Chiral host molecules racemic mixtures

A different non-classical approach to the resolution of sulphoxides was reported by Mikolajczyk and Drabowicz269-281. It is based on the fact that sulphinyl compounds very easily form inclusion complexes with /1-cyclodextrin. Since /1-cyclodextrin as the host molecule is chiral, its inclusion complexes with racemic guest substances used in an excess are mixtures of diastereoisomers that should be formed in unequal amounts. In this way a series of alkyl phenyl, alkyl p-tolyl and alkyl benzyl sulphoxides has been resolved. However, the optical purities of the partially resolved sulphoxides do not exceed 22% after... 

Molecules in this section fall into two general categories those that are inherently achiral, and those that, in their most stable thermodynamic state, are racemic mixtures. All have at least one chromophore. The analytical approach is the same for both. They are made to form derivatives by reaction with a chiral host. In the case of racemates the derivatives are diastereomers and methods for their analytical applications have been referenced in earlier sections. Regardless of the distinction between racemic and non-racemic substances, the induced CD effects are uniformly smaller than the effects inherent to chiral molecules by one to two orders of magnitude. 

We highlight here a few studies in which the synthesis of chiral molecules has been achieved through the use of organic crystals in the hopes that this will prove a useful incentive and review. The reported studies fall into two natural categories. In the one case one starts with racemic mixtures or optically inactive compounds, crystallize these materials into chiral crystals and finally by subsequent reactions, trap this chirality in the final chemical products. In the second category one forms host-guest inclusion compounds in which the host is already an optically resolved compound. This in turn leads to the formation of optically active guest molecules. 

Using enantiomeric pure chiral hosts, the separation of racemic substrates hopefully will be possible soon. It seems of great advantage in this context that the guests must not necessarily bear functions, but can be hydrocarbons, heterocycles, and so on. This will also include immobilized hosts bound to resins. Then not only enantiomer separations will be possible, but affinity-type chromatography will allow more specific separations of guests, which can be separated of a mixture of similar substrate molecules. 

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