Chiral barrier hosts

Attachment of bulky chiral barriers to achieve enantiomeric selection is exemplified by ligands (61),204 (63)69 and (64).19 Ligands of type (61) are especially efficient chiral compounds with binap-thyl hinges . The orientation of the binaphthyl rings for (61) is shown more explicitly in (62).234 With optically pure (61), complete enantiomeric separation of ammonium salt racemates such as (67) is obtained.235-237 The structure has been determined crystallographically for the less stable d form of the PF6- salt of (67) with the (S,S) host (61). The results indicate that the receptor, substrate, counterion and solvent have undergone several steric concessions in order to relieve the strain imposed on the system by the visit of an unwanted guest.238... 

Peacock, S. C., Domeier, L. A., Gaeta, F. C. A., Helgeson, R. C., Timko, J. M., Cram, D. J., Host-guest complexation. 13. High chiral recognition of amino esters by dilocular hosts containing extended steric barriers. J. Am. Chem. Soc. 1978, 100, 8190-8202. 

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