Optically pure calix[6]tris-ammoniums: syntheses and host-guest properties toward neutral guests.
[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.
Darbost
Ulrich
U
URCOM, Université du Havre, FST, 25 rue Philippe Lebon, BP 540, 76058 Le Havre Cedex, France.
Zeng
Xianshun
X
Giorgi
Michel
M
Jabin
Ivan
I
D047250:Calixarenes / Q000138:chemical synthesis* / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D010636:Phenols / Q000138:chemical synthesis* / Q000737:chemistry; D000644:Quaternary Ammonium Compounds
D010636:Phenols; D000644:Quaternary Ammonium Compounds; C476540:calix(6)arene; D047250:Calixarenes