Corands chiral

Also of interest is the different behavionr observed in the comparison of the macrocycle 12.11 with the podand 12.12 (Table 12.1). 

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This technique is now used extensively to assess enantiomeric excesses in organic reactions and separate small quantities of enantiomers. Closely related chiral corands are particularly useful in assessing the optical purity of amino acids, although modern chiral columns for HPLC may cost in excess of US 2000. 

We saw in Section 12.3.1 the use of the cyclodextrins as mimics for transacylases, a well-understood class of enzymes that perform the task of transferring an acyl group from one substrate to another (e.g. from an ester to water). Transacylase chemistry has also been addressed by Cram,5 who used chiral corands, such as 12.11, related to 3.106 bearing thiolate nucleophiles situated above and below the plane of the macrocycle. An acyclic analogue 12.12 was also prepared for comparison. The salient features of 12.11 are shown diagrammatically in Figure 12.4. 

Figure 12.4 Key features of chiral corand transacylase mimics.

The chiral recognition ability of 4.2 was examined by two-phase liquid-liquid extraction experiments of racemic mixtures of amino acids and ester guests. Cram and coworkers developed an enantiomer-resolving machine based on such chiral corand receptors and found that the /f,/f-host was selectively able to extract the D-enantiomer of amino acids. 

In the solid state one half of the 16-membered cycle, -CH2P(CH2)3PCH2N-has a distorted crown conformation with a nearly axial orientation of the lone pairs of the heteroatoms, and the other half is its inverted (or pseudoinverted for the chiral corands 103) with the lone pairs pointing to the opposite sides of the macrocycle. Two phosphorus atoms of each half have different configurations [95, 96, 98]. 

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