Stereoisomers, also

Most enolates can exist as two stereoisomers. Also, most aldol condensation products formed from a ketone enolate and an aldehyde can have two diastereomeric structures. These are designated as syn and anti. The cyclic-transition-state model provides a basis for understanding the relationship between enolate geometry and the stereochemistry of the aldol product. 

Glyceraldehyde has a single asymmetric carbon atom (the central one) and so two stereoisomers (also called optical isomers) are possible, that is two forms of glyceraldehyde, denoted as d- and L-glyceraldehyde, which are mirror images of each other (Fig. 2). Stereoisomers also exist for amino acids (see Topic Bl). 

The molecule 1 has two stereogenic centres and has a different set of substituents at each centre accordingly, there are four stereoisomers of 1. Four stereoisomers also result if in 1 one replaces, say, an NH2 by an... 

Stereoisomers also arise when two rings share a common bond. In the cis isomer both branches of the fused ring are on the same side. In the trans isomer they are on opposite sides. 

Unlike the atoms in constitutional isomers, the atoms in stereoisomers are connected in the same way. Stereoisomers (also called configurational isomers) differ in the way their atoms are arranged in space. Stereoisomers are different compounds that do not readily interconvert. Therefore, they can be separated. There are two kinds of stereoisomers cis-trans isomers and isomers that contain chirality (ky-RAL-i-tee) centers. 

Table I shows that the olefin stereoisomers also give diflFerent 3-heptene ozonide cis-trans ratios when they react with the aldehyde only. The greatest diflFerence is in the hexenes where the cis isomer gives 52% ci5-3-heptene ozonide in this reaction, whereas the trans isomer gives only 40% cw-3-heptene ozonide.

OTONECINE ( 3 OTHER STEREOISOMERS ALSO OCCUR NATURALLY)... 

Detailed interaction studies have a been performed with the neuroleptic drugs cis- and trara-flupentixol. These stereoisomers also act like trifluoperazine as modifiers of multidrug resistance in tumor cell lines where it has been observed that the rra i-isomer is... 

Differentiation between stereoisomers also occurs in drug disposition and is of particular significance for those processes that depend upon a direct interaction between the drug and a chiral biological macromolecule, e.g., active transport processes, binding to plasma and tissue proteins, and drug metabolism [89-95]. 

When the biscobalthexacarbonyl complex 17, prepared from 2-acetoxy-3,4,6-tri-O-acetyl-D-glucal, was treated with NMNO, a Pauson-Khand reaction occurred to give the fused cyclopentenone 18 in virtually quantitative yield. Treatment of 17 with triflic add caused epimerization at C-1, and the stereoisomer also underwent Pauson-Khand cyclization eflfidently. ... 

Cis-trans isomers of cycloalkanes are a type of stereoisomer, also called geometric stereoisomers, in which substituents are on the same side ids) or on the opposite sides trans) of the ring. 

Stereoisomers also include conformational isomers, in which different isomers are generated through rotations about bonds. Conformational isomers are often called con-formers. Eclipsed and staggered ethane are typical examples. Note that a conformational isomer need not be an energy minimum— the eclipsed conformation of ethane is an energy maximum, for example. Conformational isomers can be either enantiomeric or diastereomeric.The two gauche forms of butane are conformational enantiomers, but the gauche and the anti forms of butane are conformational diastereomers (Rg. 4.57). 

More importantly the amines are also responsible for activity in cisplatin-resistant cells (Figure 2.4). This has important clinical implications and the demonstration of the lack of cross resistance in 1,2-diamino-cyclohexane complexes [51] is responsible for much of the interest in developing these particular complexes. This ligand is also of interest from a mechanistic point of view because of the demonstration that different stereoisomers also have different biological properties. 

Cocaine is a tropane-type derivative with several chiral centres and only the (lJ ,2f ,3S,5S)-stereoisomer, also called (—)-cocaine, appears to have appreciable psychostimulant properties. Its primary mode of action is based on blockage of the dopamine transporter which is defined as a directly mediated mechanism. Cocaine is therefore classified as a dopamine blocker rather than dopamine releaser, as is the case with some amfetamines. A large number of 2P,3P-cocaine analogues are well tolerated by the dopamine transporter but not necessarily by the noradrenaline or serotonin counterparts. This means that dopamine reuptake blockage alone is not necessarily sufficient to reinforce consumption, and appropriate stmctural modifications could lead to the development of treatment options for cocaine abuse. 

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