Resolution by preferential crystallisation

About 90% of all racemates are racemic compounds, consisting of a perfectly ordered array of R and S molecules. Conglomerates, on the other hand, consist of a mixture of crystals of the two enantiomers in equal amounts. Conglomerates are in principle amenable to resolution by direct crystallisation, a simple, non-waste producing technique. Where the racemate is available, the existence of conglomerates should always be 

Direct crystallisation techniques are widely used for industrial scale resolution, e.g. the manufacture of a-methyl-L-dopa [21] and chloramphenicol [22]. 

The method is particularly attractive when it is accompanied by spontaneous in situ racemisation of the enantiomer remaining in solution, allowing for a theoretical yield of 100%. This is often referred to as deracemisation. An industrially relevant example is shown in equation 7.2, whereby a synthetic pyrethroid undergoes base catalysed epimerisation at the CH group adjacent to the cyano group, precipitating the S,R,R, isomer exclusively, in a 98.6% yield [23]. 

Clearly, then, industrial scale deracemisation processes are attractive both economically, and in the sense that no extra waste is generated. 

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