Conformational analysis of calix[n] arenes with chiral substituents by using circular dichroism

Calix [n] arenes (n= 4 and 6) bearing chiral substituents (R^*) on the lower rim (1_n) or on the upper rim (4_n) were synthesized. The isomers derived from 1₄ were conformationally immobile and showed different circular dichroism (CD) spectra. In the ¹H NMR spectra the proton signals in R^* shifted to higher magnetic field when the lower rim is sterically crowded and in the CD spectra a strong CD band appeared. Therefore, the change in the CD spectra is rationalized in terms of R^*…π interactions. In contrast, compound 4₄ was not conformationally immobilized and the CD spectra changed in response to the metal binding to the methoxy oxygens arranged on the lower rim. The specific metal interaction enabled the metal binding event to be detected by the CD technique.