A “Classical” Tetrahydroxycalix[4]arene Adopting the 1,2-Alternate Conformation

The first example of a “classical” tetrahydroxycalixarene, which adopts the 1,2-alternate conformation both in solution and in the crystal, is described. Calixarene derivatives with two distal
methylene groups substituted in a trans fashion by phenyl (5a) or mesityl (5b) groups were
synthesized via addition of PhMgBr/CuCN or MesMgBr/CuCN to the bis(spirodiene) derivative 3.
Whereas the phenyl-substituted calixarene derivative 5a adopts the usual “cone” conformation,
solution NMR data and X-ray crystallography indicate that the more crowded mesityl derivative
5b adopts a 1,2-alternate conformation with the two mesityl groups located at isoclinal positions
of the macrocycle.