Stereochemical information

To code the configuration of a molecule various methods are described in Section 2.8. In particular, the use of wedge symbols clearly demonstrates the value added if stereodescriptors are included in the chemical structure information. The inclusion of stereochemical information gives a more realistic view of the actual spatial arrangement of the atoms of the molecule imder consideration, and can therefore be regarded as between the 2D (topological) and the 3D representation of a chemical structure. 

Clearly, the next step is the handling of a molecule as a real object with a spatial extension in 3D space. Quite often this is also a mandatory step, because in most cases the 3D structure of a molecule is closely related to a large variety of physical, chemical, and biological properties. In addition, the fundamental importance of an unambiguous definition of stereochemistry becomes obvious, if the 3D structure of a molecule needs to be derived from its chemical graph. The moleofles of stereoisomeric compounds differ in their spatial features and often exhibit quite different properties. Therefore, stereochemical information should always be taken into ac-count if chiral atom centers are present in a chemical structure. 

The reaction of vinyloxiranes with malonate proceeds regio- and stereose-lectively. The reaction has been utilized for the introduction of a 15-hydroxy group in a steroid related to oogoniol (265)(156]. The oxirane 264 is the J-form and the attack of Pd(0) takes place from the o-side by inversion. Then the nucleophile comes from the /i-side. Thus overall reaction is sT -StM2 type, in the intramolecular reaction, the stereochemical information is transmitted to the newly formed stereogenic center. Thus the formation of the six-membered ring lactone 267 from 266 proceeded with overall retention of the stereochemistry, and was employed to control the stereochemistry of C-15 in the prostaglandin 268[157]. The method has also been employed to create the butenolide... 

Enantioselective processes involving chiral catalysts or reagents can provide sufficient spatial bias and transition state organization to obviate the need for control by substrate stereochemistry. Since such reactions do not require substrate spatial control, the corresponding transforms are easier to apply antithetically. The stereochemical information in the retron is used to determine which of the enantiomeric catalysts or reagents are appropriate and the transform is finally evaluated for chemical feasibility. Of course, such transforms are powerful because of their predictability and effectiveness in removing stereocenters from a target. 

In summary, modem synthetic methodology allows the stereoselective generation of one, two, or even more stereocenters in a single reaction with or without spatial control by the substrate. The application of transforms to retrosynthetic simplification of stereochemistry requires the selection of transforms on the basis of both structural and stereochemical information in the target and also validation of the corresponding synthetic processes by analysis for both chemical feasibility and stereoselectivity. 

Stereochemistry deals with the three-dimensional ariangement of a molecule s atoms, and we have attempted to show stereochemistry with wedge-and-dash drawings and computergenerated models. It is possible, however, to convey stereochemical information in an abbreviated form using a method devised by the German chemist Emil Fischer. 

On the other hand, the fluorine-induced addition of the diastereomeric silyl-subsliluted sulfides 36 A and 36B to benzaldehyde proceeds without loss of stereochemical information and with retention of configuration32. Since, however, the anionic reagent 35A/35B is known to be configurationally labile, the observed retention of configuration in the fluorine-induced desi-lylative hydroxy alkylation lends experimental evidence to the notion that these reactions proceed via hypervalent silicon species rather than anionic reagents. 

It should be stressed that this treatment of polymer stereochemistry only deals with relative configurations whether a substituent is "up or down" with respect to that on a neighboring unit. Therefore, the smallest structural unit which contains stereochemical information is the dyad. There are two types of dyad meso (m), where the two chiral centers have like configuration, and racemic /-), where the centers have opposite configuration (Figure 4.1). 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

PROBLEMS Over the course of your studies, you will learn mechanisms for each of the following reactions. In the meantime, you will be given the stereochemical information that you need to answer each of the following problems. These problems are intended to ensure that you understand what stereochemistry means. 

Stereochemistry. The stereochemical information contained in the input data must be handled correcdy. 

Homonuclear 19F-TOCSY and 19F-NOESY experiments are utilized much less frequently than 19F-COSY, but as discussed in the Battiste and Newmark review,23 they are very much underutilized experiments, which when implemented can provide unique structural and particularly stereochemical information. 

HPLC-NMR analysis in a closed-circuit reveals the stereochemical information for elucidating the structures of unknown compounds (Albert 2002). In contrast to the technique of off-line separation, sample collection, and peak identification closed-circuit analysis guarantees the absence of isomerization and degradation. Very often only small amounts of sample are available after extraction. 

While electrochemical cleavage of several other kinds of single bonds has been reported, stereochemical information is sparse. Erickson and Fischer examined the electrochemical reduction of O-benzoylatrolactic acid (76, R = H) and its methyl ester (76, R = CH3) 69>. The products (77, R = H and CH3) of the... 

The CAS Registry numbers are the key to the full structural and stereochemical information held within the CAS chemical identification system, available both in printed form and on-line via several hosts worldwide. 

As seen from the above theoretical developments, accessing geometrical (and stereochemical) information implies at least an estimation of the dynamical part of the various relaxation parameters. The latter is represented by spectral densities which rest on the calculation of the Fourier transform of auto- or cross-correlation functions. These calculations require necessarily a model for describing molecular reorientation... 

From a cholesteric induction experiment, one can obtain chiral information on the induced cholesteric (namely, pitch and handedness) and therefore the helical twisting power of the dopant in that solvent (at a certain temperature). If a model or molecular theory relating molecular chirality to mesophase chirality is available, one can infer stereochemical information about the dopant (absolute configuration, preferred conformation). 

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