be nonstereoselective. A consequence of these definitions is that if a reaction is carried out on a compound that has no stereoisomers, it cannot be stereospecific, but at most stereoselective. For example, addition of bromine to methylacetylene could (and does) result in preferential formation of trans-1,2-dibromopropene, but this can be only a stereoselective, not a stereospecific reaction. 

Although this was not appreciated at the time, these compounds were first handled by Busch and Becker in 1896. Their study continued until a more extensive report by Busch and Schmidt was published in 1929,195 However, Busch and his co-workers were apparently unaware of some closely related work reported by von Pechmann in 1896. These earlier studies have been interpreted by Farrar, extended by Christopherson and Treppendahl, and brought to a definitive conclusion by the Sheffield group. This history is an interesting facet of the development of the chemistry of meso-ionic heterocycles. 

In the definition of the term meso-ionic as originally proposed by Baker and Ollis, it was stated that a compound may be appropriately called meso-ionic if it is a five- or possibly a six-membered heterocyclic compound which cannot be represent satisfactorily by any one covrdent or polar structure and possesses a sextet of electrons in association with the atoms comprising the ring. This definition has formed the basis of this review, but we now propose a modification. 

One hundred and forty-four meso-ionic heterocycles of type A (13, 19) and 84 meso-ionic heterocycles of type B (14, 20) are possible. The numbers of these two types which are now known (Table I type A, 44 representatives) and (Table II type B, 8 representatives) encourage us to put forward the proposal that the term meso-ionic should in future be restricted to five-membered heterocycles belonging to type A (13, 19) and type B (14,20). This clear restriction upon the use of the term meso-ionic has obvious advantages. It still embraces 228 different classes of heterocycles with a common structural characteristic, and the many types of meso-ionic compounds included in the authoritative review by Ohta and Kato " are included. Needless to say, the restriction upon the definition of the term meso-ionic to five-mem red heterocycles of type A and type B still includes, for example, benz derivatives such as the compounds 67, 71, 110, 123, 133, 151, 206, 209, 226, 255, and 258. 

If our proposal for the restriction of the term meso-ionic finds general support and it is agreed that compounds exemplified by the structures (456-478) should not be called meso-ionic, then the definition we would wish to propose is as follows ... 

The compounds 513-517 have recently been described as meso-ionic. " According to our modified definition of the term meso-ionic (page 110) these interesting compounds (513-517) should now be described as mesomeric betaines. 

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