Regioselective double intramolecular bridging of p-tert-butylcalix[7]arene

The first examples of doubly bridged calix7]arenes 2a-h have been obtained by base-promoted O-alkylation of p-tert-butylcalix7]arene 1 or 1,4-tetramethylene-bridged calix7]arene 3a with a variety of bis-electrophiles including BrCH₂Cl, oligoethylene glycol ditosylates, and 1,2-bis(bromomethyl)benzene. In the presence of Cs₂CO₃ as the base, in acetone, the syn-1,4:2,3-bis-bridged regioisomer was obtained in yields up to 76%. Assignment of bridging pattern was based on chemical shift of OH groups in conjunction with chemical correlations with known compounds. Stereochemical and conformational features were investigated with the aid of 2D and Dynamic NMR studies and MM3 calculations.