Structure and equilibration studies of bis- and tris(spirodienone) derivatives of medium-sized calixarenes

The isomerism of the tris(spirodienone) derivatives of p-tert-butylcalix[6]arene and bis(spirodienone) derivatives of p-tert-butylcalix[5]arene is analyzed. Oxidation of p-tert-butylcalix[6]arene with phenyltrimethylammonium tribromide/base afforded two chiral tris(spirodienone) calix[6]arene derivatives (3 and 4) which react with excess benzyne yielding the corresponding all-exo tris adducts 5 and 6. The structural characterization of the tris(spirodienone) derivatives was achieved by 2D NMR methods and in the case of 3, by X-ray crystallography. The main product 3 possess an alternant disposition of the ether and carbonyl functional groups and three spiro stereocenters of dissimilar configurations (RRS/SSR), while the disposition of the functional groups in 4 is nonalternant. Equilibration studies in toluene at 85°C indicate that 3 is the thermodynamically most stable isomer. The tris adduct 5 exists in two nearly equally populated conformations which interconvert with a barrier of 12.1 ± 0.2 kcal mol^-1. A bis(spirodienone) derivative of p-tert-butylcalix[5]arene was synthesized by oxidation (K₃Fe(CN)₆/base) of the corresponding monospirodienone derivative. The product obtained is more likely the cycloenantiomeric pair which isomerizes in toluene solution to an isomeric mixture consisting mainly of a heterodirectional isomer of C₂ symmetry.