Expanding cavitand chemistry: The preparation and characterization of [n]cavitands with n >= 4

The preparation of cavitands composed of 4, 5, 6, and 7 aromatic subunits ( [n]cavitands, n = 4-7) is described. The simple, two-step synthetic procedu re utilized readily available starting materials (2-methylresorcinol and di ethoxymethane). The two cavitand products having 4 and 5 aromatic subunits exhibited highly symmetric cone conformations, while the larger cavitands ( n = 6 and 7) adopt conformations of lower symmetry, H-1 NMR spectroscopic s tudies of [6]cavitand and [7]cavitand revealed that these hosts undergo exc hange between equivalent conformations at room temperature. The departure o f these two cavitands from cone conformations is related to steric crowding on their Ar-O-CH2-O-Ar bridges and is predicted by simple molecular mechan ics calculations (MM2 force field). X-ray diffraction studies on single cry stals of the [4]cavitand, [5]cavitand, and [6]cavitand hosts afforded addit ional experimental support for these conclusions.