Catalytic Asymmetric Synthesis Sharpless Oxidations of Allylic alcohols

6 Catalytic Asymmetric Synthesis Sharpless Oxidations of Allylic alcohols 

Asymmetric syntheses can be carried out even more easily and elegantly than by reacting achiral substrates with enantiomerically pure chiral reagents if one allows the substrate to react with an enantiomerically pure species formed in situ from an achiral reagent and an enantiomerically pure chiral additive. The exclusive reaction of this species on the substrate implies that the reagent itself reacts substantially slower with the substrate than its adduct with the chiral additive. If high stereoselectivity is observed, it is exclusively due to the presence of the additive. The chiral additive speeds up the reaction. This is an example of ligand accelerated asymmetric catalysis. 

This type of additive (or ligand) control of stereoselectivity has three advantages. First of all, after the reaction has been completed, the chiral additive can be separated from the product with physical methods, for example, chromatographically. In the second place, the chiral additive is therefore also easier to recover than if it had to be first liberated from the product by means of a chemical reaction. The third advantage of additive control of enantioselectivity is that the enantiomerically pure chiral additive does not necessarily have to be used in stoichiometric amounts catalytic amounts may be sufficient. This type of catalytic asymmetric synthesis, especially on an industrial scale, is important and will continue to be so. 

The most important catalytic asymmetric syntheses include addition reactions to C=C double bonds. One of the best known is the Sharpless epoxidations. Sharpless epoxidations cannot be carried out on all alkenes but only on primary or secondary allylic alcohols. Even with this limitation, the process has seen a great deal of application. 

There are two reasons for this. First, the Sharpless epoxidation can be applied to almost all primary and secondary allylic alcohols. Second, it makes trifimctional compounds accessible in the form of enantiomerically pure ,/1-epoxy alcohols. These can react with a wide variety of nucleophiles to produce enantiomerically pure second-generation products. Further transformations can lead to other enantiomerically pure species that ultimately may bear little structural resemblance to the starting a,/1-epoxy alcohols. 

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