Stereoisomerism and complexation behaviour of functionalized p-tert-calix[6]-1,4-2,5-biscrown-4

p-tert-Calix6]-1,4-2,5-biscrown-4 was subjected to functionalization by benzyl bromide or ethyl bromoacetate. Two pairs of disubstituted calix6]biscrown stereoisomers were obtained. Their structures had been deduced from ¹H NMR and ESI-MS (electrospray ionization mass spectroscopy). One of the bisethyloxycarbonylmethylated derivatives 3a was further investigated by X-ray crystallographic analysis. Two-phase extraction experiments indicated that bisethyloxycarbonylmethylated derivatives exhibited high Cs⁺/Na⁺ selectivity. By ESI-MS and ¹H NMR experiments it was confirmed that 3a formed 1:1 complex with Cs⁺.