Chirality control principles

As in the case of P-3CRs, any of the four components can in principle, if chiral, control the generation of the new stereogenic center (with the exception of the isonitrile if mechanism B is operating). To date most efforts have been carried out with chiral amines, partly because removal of the chiral auxiliary is in this case easier and leads to synthetically useful secondary amides (instead of the tertiary amides usually obtained by the classical U-4CR). 

Green, B.S., Lahav, M., and Schmidt, G.M.J., Reactions in chiral crystals. Principles governing asymmetric synthesis via topochemicaUy controlled sohd-state photodimerization, Mol. Cryst. Liq. Cryst, 29, 187,1975. 

The stereogenic centers may be integral parts of the reactants, but chiral auxiliaries can also be used to impart facial diastereoselectivity and permit eventual isolation of enantiomerically enriched product. Alternatively, use of chiral Lewis acids as catalysts can also achieve facial selectivity. Although the general principles of control of the stereochemistry of aldol addition reactions have been well developed for simple molecules, the application of the principles to more complex molecules and the... 

In principle, asymmetric synthesis involves the formation of a new stereogenic unit in the substrate under the influence of a chiral group ultimately derived from a naturally occurring chiral compound. These methods can be divided into four major classes, depending on how this influence is exerted (1) substrate-controlled methods (2) auxiliary-controlled methods (3) reagent-controlled methods, and (4) catalyst-controlled methods. 

According to this correlation model, in which the principles of steric control of asymmetric induction at carbon (40) are applied, the stereoselectivity of oxidation should depend on the balance between one transition state [Scheme 1(a)] and a more hindered transition state [Scheme 1(6)] in which the groups and R at sulfur face the moderately and least hindered regions of the peroxy acid, respectively. Based on this model and on the known absolute configuration of (+)-percamphoric acid and (+)-l-phenylperpropionic acid, the correct chirality at sulfur (+)-/ and (-)-5 was predicted for alkyl aryl sulfoxides, provided asymmetric oxidation is performed in chloroform or carbon tetrachloride solution. Although the correlation model for asymmetric oxidation of sulfides to sulfoxides is oversimplified and has been questioned by Mislow (41), it may be used in a tentative way for predicting the chirality at sulfur in simple sulfoxides. 

To illustrate this point it is particularly instructive to examine the configurational defects existing in isotactic polypropylene. The steric control during polymerization can in principle be attributed to two different factors the influence of the chirality of the last entered unit (this case falls within the discussion of symmetric chains reported previously) or that of the catalytic site. The chirality of the latter may be preexisting or may arise at the moment of polymer-... 

The overall observed retention of the enantiomers, and thus the elution order, is based on several kinetically and thermodynamically controlled parameters concerned with stereorecognition nonstereoselective interactions of all partners SO(R), SA(R S), and particularly of the [SO(RI-SA(KI] and [SO(K)-SA(Si] complexes with the achiral stationary phase, also play a role (Figure 21). Therefore the retention order may be reversed for a specific pair of enantiomers depending on whether a covalently bound CSP or a CMPA is applied, but using the same chiral molecule (part) as chiral selector. These general principles, shown schematically for a CLEC system, are further complicated by the complexity of the entire system, hence they are difficult to anticipate and each case must be studied individually. 

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