Chiral auxiliary

A chiral auxiliary is a stereogenic group or unit that is temporarily incorporated into an organic compound in order to control the stereochemical outcome of the synthesis. The chirality present in the auxiliary can bias the stereoselectivity of one or more subsequent reactions. The auxiliary can then be typically recovered for future use.

Most biological molecules and pharmaceutical targets exist as one of two possible enantiomers; consequently, chemical syntheses of natural products and pharmaceutical agents are frequently designed to obtain the target in enantiomerically pure form. Chiral auxiliaries are one of many strategies available to synthetic chemists to selectively produce the desired stereoisomer of a given compound.

Chiral auxiliaries were introduced by E.J. Corey in 1975 with chiral 8-phenylmenthol and by B.M. Trost in 1980 with chiral mandelic acid. The menthol compound is difficult to prepare and as an alternative trans-2-phenyl-1-cyclohexanol was introduced by J. K. Whitesell in 1985.

Chiral auxiliaries are incorporated into synthetic routes to control the absolute configuration of stereogenic centers. David Evans' synthesis of cytovaricin, considered a classic, utilizes oxazolidinone chiral auixiliaries for one asymmetric alkylation reaction and four asymmetric aldol reactions, setting the absolute stereochemistry of nine stereocenters.

A typical auxiliary-guided stereoselective transformation involves three steps: first, the auxiliary is covalently coupled to the substrate; second, the resulting compound undergoes one or more diastereoselective transformations; and finally, the auxiliary is removed under conditions that do not cause racemization of the desired products. The cost of employing stoichiometric auxiliary and the need to spend synthetic steps appending and removing the auxiliary make this approach appear inefficient. However, for many transformations, the only available stereoselective methodology relies on chiral auxiliaries. In addition, transformations with chiral auxiliaries tend to be versatile and very well-studied, allowing the most time-efficient access to enantiomerically pure products.

Furthermore, the products of auxiliary-directed reactions are diastereomers, which enables their facile separation by methods such as column chromatography or crystallization.

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