Stereochemical/geometric isomers

This was ihe first investigation of stereoselective formation of geometric isomers of tertiary amines. The authors suggested most enamine systems should be subject to considerable stereochemical control. 

The same stereochemical principles apply to both acyclic and cyclic compounds. With simple cyclic compounds that have little or no conformational mobility, it can even be easier to foUow what is going on. Let us first look at cyclopropane-1,2-dicarboxylic acid. These compounds were considered in Section 3.4.3 as examples of geometric isomers, and cis and trans isomers were recognized. 

The loss of stereochemical memory in the non-vertical excited state implies that stereospecific, concerted reactions of an alkene singlet state may take place from the vertical state. Of particular importance is that the change from cis or trans geometry to something in between opens up a route for converting one geometrical isomer of an alkene to another, and this is a photoisomerization reaction that will be described in the next section. 

The assignment of descriptors and configuration is sometimes arbitrary, at best, when based on model structures and pseudo-atom coordination numbers. A more explicit stereochemical notation is achieved by using this notation, which states within the stereodescriptor the model structure on which the notation is based. In this notation the pseudo-square pyramidal structure is [5PF-5-14-C(i )]. This structure can be expected to result in geometric isomers when one of the... 

The allene complexes 123-125 were also determined to adopt trigonal bipyramidal structures with the allene unit assuming a stereochemically rigid position in the trigonal plane, even at 110 °C. In the case of 1,1 -dimethylallene (124), coordination is exclusively through the CH2=C-double bond,54,55 while for methylallene (125) at least two of the four possible geometric isomers were obtained, though their stereochemistry remains undetermined, due to the complexity of the spectra (Chart 1). 

It is often more convenient, as in the Heck reaction, to use a stable and soluble Pd(II) derivative such as bis(triphenylphosphine)palladium(II) chloride instead of Pd(0). This is rapidly reduced in situ to give a coordinatively unsaturated, catalytically active, palladium(O) species. The geometry of the alkene is generally preserved so that cis (Z) and trans (E) dichloroethylene give the two different geometrical isomers of the enyne below in >99% stereochemical purity as well as excellent yield. 

The chemical and stereochemical purities of synthetic pheromones are often of critical importance for their use in survey and monitoring traps (2,5). The problems associated with inhibitors requires, in general, that the synthetic products have high chemical purity. The fact that a precise mixture of geometrical isomers is usually essential if the synthetic material is to be an effective attractant in the field requires that the synthetic route give a product of reproducible and predictable stereochemical composition. 

The recorded reports of the synthesis of enamines which can exist as geometric isomers are generally characterized by the absence of discussion of the stereochemical constitution of the products. It is likely that, where possible, mixtures of stereoisomers are obtained when employing the general procedures, whose composition is the result of thermodynamic control. 

A stereoselective tandem Sakurai-carbonyl-ene reaction for the synthesis of steroid derivatives has been reported [48]. When LtAlCl2 and la were employed in this cyclization, stereochemical control was different. The cyclization product obtained with la is only 19 (Sch. 17), even though the starting material contained all four geometrical isomers use of LtAlCl2 resulted in a mixture of two different stereoisomers in lower yield. 

The uncatalyzed Diels-Alder reaction is well known to be highly stereospecific, preferentially occurring via syn addition to both the diene and dienophilic components. Stereochemical studies of the cation radical Diels-Alder reaction have confirmed an analogous stereospecificity in two distinctly different systems. The initial study was carried out using the cycloaddition of the three geometric isomers of 2,4-hexadiene as dienophilic components and 1,3-cyclohexadiene as the diene component [39]. Each of the three isomers of the acyclic diene was found to add stereo-specifically to cyclohexadiene. In a more recent study, the cis and trans isomers of 1,2-diaryloxyethenes were found to add stereospecifically to 1,3-cyclopentadiene (Scheme 17) and also to 2,3-dimethyl-l,3-butadiene [46]. 

Azotnethine ylides are planar molecules composed of one nitrogen and two terminal sp carbons. At most, four geometrical isomers are possible for these transient molecules. Their cycloadditions to olefin or acetylene dipo-larophiles give rise to the formation of two sets of carbon-carbon bonds in a single step. Because of the structural complexity of azomethine ylide itself compared to other dipoles and the stereochemical selectivity in the cycloadditions, a number of stereoisomers are possible for the cycloadducts. These two points make azomethine ylides one of the most attractive 1,3-dipoles, both in the fields of ylide chemistry and synthetic organic chemistry. 

Because diastereomeric impurities and degradation products, as well as geometric isomers, present an analytical challenge that is substantially similar to that of other impurities, the FDA policy statement specifically excludes these stereochemical problems. The considerations here are limited to those that are the direct consequence of the analytical problem of simultaneously quantitating molecules that are identical in their physical (except for the rotation of the plane of polarization of light) and most chemical properties. 

Hydrometallation or carbometallation of alkynyl silanes 186 is easier because of the presence of the silicon atom as the metal prefers to be at the silylated end of the resulting alkene 187. Trapping these intermediates with electrophiles allows the stereochemically controlled synthesis of almost any vinyl silane 188. The metal can be Li, Mg, Al, or Cu and both 187 and 188 are single geometrical isomers (whether E or Z depends on priorities of M and the various R groups).30... 

The exclusive formation of the ( )-enamine in spite of the double bond geometry of the starting substrate is another noticeable feature of the isomerization. The present enantioselective isomerization requires prochiral allylamines free from geometrical isomers. In the isomerization, one specific feature is the stereochemical correlation between substrate geometries, product configurations, and the ligand chirality, as shown in Scheme 9. 

A pair of geometric isomers about the alkene can result from the condensation process. Eschenmoser demonstrated that sulfide contraction on the secondary thiolactam produced exclusively the (Z)-isomer (equation 3). Other studies have indicated sole formation of the ( )-isomer using tertiary thiolactams and bromoacetates or a mixture of isomers when a-substituted electrophiles were employed (vide infra). However, most synthetic applications of the Eschenmoser reaction have not stringently identified the geometry about the resulting alkene since the double bond is later reduced or equilibrated in the final product. A systematic investigation to identify factors that may influence the stereochemical outcome has not been reported. 

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