Conformation and Stereodynamics of Monodioxamethylene Calix[4]arene Derivatives

Reaction of the monospirodienone derivative of p-tert-butylcalix[4]arene with methylene chloride/base in the presence of a phase transfer catalyst yielded the bridged monodioxamethylene derivative 6 which was characterized by X-ray crystallography. Treatment of 6 with NaBH₄ resulted in exo-reduction of the carbonyl group and afforded the spiro alcohol 7, which rearranged thermally to the monodioxamethylene bridged calix[4]arene 3. Calixarene 3 exists in solution (CD₂Cl₂, 198 K) in three different conformations (a cone form with the dioxocine unit in a distorted boat conformation, and cone and partial cone forms with a boat-chair arrangement of the dioxocine) which interconvert with barriers in the 13.5-14.3 kcal mol^-1 range. Molecular mechanics calculations indicate that the rate-limiting steps for the conformational interconversions of 3 involve transitions of the dioxocine subunit.