Convenient direct synthesis of bisformylated calix[4]arenes via ipso substitution

A facile synthesis of bisformylated calix[4]arenes via ipso substitution of p-tert-butylcalix[4] arenes through treatment with hexamethylenetetramine/ trifluoroacetic acid is described. Under identical conditions, p-tert-butylcalix[4] arene tetramethyl ether 4 gives proximally substituted bisformylated derivative 4a in a pinched cone conformation.