CHAPTER 5: Question 6
Simple Answers
The initial reaction is a straight forward resolution in which the salt
of the (S)-enantiomer of the amino ester and the (R,R)-isomer
of the tartaric acid derivative has a low solubility in water and so crystallizes
from the solution. The diastereomeric salt formed from the (R)-enantiomer
of the amino ester and the (R,R)-dibenzoyl tartrate is more soluble
in water and so does not precipitate from the reaction mixture. The addition
of benzaldehyde to the resulting solution (which will mostly consist of
the (R)-enantiomer of the amino ester) reversibly forms an imine
with the amino group of the amino ester. This increases the acidity of
the hydrogen atom attached to the stereocentre to such an extent that rapid
racemization occurs under the reaction conditions. This forms more of the
(S)-enantiomer of the amino ester which again precipitates from
solution. It is theoretically possible for this process to give a 100%
yield of the (S)-enantiomer of the amino ester complexed to (R,R)-dibenzoyl
tartrate, but in practice a 70% yield of the salt is obtained.
This example is taken from: J.D. Moseley, B.J. Williams, S.N. Owen,
and H.M. Verrier; Tetrahedron: Asymmetry, 1996, 7,
3351-2.
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