Selective Hydroxyl Replacement in Calixarenes: Amino-, Azo-, and Xanthenocalixarene Derivatives

The synthesis of monoamino-, azo-, and xanthenocalixarenes and dehydroxylated calixarenes via condensation, reduction, or rearrangement of monospirodienone calixarene derivatives is described. Mild oxidation of p-tert-butylcalix[5]arene and p-tert-butylcalix[6]arene yielded their corresponding monospirodienone derivatives 4b and 4c. Monospirodienone 4b was characterized by X-ray crystallography. Reaction of 4b with (2,4-dinitrophenyl)-hydrazine gave a (dinitrophenyl)azo derivative (8), which by reaction with HI gave the monoaminotetrahydroxycalix[5]-arene 10. Reaction of 4b with hydrazine/base yielded in a Wolff–Kishner-type reaction, the monodehydroxylated calix[5]arene 8. Reaction of 4b with a methanolic solution of hydrazine at room temperature yielded a derivative characterized by X-ray crystallography as a system with an azo bridge (15). Spirodienone 4b rearranges in MeOH/H⁺, yielding the xanthene derivative 16 as the main product, together with the linear pentamer 17. The larger spirodienone 4c rearranges by treatment with MeOH/H⁺, yielding the xanthenocalix[6]arene 18. X-ray diffraction of a crystal of 18 grown in MeCN shows that in the crystal the molecules form intermolecular hydrogen-bonded dimers in which the two molecules mutually include each other; i.e., one of the protuberances of each molecule is located in the V-shaped cavity of its neighboring molecule. Molecules 16 and 18 represent the first examples of calixarene systems incorporating a xanthene moiety.