Configurations of stereocenters

Stereochemistry is all about configurations of stereocenters (R vs. S) and double bonds (E vs. Z). Whenever we have a reaction where we are forming a stereocenter, we need to ask whether we get a racemic mixture (equal amounts of R and S) or only one configuration. And, if so, why Also, whenever we form a double bond, we need to ask whether we get both E and Z isomers or only one of them And, if so, why ... 

The related configurations of stereocenters in substituted cyclic nitronates can be determined by analyzing the spin—spin coupling constants between the vicinal protons in the stereoisomer discussed (Chart 3.7) (276). If needed, the results of this analysis are supplemented by special NOE experiments. 

Deoxygenation of Cyclic Nitronates Since cyclic nitronates are readily available and are rather stable compounds (see Sections 3.2 and 3.3), simple approaches for their deoxygenation with retention of the configuration of stereocenters can be used for the synthesis of cyclic esters of functionalized oximes. 

Reactions of Cyclic Conjugated Ene Nitroso Acetals with Nucleophiles The reaction of six-membered cyclic ene nitroso acetals (475) with nucleophiles has a high potential and will probably provide the basis for a promising procedure for the synthesis of polyfunctional compounds from very simple precursors, the more so that the configurations of stereocenters in the starting nitronate (475) can be retained in particular transformations (Scheme 3.253). Unfortunately, the available data (264) are insufficient to elucidate the complete mechanism of this process. 

It is very likely, that this reaction occurs due to the equilibrium between trimethylsilyl halide and a nitrogen-containing nucleophile, which increases the electrophilicity of silyl Lewis acids. It should be noted that the configuration of stereocenters at the carbon atoms of the oxazine ring is partially distorted. Hence, it is assumed that the reaction proceeds through the intermediate cation B, which is partially isomerized into the stereoisomeric cation B through the open chain cation B". 

In practice, both the conformation and configuration of stereocenters of a drug are often studied together when the shape of a molecule is being idealized for target binding. An excellent example of this type of problem is the study of cholinergic activity of acetylcholine. 

The most obvious way to synthesize these compounds is through co-halocarboxamides as key intermediates. Such a reaction has been performed by retention of configuration of stereocenters within the molecule by treatment of the halide with a base (equation 168)928-930. Better yields and lactams with larger rings have been prepared using PTC931. 

Concerning the relative configuration of stereocenters in the products, the nomenclature used here is (usually) the syn/anti convention currently used for acyclic aldols and related compounds (6-8). To apply this convention, the carbon backbone is drawn in its longest extended (zigzag) form such as for 1.1 (Scheme 1). If both substituents on the chain project in the same direction from the plane of the carbon backbone as in 1.2 or 1.3, then the descriptor is syn. If the two substituents project in opposite directions from the plane of the carbon backbone as in 1.4 or 1.5 the descriptor is anti.f... 

Homofacial, heterofacial The relative configuration of stereocenters (in different molecules) having three identical ligands and one different is homofacial if the fourth ligand is on the same heterotopic face in both, and heterofacial if on opposite faces, as shown below. [91,92,101]. See also relative configuration. 

Tacticity describes only local, relative configurations of stereocenters. The terms are best defined pictorially, as in Figure 6.16. Thus, isotactic polypropylene has the same configuration at all stereocenters. Recall the two faces of propylene are enantiotopic, and the isotactic polymer forms when all new bonds are formed on the same face of the olefin. If, instead, there is an alternation of reactive faces, the polymer stereocenters alternate, and a syndio-tactic polymer is produced. Finally, a random mixture of stereocenters produces atactic polymer. 

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