2” rule meso compounds

In 2,4-dichloropentane, for example, there are two asymmetric carbon atoms with four ligands having the seniorities Cl > CH2—CHCl—CH3 > CH3 > H (Figure 3-5). Molecule I has, according to the chirality rules, the configuration RS. The two asymmetric central atoms have opposing configurations so, the molecule is a meso compound. Since molecules I and III can be converted into each other by a rotation about 180°, these two molecules must be identical. Molecules II and IV cannot be transformed into each other by a symmetry operation, and so, they must be enantiomers a 1 1 mixture of these two is therefore racemic. 

Locate all stereocenters and use the 2" rule to determine the maximum number of stereoisomers possible. Determine which, if any, of the possible stereoisomers are meso compounds. 

Cyclohexanediol has two stereocenters, and, according to the 2" rule, a maximum of 2 = 4 stereoisomers is possible. The trans isomer of this compound exists as a pair of enantiomers. The cis isomer has a plane of symmetry and is a meso compound. Therefore, although the 2" rule predicts a maximum of four stereoisomers for 1,3-cyclohexanediol, only three exist—one pair of enantiomers and one meso compound ... 

When a molecule has a structure with identical atoms or groups on the top and bottom (one side can be perfectly reflected into the other side), that molecule has a plane of symmetry (see Figure 9.9). When such symmetry occurs, the mirror image of one stereoisomer is superimposable on itself. Such a stereoisomer is called a meso compound. Because 2,3-dibromobutane has two stereogenic centers, the 2" rule predicts a maximum of four stereoisomers. Symmetry in one stereoisomer means that 46 is a meso compound, so there are only three stereoisomers (the two enantiomers 43 and 44 and the meso compomid 46). It is important to point out that 46 and 43 are diastereomers. Likewise, 46 and 44 are diastereomers, but 43 and 44 are enantiomers. 

Further analysis of the products from cyclodimerization revealed that each of the cis- and trans- stereoisomers has two enantiomers. As can be seen in Scheme 14.10, the cis- product has two enantiomers (a) and (b), being (S, R) and (R, S) configuration, respectively, according to the Cahn-Ingold-Prelog priority rules. Both the cis- enantiomers are meso compounds and optically inactive. The trans- product also has two enantiomers, being (S, S) and (R, R), which exist as non-superimposable mirror images of each other. Enantiomers (c) and (d) should have identical physical and chemical properties in the absence of a chiral environment, while the trans and cis diastereomers will display differences in some physical and chemical properties, such as reactivity. 

When counting isomers it is tempting to use the 2" rule without thinking. You must be mindful of S5unmetrical molecules with the possibility for meso compounds among the configurational stereoisomers. 

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. 

A systematic investigation of the free amino acids of the Leguminosae led to the isolation of a novel ninhydrin-positive compound from the leaves of Derris elliptica Benth. (Papilionidae) (93). This substance was analyzed as C6H,3N04 (microanalysis and high resolution mass spectrometry) and was shown to be an amino alcohol. The absence of a carbonyl in the 1R, the loss of 31 mass units in the mass spectrum, and a positive periodate cleavage reaction were best embodied into a dihydroxydihydroxymethylpyrrolidine structure. The relative simplicity of the NMR spectra (three peaks in the 13C spectrum four spin-system in the H spectrum) pointed out a symmetrical structure. Inasmuch as the material was optically active ([a]D 56.4, c = 7, H20), meso structures were ruled out, and the 2R, 3R, 4R, 5R relative configuration was retained (93). This structure (53) was further confirmed by an X-ray determination (94). 

The nomenclature of porphyrins, which belong to the larger class of tetrapyrrole compounds, is sometimes obscured by historical remnants (e.g. chlorin which does not contain any chlorine substituent, see H22c in fig. 10, or 2,4-di(o -methoxyethyl)-deuteroporphyrin for TMb, see fig. 11 below). IUPAC has published nomenclature rules in 19863 and the numbering adopted for the ring is given in fig. 10. The 5, 10, 15, and 20 positions are commonly referred to as meso positions the roman number after a name (I though IV) denotes the relative positions of substituents a and b. 

Rule 12. Designation of optical isomerism. Where optical isomerism can occur, the optically active compound is designated by (+) or (—) depending upon the sign of rotation alternatively, d- or /- may be used. The racemic mixture is designated by (db) or dl- and the inactive form by meso. 

The nature and the number of the saccharide substituents can rule the physical properties of such compounds furthermore, the presence of hydrophobic raeso-phenyl, meso-pentafluorophenyl or meso-alkyl substituents could increase the interactions of porphyrins with the hpid parts of cell membranes whereas the glycosyl moieties could become fimctional components involved in cell recognition [118,119). 

Cyclopentanediol also has two chiral centers therefore, the 2" rule predicts a maximum of 2 = 4 stereoisomers. As shown in the following stereodrawings, only three stereoisomers exist for this compound. The ds isomer is achiral (meso) because it and its mirror image are superposable. An alternative way to identify the ds isomer as achiral is to notice that it possesses a plane of symmetry that bisects the molecule into two mirror halves. The trans isomer is chiral and exists as a pair of enantiomers. 

---