Meso molecule

Meso molecules such as S-I.2-dichloro-I,2-difluorc>clhane are usually cited as achiral because they possess a mirror plane (when in the perfectly eclipsed conformer, a stipulation that is often omitted). Whal is the symmetry of this conformer In other conformations Discuss why meso molecules show no optical activity. 

A molecule that has two or more chiral centers so arranged that one-half of the molecule is the mirror image of the other half is achiral. Such a molecule is called a meso molecule. 

Meso is a prefix which, by strict definitions, indicates the presence of a second chiral center. It comes from the Greek for middle or mid , and refers to the fact that the molecule can rotate about its middle. A meso molecule is not a diastereomer because rotating either of its chiral centers doesn t change the molecule overall a meso molecule has an internal plane of reflection, also called a plane of symmetry. 

DMAP is an efficient desynmetrisation catalyst. [Wilhs J Chem Soc, Perkin 1 1765 7999.] Thus it allows the use of an achiral or meso molecule, e.g. 3-substituted gjutaric anhydride to react with chiral alcohols e.g. 7 -l-(l -naphthyl)ethanol to produce (l 7 ,37 )-l(r-naphthyl)ethyl methyl 3-substituted-pentanoate di-esters (after treatment with diazomethane), which are now chiral at C-3. The achiral carbon atom at C-3 in the glutaric anhydride becomes chiral in the pentanoate (mono-ester of glutarate). [Theisen Heathcock J Org Chem 53 2374 1988, Theisen Heathcock J Org Chem 58 142 7995.]... 

Furukawa et al. explored the use of methanol and ethanol as additives for diethylzinc-based epoxide polymerization systems, and found that both the yield and crystallinity of the resulting polymers were inferior to those for polymers synthesized with the ZnEt2/H20 system. The use of achiral alcohols as cocatalysts was revisited in 1994 when Kuran and Listos reported the polymerization of propylene oxide and cyclohexene oxide (a meso molecule) with ZnEt2/polyhydric phenol (such as 4-tert-butyl-catechol), phenol, or l-phenoxy-2-propanol. The poly(propylene oxide) formed from these systems contained mostly isotactic dyads (72% m), whereas the poly(cyclohexene oxide) contained mostly syndiotactic dyads (80% r) (Scheme 24.8). 

Dendritic molecule, dendrimer A multibranched meso-molecule prepared by either an iterative or a convergent methodology. Typically such structures are very highly branched and have been described as multiarmed . Dendritic molecules may be perfectly monodisperse, or may contain structural defects. A variety of synonyms are in use cascade molecule, dendrimer, arborol, cascadol, molecular fractal, starburst dendrimer, etc. 

There is significant interest in controlling the absolute stereochemistry of ring opening in epoxide/C02 copolymerization. Cyciohexene oxide, a meso molecule, is an ideal substrate for desymmetrization using chiral catalysts. In 1999, Nozaki et al7 reported that a 1 1 mixture of ZnEt2 and (S)-diphenyl (pyrrolidine-2-yl)methanol (11) (Scheme 13) was active for stereoselective cyciohexene oxide/C02 copolymerization at 40 °C and 30 atm. CO2 (Scheme 14). The resultant polycarbonate contained 100% carbonate linkages, had an Ain of... 

Label A-E in Model 8 with the terms cis, trans, and/or meso, as appropriate. Two terms may apply. (Review) meso = molecule with chiral centers and an internal mirror plane such that it is not chiral. 

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... 

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... 

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... 

Merrifield method See solid phase peptide synthesis Meso stereoisomer (Section 7 11) An achiral molecule that has chirality centers The most common kind of meso com pound IS a molecule with two chirality centers and a plane of symmetry... 

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

The all trans" stmcture in Figure 4.9(a) is not chiral as it has an inversion centre i. This is called a meso stmcture. Although each CFIFCFI3 group rotates the plane of plane-polarized light in one direction, the other group rotates it an equal amount in the opposite direction. The result is that there is no rotation of the plane of polarization and, as the presence of an inversion centre tells us, the molecule is achiral. 

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). 

The situation is different if the substrate is a prochiral or meso compound. Since these molecules have a center or plane of symmetry the binding of pro-S or pro-R forms is equivalent. The chirahty appears only as a result of the transformation. Hence, at least theoretically, the compound can be converted to one enantiomer quantitatively. 

The calcium salt of the principal product, d/-tartaric acid, crystallizes with four molecules of water, while the secondary product, meso-tartaric acid, forms a calcium salt which crystallizes with three molecules of water. The amount of sulfuric acid actually required may readily be calculated from the percentage of calcium found on analysis in the regular way or it may be estimated by igniting a sample, and titrating the residue with standard acid. 

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... 

Only three, not four, stereoisomeric 2,3-butanediols aie possible. These three aie shown in Eigure 7.10. The (2R,3R) and (2.S,3.S) fonns aie 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 molecules that have chirality centers meso forms. The meso form in Eigure 7.10 is known as iwe50-2,3-butanediol. 

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 three water ligands located at meridional positions of the J ,J -DBFOX/Ph aqua complexes may be replaced by another molecule of DBFOX/Ph ligand if steric hindrance is negligible. Based on molecular model inspection, the hetero-chiral enantiomer S,S-DBFOX/Ph looks like a candidate to replace the water ligands to form the heterochiral meso-2 l complex J ,J -DBFOX/Ph-S,S-DBFOX/... 

Figure 9.11 A symmetry plane through the C2-C3 bond of meso-tartaric acid makes the molecule achiral. H ho co2h...

To see whether a chirality center is present, look for a carbon atom bonded to four different groups. To see whether the molecule is chiral, look for the presence or absence of a symmetry plane. Not all molecules with chirality centers are chiral overall—meso compounds are an exception. 

A look at the structure of cis-1,2-dimethylcyclobutane shows that both methyl-bearing ring carbons (Cl and C2) are chirality centers. Overall, though, the compound is achiral because there is a symmetry plane bisecting the ring between Cl and C2. Thus, the molecule is a meso compound. 

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

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... 

In open-chain compounds, the molecule can usually adopt that conformation in which H and X are anti periplanar. However, in cyclic systems this is not always the case. There are nine stereoisomers of 1,2,3,4,5,6-hexachlorocy-clohexane seven meso forms and a dl pair (see p. 161). Four of the meso compounds and the dl pair (all that were then known) were subjected to elimination of HCl. Only one of these (1) has no Cl trans to an H. Of the other isomers, the fastest elimination rate was about three times as fast as the... 

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