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  • Title: Optically pure calix[6]tris-ammoniums: syntheses and host-guest properties toward neutral guests.
    Author: Darbost U, Zeng X, Giorgi M, Jabin I.
    Journal: J Org Chem; 2005 Dec 09; 70(25):10552-60. PubMed ID: 16323870.
    Abstract:
    [reaction: see text] Optically pure calix[6]arenes bearing chiral amino arms 4, 7, and 10 have been synthesized in high yields from the known symmetrically 1,3,5-trismethylated calix[6]arene. For both compounds 7 and 10, the key step consists of an efficient selective alkylation on the narrow rim of the calix[6]arene with Ba(OH)2 as the base. All of these chiral calix[6]tris-amines possess a similar flattened cone conformation with the cavity occupied by the methoxy groups. In contrast to 4 and 7, upon protonation, the enantiopure calix[6]arene 10 can switch to the opposite flattened cone conformation through self-assembly of its ammonium arms in an ion-paired cap which closes the cavity. As shown by NMR host-guest studies and an X-ray structure, the obtained polarized host (10 x 3H+) behaves as a remarkable endo-receptor for small polar neutral molecules. Thanks to the tris-cationic chiral binding site of 10 x 3H+, it was shown that the guests experience a chiral environment upon inclusion. Finally, the first example of enantioselective molecular recognition inside the cavity of a calix[6]arene has been evidenced with a racemic 1,2-diol guest.
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