how is their use to synthesize the desired enantiomer of a drug

not positive, but i believe that the auxiliary is added in the middle of the process to form an "unnecessary" intermediate. by unnecessary i just mean, not needed to produce a racemic mixture. Then, this auxiliary is then removed by substitution to form the desired product. When this is done, the auxiliary only allows one enantiomer to primarily form because of steric hindrance - in this case only allowing the substitution to occur in one direction.

This post was edited by cialda on Apr 9 2012 07:17am

Pretty good summary. I'll see if i can add anything maybe

Basically, its impossible to make a chiral molecule without the influence of some chiral molecule to begin with, mostly because of the randomization of stereochemistry brought about in many forms of substitution, etc. The method Rice speaks of substitutes an easy to make chiral molecule to guide the synthesis of a more complex one (ex: the one changes the course of the reaction by making the original reactant bend in a certain way that allows the formation of only one chiral product).

This post was edited by EndlessSky on Apr 9 2012 07:03pm