Example crystallization stereoisomer separation

Indeed, the concept of stereochemistry is almost as old as organic chemistry itself. In 1848, Louis Pasteur (France 1822-1895) found that tartaric acid (9) existed in two forms that we now know differ only in their ability to rotate plane-polarized light in different directions (they are examples of stereoisomers). Pasteur observed that the crystals had a different morphology, defined here as their external structure, and he was able to separate these two forms by peering through a microscope and using a pair of tweezers to separate them physically. Because of this difference, the two forms of tartaric acid are considered to be different compounds, now called enantiomers (see Chapter 9). 

Impurities, added or unintentional, can have a major effect on rates of nucleation and crystal growth. Table 4-1 shows the effect of an impurity, structurally similar to the crystallizing solute, added to an all-growth crystallization (separation of stereoisomers. Examples 7-6 and 11-6). The data for a continuous stirred tank (CSTR) operation show a sevenfold decrease in the first order growth rate constant as a result of addition of this impurity to prevent nucleation of the undesired isomer. 

The diastereomeric differentiations in the preceding cases involve physical interactions in chromatography or crystallization. Another possibility is to use a chemical reaction that is fast with one stereoisomer and slow with the other (kinetic resolution). A strain of baker s yeast engineered to express cyclohexanone monooxygenase was used to effect a Baeyer Villiger reaction on a variety of 2-alkylcyclohexanones, an example of which is shown in Equation 3.4 [23]. The ketone and lactone are readily separated by silica gel chromatography. 

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