Immolative process chiral auxiliaries

In general, the recycling is desirable, so the chiral auxiliary should be easy to recover without any loss of enantiomeric purity. However, if the auxiliary is inexpensive, it need not be recycled. In some cases, the chiral auxiliary is actually destroyed during the cleavage reaction to produce the final product. Such transformations are called immolative processes. 

If the auxiliary is bound covalently to the enolate, the cleavage from the product is frequently a crucial step and as important as the asymmetric synthesis itself. Clearly, any racemization or epimerization of the newly created stereogenic centers but also in the auxihary has to be avoided, and the cleavage should be feasible in nearly quantitative yield. Equally important is the recovery of the chiral auxiliary that should be reusable for reasons of sustainability. This holds in particular for larger-scale processes. Early immolative methods that destroy the auxiliary and produce extra waste for disposal will be mentioned only briefly, as they are considered as obsolete and hardly acceptable under the aspect of chiral economy [1]. 

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