Meso- form

H02CC(H)(NH2)(CH2)3C(H)(NH2)C02H. M.p. at least 305"C. The o, l and meso forms are all isolated from hydrolysates of bacterial proteins. It is an intermediate in the biosynthesis of lysine in many bacteria. 

The prochiral meso form of 2-cyclopenlen-1,4-diol (101) reacts with the (Z)-alkenyl iodide 102 to give the 3-substituted cyclopentanone 103 with nearly complete diastereoselectivity (98 2)[92], The reaction is used for the synthesis of prostaglandin. The alkenyl iodide 102 must be in the Z form in order to obtain the high diastereoselectivity. The selectivity is low when the corresponding (Z)-alkenyl iodide is used[93]. 

Only three not four stereoisomeric 2 3 butanediols are possible These three are shown m Eigure 7 10 The (2R 3R) and (2S 3S) forms are enantiomers of each other and have equal and opposite optical rotations A third combination of chirality centers (2R 3S) however gives an achiral structure that is superimposable on its (2S 3R) minor image Because it is achiral this third stereoisomer is optically inactive We call achiral mole cules that have chnahty centers meso forms The meso form m Eigure 7 10 is known as meso 2 3 butanediol... 

Fischer projection formulas can help us identify meso forms Of the three stereoisomeric 2 3 butanediols notice that only in the meso stereoisomer does a dashed line through the center of the Fischer projection divide the molecule into two mirror image halves... 

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... 

When two or more of a molecule s chirality centers are equivalently substituted meso forms are possible and the number of stereoisomers is then less than 2" Thus 2" represents the maximum number of stereoisomers for a molecule containing n chirality centers... 

Because there are four chirality centers and no possibility of meso forms there are 2" or 16 stereoisomeric hexoses All 16 are known having been isolated either as natural products or as the products of chemical synthesis... 

Three stereoisomers are possible a parr of enantiomers and a meso form... 

Achiral molecules that contain chirality centers are called meso forms Meso forms typically contain (but are not limited to) two equivalently substituted chirality centers They are optically inactive... 

Section 7 12 For a particular constitution the maximum number of stereoisomers is 2" where n is the number of structural units capable of stereochemical variation—usually this is the number of chirality centers but can include E and Z double bonds as well The number of stereoisomers is reduced to less than 2" when there are meso forms... 

DUactide (5) exists as three stereoisomers, depending on the configurations of the lactic acid monomer used. The enantiomeric forms whereia the methyl groups are cis are formed from two identical lactic acid molecules, D- or L-, whereas the dilactide formed from a racemic mixture of lactic acid is the opticaUy iaactive meso form, with methyl groups trans. The physical properties of the enantiomeric dilactide differ from those of the meso form (6), as do the properties of the polymers and copolymers produced from the respective dilactide (23,24). 

Tartaric acid [526-83-0] (2,3-dihydroxybutanedioic acid, 2,3-dihydroxysuccinic acid), C H O, is a dihydroxy dicarboxyhc acid with two chiral centers. It exists as the dextro- and levorotatory acid the meso form (which is inactive owing to internal compensation), and the racemic mixture (which is commonly known as racemic acid). The commercial product in the United States is the natural, dextrorotatory form, (R-R, R )-tartaric acid (L(+)-tartaric acid) [87-69-4]. This enantiomer occurs in grapes as its acid potassium salt (cream of tartar). In the fermentation of wine (qv), this salt forms deposits in the vats free crystallized tartaric acid was first obtained from such fermentation residues by Scheele in 1769. 

A rather air-stable titanocycle [76933-94-3 76933-97-6] has been prepared in meso form only (281) ... 

The reason that the third stereoisomer is achiral is that the substituents on the two asymmetric carbons are located with respect to each other in such a way that a molecular plane of symmetry exists. Compounds that incorporate asymmetric atoms but are nevertheless achiral are called meso forms. This situation occurs whenever pairs of stereogenic centers are disposed in the molecule in such a way as to create a plane of symmetry. A... 

The RR and SS forms have exactly the same values for the energy and the dipole moment. The energy difference between them and the meso form is about 2.5 millihartrees, which corresponds to about 1.5 kcal moT. This is a small but significant difference in energy. 

The meso form has no dipole moment. If we look again at the structure, this makes sense, since the molecule has a center of inversion. ... 

The meso form swaps the hydrogen and fluorine on only one of the carbons, leaving the other two unchanged with respect to the RR form. This will result in the dihedral angles for the hydrogens and fluorines having different signs on the two carbons ... 

Draw the meso form of each of the following molecules, and indicate the plane of symmetry in each ... 

Racemates may be indicated by the prefix DL-. Structures that have a plane of symmetry and are therefore optically inactive (e.g. erythritol, galactitol) are called meso forms and may be given the prefix meso- . 

L-Rhamnitol (L-Rha-ol) or 1-deoxy-L-mannitol 2-Carb-19.2. meso Forms... 

Alditols that are symmetric and therefore optically inactive - the meso forms - can be designated by the prefix meso-. 

The prefix D or L must be given when a derivative of a meso form has become asymmetric by substitution. It is also necessary to define the configurational prefixes as D or L in the case where there are more than four contiguous asymmetric carbon atoms. 

X-ray diffraction pattern, 334-335 Mass spectra, partial, per-O-trimethylsilylated dihexulose dianhydrides, 253 meso forms... 

Although four is the maximum possible number of isomers when the compound has two chiral centers (chiral compounds without a chiral carbon, or with one chiral carbon and another type of chiral center, also follow the rules described here), some compounds have fewer. When the three groups on one chiral atom are the same as those on the other, one of the isomers (called a meso form) has a plane of symmetry, and hence is optically inactive, even though it has two chiral carbons. Tartaric acid is a typical case. There are only three isomers of tartaric acid a pair of enantiomers and an inactive meso form. For compounds that have two chiral atoms, meso forms are found only where the four groups on one of the chiral atoms are the same as those on the other chiral atom. 

In most cases with more than two chiral centers, the number of isomers can be calculated from the formula 2", where n is the number of chiral centers, although in some cases the actual number is less than this, owing to meso forms.An interesting case is that of 2,3,4-pentanetriol (or any similar molecule). The middle carbon is not asymmetric when the 2- and 4-carbon atoms are both (/ ) [or both (5)] but is asymmetric when one of them is (/ ) and the other (5). Such a carbon is called a pseudoasymmetric carbon. In these cases, there are four isomers two meso forms and one dl pair. The student should satisfy himself or herself, remembering the rules... 

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. 

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