The design and synthesis of chiral crown ethers possessing a chiral recognition ability which carry great potential for separation of enantiomers and analytical purpose has become an important and rapidly growing field of host-guest chemistry. One of the advantages of crown ethers constructed using a synthetic chiral building block is that it is rather easy to modify the chiral cavity resulting in the improvement in enantiomer selectivity. Cyclohexane-1,2-diol derivatives such as cis-1-phenylcyclohexane-1,2-diol, trans-1-phenylcyclohexane-1,2-diol and trans-1,2-diphenylcyclohexane-1,2-diol are of interest as chiral building blocks of crown ethers, because incorporation of these subunits into the 18-crown-6 framework causes a reduction in conformational flexibility of the chiral cavity and fixes the phenyl chiral barrier perpendicularly above the face of crown ring to modify enantiomer recognition ability. Chiral crown ethers of the 18-crown-6 type were synthesized using these building blocks and their chiral recognition behaviour in differential enantiomer transport through bulk liquid membranes were examined. Chiral azophenolic crown ethers containing cis-1-phenylcyclohexane-1,2-diol or cis-cyclohexane-1,2-diol which can bind neutral amines to form a stable complex and exhibit enantiorecognitive coloration in complexation with chiral amines were prepared and the association constants for their complexes with chiral ethylamine and ethanolamine derivatives were determined on the basis of UV-visible absorbance. The observed enantioselectivity is rationalized in terms of complementarity between a host and a guest as indicated by CPK molecular model examination. The alternation of the position of chiral barriers resulted in a reversal of the enantioselectivity. Crown ethers with diastereotopic faces can bind a guest to each side of the diastereotopic faces to form diastereoisomeric complexes which occasionally cause troublesome "sided-ness" problems. In order to avoid the problems, most crown ethers previously prepared contained at least one C2 axis of symmetry. However, diastereotopic face selectivity in complexation does provide helpful information on the complexing behaviour of crown ethers to assist in the design of more elaborate and structured host molecules. meso-Crown ethers having cis-1-phenylcyclohexane-1,2-diol were prepared and the diastereotopic face selectivity by their diastereotopic faces in complexation with achiral amines was examined by temperature-dependent 1H nuclear magnetic resonance. Ethanolamine is attached stereoselectively to one of the faces to give one of diastereoisomeric complexes and the alternation of the position of steric barriers led to reversal of the diastereotopic face selectivity. A prediction of which diastereoisomeric complex is formed is made on the basis of CPK molecular model.
- Chiral azophenolic crown ether
- Chiral crown ether
- Diastereotopic face selectivity in complexation
- Enantioselectivity in complexation
- Meso-crown ether