Partial OH → Me Replacement in the Calixarene Scaffold: Preparation, Conformation, and Stereodynamics of Tetra-terf-butyl-25,27-dihydroxy-26,28-dimethylcalix[4]arene and Its Dimethyl Ether Derivative

The first example of the replacement of hydroxyl groups of a calixarene by methyls is described. Reaction of the bis(spirodienone) calixarene derivative 3B with MeLi afforded the bis addition product 4 which is derived, as shown by X-ray crystallography, from attack on the face of the carbonyls which is anti to the ether oxygen. The reaction of the alternant bis(spirodienone) calixarene derivative 3A with excess MeLi resulted in addition to the C=O groups, but with a concomitant cleavage of the spiro bonds. Ionic hydrogenation (CF₃COOH/Et₃SiH) of this product (5) yielded 5,11,17,23-tetra-tert-butyl-25,27-dihydroxy-26,28-dimethylcalix[4]arene (6) while ionic hydrogenation of 4 resulted in fragmentation of the macrocyclic ring. Calixarene 6 adopts a 1,3-alternate conformation both in solution and in the solid state. 6 is conformationally flexible, and an inversion barrier of 15.1 kcal mol^-1 was measured for it by DNMR. The dimethyl ether derivative of 6 (i.e., 7) exists in a partial cone (paco) conformation and undergoes two distinct dynamic processes possessing barriers of 13.3 and 18.1 kcal mol^-1. Molecular mechanics calculations predict correctly the preferred conformation of 6 and 7 and indicate that the topomerization pathways resulting in the mutual exchange of the protons within a methylene group are the following: 1,3-alt → paco(CH3) → 1,2 alt → paco(CH3)* → 1,3-alt* for 6, and paco(CH₃) → 1,3-alt → paco(OCH₃) → 1,2 alt → paco(OCH₃)* → 1,3-alt* → paco(CH₃)* for 7 with calculated barriers of 15.0 and 16.1 kcal mol^-1, respectively.