CHAPTER 5: Question 4

Simple Answers

This starts off as a simple resolution. Then, when the sodium ethoxide is added, the (S)-enantiomer which remained in solution is rapidly racemized and the (R)-enantiomer which is thus formed again precipitates from solution as a salt with (-)-quinine. Reaction schemes and energy diagrams are shown below. Note, that the (R)-acid-(-)-quinine salt may be more or less stable than the (S)-acid-(-)-quinine salt. It is not stability that determines which salt is isolated but solubility.

If (+)-quinine was used instead of (-)-quinine, then the (S)-enantiomer of the acid would crystallize out of the solution as a salt with the quinine.

This example is taken from: J.Y.L. Chung, D.L. Hughes, D.L. Zhao, Z.G. Song, D.J. Mathre, G.J. Ho, J.M. McNamara, A.W. Douglas, R.A. Reamer, F.R. Tsay, R. Varsolona, J. McCauley, E.J.J. Grabowski, and P.J. Reider; J. Org. Chem., 1996, 61, 215-22.

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