Conversion of 1,3-Alternate to Partial Cone Conformation of tetra-Substituted Calix[4]arene

Due to the nonplanar structure, calix[4]arenes can exist in one of the four conformations, and has been designated1 as cone, partial cone, 1,2-alternate, and 1,3-alternate. By plac-ing substituents at OH groups larger than methyl, conforma-tion can be locked. Very often cone and partial cone con-formers were synthesized by the alkylation2 and acylation3 reaction at the lower rim of calix[4]arene. But, 1,3-alternate4 and 1,2-alternate5 conformers also were observed under the certain reaction conditions. Rapid interconversion of confor-mation was observed with parent calix[4]arene, but tetra-substituted calix[4]arenes particularly were found to be locked in one conformation. The stability sequence for the parent calix[4]arene was predicted by calculation6 to be cone> partial cone> 1,2-alternate> 1,3-alternate, but the stability sequence of tetrasubstituted calix[4]arene was no