Optical isomerism diastereoisomers

Nonionic contrast media have a multiplicity of optical and rotational isomers and diastereoisomers (201, 261, 264-267). Optical isomerism is derived from the chiral centers in... 

Optical isomerism occurs when the compound has no plane of symmetry and can exist in left- and right-handed forms that are mirror images of each other. Such molecules always contain a CHIRAL ELEMENT. Molecules that are mirror images of each other are more properly called enantiomers. Stereoisomers that are not mirror images are called diastereoisomers. For example, ANOMERS are diastereoisomers. See also optical activity. 

Iodinated contrast agents with polyhydroxylated carbon side-chains contain a number of asymmetric carbon atoms yielding numerous optical isomers which relate to each other as enantiomers or diastereoisomers. Sterically hindered non-asymmetric carbon or nitrogen atoms might result in additional asymmetry centres while the partial double bond character of the acyl-carbon-nitrogen bond of amide functions can lead to cisitrans isomerism. Such isomers are labelled rotamers when heating in solution is able to modify their ratio. Isomerism of iodixanol has been described by Priebe et al. [122], Fossheim et al. [123] and by Molander et al. [115]. 

The synthesis of a mixture of two diastereoisomers of harringtonine has also been reported24 by esterifying cephalotaxine (9b) with the acid chloride (33) of racemic 4-benzyloxy-7,7-dimethyl-2-oxo-l-oxacycloheptane-4-carboxylic acid and converting the functionality of the acyl moiety into that of harringtonine. Isomerically pure harringtonine (9a) was made formally accessible by optical resolution of a hydroxy-acid precursor. 

It is assumed that the preferred conformation of the substructure C=C—O—C of the E -configured ester enolate in the preferred transition state of the alkylation is that depicted in the center of the left-hand column of Figure 10.37. In the projection shown, the alkylating reagent attacks the enolate from the front side for the reasons just stated. The reaction occurs with a diastereoselectivity of 97 3. Chromatography allows for the complete separation of the main diastereoisomer (97 rel-%) from the minor diastereoiso-mer. Reduction of the main diastereoisomer (for the mechanism, see Section 14.4.3) affords the alcohol B, a derivative of S-a-benzyl propionic acid, with an optical purity of 100% ee. Hydrolysis of the benzylated esters without isomerization is impossible, however, so that optically active a-benzylpropionic acids cannot be obtained in this way. 

N-methyl derivative resulted in oxidation of the ligand with concomitant reduction of Co (III) to Co (II). The preparation of tris (benzohydroxa-mato) chromium (III), Cr(benz)3, was successful and resulted in the separation and characterization of its two geometric isomers (2). The half-lives for isomerization of these complexes near physiological conditions is on the order of hours. To facilitate the separation of all four optical isomers of a simple model tris (hydroxamate) chromium (III) complex, we prepared (using Z-menthol as a substituent) the optically active hydroxamic acid, N-methyl-Z-menthoxyacethydroxamic acid (men). This resulted in the separation of the two cis diastereoisomers of tris(N-methyl-Z-menthoxyacethydroxamato) chromium (III) from the trans diastereoisomers and their characterization by electronic absorption and circular dichroism spectra. 

An excellent resolving agent for many dissymmetric anionic metal complexes is [Co(C204)(en)2]. This complex ion is prepared easily from inexpensive materials and the optical isomers are relatively stable to isomerization in aqueous solutions. Previously, the optical isomers of [Co(C204)(en)2] were separated by means of diastereoisomer formation with [Co(edta)] . This... 

A detailed investigation of the stereochemistry of cis- and tran5-[RuCl2(L-L)2] (l L = o-QH< (EMePh)2 E = As, P) has been undertaken. The optically active, racemic, meso, syn, and anti forms of the trans compound were isolated for both ligands each of these subsequently isomerized to the corresponding cis complex by reaction with AlEts and the various diastereoisomers of the latter separated and characterized. Whereas the trans isomers are relatively inert, the cis complexes readily undergo stereospecific halogen substitution by 1 and A minor product obtained... 

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