Stereochemical transformations

Up to this point, we have emphasized the stereochemical properties of molecules as objects, without concern for processes which affect the molecular shape. The term dynamic stereochemistry applies to die topology of processes which effect a structural change. The cases that are most important in organic chemistry are chemical reactions, conformational changes, and noncovalent complex formation. In order to understand the stereochemical aspects of a dynamic process, it is essential not only that the stereochemical relationship between starting and product states be established, but also that the spatial features of proposed intermediates and transition states must account for the observed stereochemical transformations. 

The Mitsunobu reaction offers a powerful stereochemical transformation. This reaction is very efficient for inverting the configuration of chiral secondary alcohols since a clean SN2 process is generally observed ( Mitsunobu inversion ). Considering the fact that Mitsunobu chemistry is typically carried out at or below room temperature, high-temperature Mitsunobu reactions performed under microwave con-... 

Scheme 5 Synthesis of a t )[CH2-S] Pseudopeptide with the Natural l-l-Amino Acid Configuration Assuming the Stereochemical Transformations as Indicated 36 ...

In analogy to the transformations observed for other compounds, a similar process was proposed for the interconversion of thiaspirane sulfoxides and to explain the dependence of the thermolytic process on sulfoxide stereochemistry. Scheme 1 shows the probable mechanism of the transformation. Stereochemical transformations of the C30 Nuphar thiaspiranes have been observed on quarter-nization of thiobinupharidine (16) (equatorial sulfur atom). The quaternized quinolizidine system is transformed from the trans to the cis form with inversion of the relative configuration of the corresponding C-7 or C-7. ... 

An analogous approach was used to show that galactose-1-P uridylyltransferase catalyzes the interconversion of UDP-glucose and galactose- -P with UDP-galactose and glucose-1-P with retention of configuration at PQ [21]. The stereochemical transformations used in the analysis are shown in Equation 14. 

Figure 10.46 Combinatorial biosynthesis manipulation of the ring size and the stereochemical transformations of DEBS modules to produce analogues 10.77 and 10.78.

Ill the presence of hydrogenation catalysts, unsaturated compounds undergo not only hydrogenation but also isomerization—shift of double bonds, or stereochemical transformations—which also affects physical and chemical properties.)... 

After the necessary stereochemical transformation is performed, the newly formed double bond is reduced and the ketone converted to a tribromo derivative. The bromine at Cs will be displaced by the adjacent hydroxyl group in a nucleophilic reaction, forming a stable five-membered ring. The double bond between C and Ca was formed in the same way as before. A possible mechanism for this transformation is as follows ... 

To allow for better handling of stereochemical transformations, such as epimeris-ations. 

Stereochemical transformations are important reactions in modem chemistry databases, and so two new properties, associated with atom-atom mapping, were... 

The existence of atom-atom mappings allowed the REACCS development team to implement other related searching features as well, including reactions involving hydrogens and stereochemical transformations. These features have made searching for such reactions much more convenient or, in some cases, made available searches which were previously impossible in REACCS. 

The abnormal ratio of the stepwise constants in dimethyl sulphoxide solution is a result of a stereochemical transformation caused by the complex formation. 

The stereochemical transformation of the mercury halide complexes in the course of the stepwise complex formation could also be followed by means of X-ray examinations [Ga 68]. The results at times cast doubt on the correctness of the conclusions derived from the equilibrium data. In the case of mercury halides, the stability of the third stepwise-formed complex is much higher than that of the fourth, and the value of X3/X4 is therefore large. This would indicate that the former complex already has tetrahedral symmetry. According to the X-ray examinations, however, the reason for the anomalous stability sequence is that the second complex is linear, the third is pyramidal and only the fourth has tetrahedral symmetry [Ga 68]. 

An interesting aspect of the Ni(II) complexes is that stereochemical transformations occur rather easily among them, since the free energy differences between the various stereochemistries are small. Therefore, equilibria among different structural types, which are temperature and concentration dependent, often exist in solution. 

The way in which stereochemical transformations effect chemical rebonding... 

Aldose-Ketose Isomerases.—The reactions catalysed by aldose-ketose isomerases, including proton-migration, stereochemical transformations, amination, and deamination, etc., have been reviewed. ... 

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