Stereochemistry of a spherand-type calixarene

The stereochemistry of the spherand-type calixarene 2a is analyzed in terms of the configuration of the three 2,2′-dihydroxybiphenyl subunits (R or S) and the disposition of the methylene groups (crown or twist). X-ray crystallography indicates that the neutral 2a and its mono- or dianion (in form of the salts 2a^-·C₅H₅NH⁺ and 2a^2-·2Et₃NH⁺) exist essentially in the same conformation (RRS/SSR-twist). This asymmetric RRS/SSR-twist form is the lowest energy conformer according to molecular mechanics calculations. Low-temperature ¹H NMR data indicate the presence of a major conformer of C₁ symmetry, in agreement with a RRS/SSR-twist form. The lowest energy topomerization pathway mutually exchanges two pairs of methylene protons and is ascribed to an enantiomerization process involving rotation around an Ar-Ar bond.