Diastereomers nomenclature

Organic chemists use an informal nomenclature system based on Eischer projections to distinguish between diastereomers. When the caibon chain is vertical and like substituents ar e on the sane side of the Eischer projection, the molecule is described as the erythro diastereomer. When like substituents are on opposite sides of the Eischer projection, the molecule is described as the three diastereomer. Thus, as seen in the... 

In the 1,3-dipolar cycloaddition reactions of especially allyl anion type 1,3-dipoles with alkenes the formation of diastereomers has to be considered. In reactions of nitrones with a terminal alkene the nitrone can approach the alkene in an endo or an exo fashion giving rise to two different diastereomers. The nomenclature endo and exo is well known from the Diels-Alder reaction [3]. The endo isomer arises from the reaction in which the nitrogen atom of the dipole points in the same direction as the substituent of the alkene as outlined in Scheme 6.7. However, compared with the Diels-Alder reaction in which the endo transition state is stabilized by secondary 7t-orbital interactions, the actual interaction of the N-nitrone p -orbital with a vicinal p -orbital on the alkene, and thus the stabilization, is small [25]. The endojexo selectivity in the 1,3-dipolar cycloaddition reaction is therefore primarily controlled by the structure of the substrates or by a catalyst. 

Whereas, from all of these informative 1H-PHIP-NMR spectra, the structure of the dihydride intermediate (including geometric details about peculiar bonding therein) can be determined rather exactly and reliably, a degree of uncertainty remains as to whether this intermediate represents the major or the minor diastereomer according to the nomenclature of Halpern [27]. This is the consequence of different kinetic constants associated with the two alternative cycles with different stereochemistry, and which accounts for the major and minor reaction product (Fig. 12.18). In fact, it is the difference in the rate... 

The adduct with the re face bound to Rhodium, leading to the 5 enantiomer, is more stable than the Pro-R -coordinated adduct, and so would be the predominant diastereomer in solution. In our original papers on this work [71, 72], we labeled the more stable Pro-5 diastereomer MAJ and the Pro-/ diastereomer MIN in analogy to the nomenclature from Figure 2. For consistency with our later work, however, we use the designations PRO-R and PRO-5 in this chapter. 

Some nomenclature note that in the trans isomer, the two methyl groups are on opposite sides of the double bond, or across from each other. In the cis isomer, they are on the same side of the double bond. Like constitutional isomers, diastereomers have different physical and chemical properties. 

Just as it is convenient to distinguish enantiomers and diastereomers by nomenclature symbols (R, S, E, Z, etc.) it is desirable to provide names for stereoheterotopic ligands or faces. The basic nomenclature to this end has been provided by Hanson 4,6) and is closely related to the nomenclature of stereoisomers. 

The compound has two chirality centres and three pseudo chirality centres. There is however, only one (achiral) diastereomer of the compound shown in the question. The two isomers can be distinguished from one another solely on the relative position of the chlorine or bromine atoms which lie in a plane which also happens to be the plane of symmetry of the molecule (this is the only symmetry element present, therefore the symmetry point group is Cs). It is possible in this instance to specify the configuration unequivocally using the descriptors E and Z. However, in systematic nomenclature the complete configuration of all the stereogenic centres is specified. Thus the (so-called) Z isomer is (ls,3r,5 ,6r,7S)-l,6-dibromo-3,6-dichloroadamantane and the isomer is (ls,3r,5 ,6s,7S)-l,6-dibromo-3,6-dichloroadamantane, i.e. the two isomers can be distinguished simply by the descriptor used for position 6. 

The olefinic substrates that are cis-trans isomers are by modern stereochemical nomenclature more generally termed diastereomers. That is, they are stereoisomers that are not enantiomers. The fact that they contain no asymmetric carbons is irrelevant to this classification. 

Fig. 23. Eight geometrical isomers of a ferric tri-chelate complex, involving three unsymmetrical bidentate ligands attached to an asymmetric backbone (e.g. coprogen). The optical conformation at the metal is A. Not shown is the set of eight A diastereomers. See text for nomenclature...

According to lUPAC the term geometric isomerism" is an obsolete synonym of cis-trans isomerism and its use is strongly discouraged. Sometimes the term geometric isomerism has been used as a synonym of stereoisomerism, i.e. optical isomers being considered to be geometric isomers. This, however, is not consistent with current standard chemical nomenclature. The exact term for stereoisomers that are not optical isomers is diastereomers. 

The number of possible diastereomers depends on the variety of ligands and sometimes requires use of the one-letter code (cis/trans is noted c/t). This nomenclature may be applied to square planar complexes and to square planar pyramidal and octahedral complexes, but not to tetrahedral complexes where a given position is equivalent to any other. Moreover, geometric isomerism often implies the existence of optical isomerism. 

Reaction of an a-substituted enolate with an aldehyde or ketone can give two pairs of aldol dia-stereomers, which are conveniently designated as the syn form (17) and the anti form (18), where is part of the parent chain in lUPAC nomenclature (equation 29). For simplicity, only one enantiomer of each pair will usually be shown throughout this section. The syn/anti notation for aldol diastereomers has been described in detail by Heathcock. ... 

The thermal hetero-Diels - Alder reaction of diene 1 a to butyl glyoxylate (2 a) leads to a mixture of four possible diastereomers 3-6 in good yield with a predominance of enrfo-attack (jS-sugar) and a d.r. 69 31 in favor of the L-sugar. Dienes lb andc, in the same reaction, produce almost equal amounts of endo- and exo-products with a low selectivity in the formation of new stereogenic centers at C-5 (carbohydrate nomenclature)23-25. 

See also Monosaccharide Nomenclature, Diastereomers, Saccharides, Mannitol... 

See also Saccharides, Oligosaccharides as Cell Markers, Biosynthesis of Glycoconjugates (from Chapter 16), Diastereomers, Monosaccharide Nomenclature... 

See also Galactose Operon, Galactose Metabolism, Diastereomers, Saccharides, cAMP Receptor Protein, Monosaccharide Nomenclature... 

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