Partially OH Depleted Calix[4]arenes

The partially OH depleted dihydroxy- and trihydroxycalix[4]arenes 2 and 3 were synthesized in order to assess the relative importance of the cyclic hydrogen bond on the conformation of p-tert-butylcalix[4]arene. Both compounds were prepared from the same diethyl diphosphate ester precursor under different reductive cleavage conditions. The ring inversion barriers of 2 and 3 are <10 and 11.6 kcal mol^-1, respectively. The completely OH depleted calixarene 5 and the MeOH solvate of dihydroxycalixarene 2 exist in the crystal in a 1,2-alternate conformation. Trihydroxycalixarene 3 crystallizes as a 1:2 pyridine solvate in which 3 adopts a cone conformation. One pyridine molecule is hydrogen bonded to one OH group of 3, while simultaneously being included into the cavity of another molecule of 3. It is concluded that three phenolic units are sufficient to stabilize the cone conformation.