Synthesis and conformational property of tannin-like p-tert-butylcalix[4]arene 1,3-diesters stabilized by intramolecular hydrogen bonds

Tannin-like-p-tert-butylcalix[4]arene 1,3-digallate was synthesized, and it s conformational property was investigated by dynamic H-1 NMR and X-ray cry stallography. It was found that the 3-OH (or 5-OH) group of the galloyl gro up in p-tert-butylcalix[4] arene 1,3-digallate is placed at the position wh ere an unusual nonbonded close contact is observed between the OH group and the aromatic ring of the galloyl group facing each other. The calixarene 1 ,3-diesters of various hydroxybenzoic acids were also prepared, and the con formational properties of those calixarenes were compared with that of p-te rt-butylcalix[4]arene 1,3-digallate. A significant contribution of the 3- a nd 5-OH groups in pendant groups toward the close contact was found. It was suggested that the conformation of p-tert-butylcalix[4]arene 1,3-digallate was stabilized by intramolecular hydrogen bonds including (OHO)-O-... and OH-pi interactions.