Diastereomer

If compounds have the same topology (constitution) but different topography (geometry), they are called stereoisomers. The configuration expresses the different positions of atoms around stereocenters, stereoaxes, and stereoplanes in 3D space, e.g., chiral structures (enantiomers, diastereomers, atropisomers, helicenes, etc.), or cisftrans (Z/E) configuration. If it is possible to interconvert stereoisomers by a rotation around a C-C single bond, they are called conformers. 

Diastereomers= stereoisomers which are not rnirror images usually have different physical properties... 

Sharpless Asymmetric Dihydroxylation (AD) - Ligand pair are really diastereomers ... 

Clearly, there is a need for techniques which provide access to enantiomerically pure compounds. There are a number of methods by which this goal can be achieved . One can start from naturally occurring enantiomerically pure compounds (the chiral pool). Alternatively, racemic mixtures can be separated via kinetic resolutions or via conversion into diastereomers which can be separated by crystallisation. Finally, enantiomerically pure compounds can be obtained through asymmetric synthesis. One possibility is the use of chiral auxiliaries derived from the chiral pool. The most elegant metliod, however, is enantioselective catalysis. In this method only a catalytic quantity of enantiomerically pure material suffices to convert achiral starting materials into, ideally, enantiomerically pure products. This approach has found application in a large number of organic... 

A more eflicient and general synthetic procedure is the Masamune reaction of aldehydes with boron enolates of chiral a-silyloxy ketones. A double asymmetric induction generates two new chiral centres with enantioselectivities > 99%. It is again explained by a chair-like six-centre transition state. The repulsive interactions of the bulky cyclohexyl group with the vinylic hydrogen and the boron ligands dictate the approach of the enolate to the aldehyde (S. Masamune, 1981 A). The fi-hydroxy-x-methyl ketones obtained are pure threo products (threo = threose- or threonine-like Fischer formula also termed syn" = planar zig-zag chain with substituents on one side), and the reaction has successfully been applied to macrolide syntheses (S. Masamune, 1981 B). Optically pure threo (= syn") 8-hydroxy-a-methyl carboxylic acids are obtained by desilylation and periodate oxidation (S. Masamune, 1981 A). Chiral 0-((S)-trans-2,5-dimethyl-l-borolanyl) ketene thioketals giving pure erythro (= anti ) diastereomers have also been developed by S. Masamune (1986). 

Stereoisomer I is not a mirror image of III or IV so it is not an enantiomer of either one Stereoisomers that are not related as an object and its mirror image are called diastereomers diastereomers are stereoisomers that are not enantiomers Thus stereoisomer I is a diastereomer of III and a diastereomer of IV Similarly II is a diaste reomer of III and IV... 

Because diastereomers are not mirror images of each other they can have quite different physical and chemical properties For example the (2R 3R) stereoisomer of 3 ammo 2 butanol is a liquid but the (2R 3S) diastereomer is a crystalline solid... 

A good thing to remember is that the cis and trans isomers of a particular compound are diastereomers of each other... 

Turning to cyclic compounds we see that there are three not four stereoisomeric 1 2 dibromocyclopropanes Of these two are enantiomeric trans 1 2 dibromocyclo propanes The cis diastereomer is a meso form it has a plane of symmetry... 

Steroids are another class of natural products with multiple chirality centers One such compound is cholic acid which can be obtained from bile Its structural formula IS given m Figure 7 12 Cholic acid has 11 chirality centers and so a total (including cholic acid) of 2" or 2048 stereoisomers have this constitution Of these 2048 stereoiso mers how many are diastereomers of cholic acid s Remember Diastereomers are stereoisomers that are not enantiomers and any object can have only one mirror image Therefore of the 2048 stereoisomers one is cholic acid one is its enantiomer and the other 2046 are diastereomers of cholic acid Only a small fraction of these compounds are known and (+) cholic acid is the only one ever isolated from natural sources... 

A reaction that introduces a second chirality center into a starting material that already has one need not produce equal quantities of two possible diastereomers Con sider catalytic hydrogenation of 2 methyl(methylene)cyclohexane As you might expect both CIS and trans 1 2 dimethylcyclohexane are formed... 

The double bond m 2 methyl(methylene)cyclohexane is prochiral The two faces however are not enantiotopic as they were for the alkenes we discussed m Section 7 9 In those earlier examples when addition to the double bond created a new chirality cen ter attack at one face gave one enantiomer attack at the other gave the other enantiomer In the case of 2 methyl(methylene)cyclohexane which already has one chirality center attack at opposite faces of the double bond gives two products that are diastereomers of each other Prochiral faces of this type are called diastereotopic... 

Dissociate diastereomer to single enantiomer recover resolving agent... 

This method is widely used for the resolution of chiral amines and carboxylic acids Analogous methods based on the formation and separation of diastereomers have been developed for other functional groups the precise approach depends on the kind of chem ical reactivity associated with the functional groups present m the molecule... 

The CIS and trans isomers of 4 methylcyclohexanol are stereoisomers but they are not related as an ob ject and its mirror image they are diastereomers... 

Stereoisomers that are not enantiomers are classified as diastereomers Each enantiomer of erythro 3 bromo 2 butanol is a diastereomer of each enantiomer of threo 3 bromo 2 butanol... 

Section 7 14 Resolution is the separation of a racemic mixture into its enantiomers It IS normally carried out by converting the mixture of enantiomers to a mixture of diastereomers separating the diastereomers then regenerating the enantiomers... 

If they are stereoisomers are they enantiomers or diastereomers" (Molecular models may prove useful in this problem )... 

Streptirnidone is an antibiotic and has the structure shown How many diastereomers of streptimidone are possible" How many enantiomers" Using the E Z and R S descnptors specify all essential elements of stereochemistry of streptimidone... 

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