Synthesis and ion selectivity of conformers of tetraalkyl esters derived from 9,16,25,32-tetrahydroxy[3.1.3.1]metacyclophane

Abstract

Attempted O-alkylation of the flexible macrocycle 1 with alkyl bromoacetates in the presence of NaH or Cs₂CO₃ gave only one pure stereoisomer, 3a–c, while other possible isomers were not observed. In contrast, a significant amount of 1,3-O-disubstituted product 2 was produced when Na₂CO₃ was used as the base. The structural characterization of these products is discussed. The two-phase solvent extraction data indicates that the tetraalkyl esters 3b,c show strong Rb⁺ affinities comparable with that for 18-crown-6, although the extractabilities are somewhat lower than that for the corresponding calix[6]arene tetraethyl ester 6. A high Rb⁺ selectivity was observed for tetraethyl ester 3b. However, no significant high ion selectivity was observed with tetramethyl ester 3a. ¹H NMR titration of tetra-tert-butyl ester 3c with KSCN clearly demonstrates that a 1:1 complex is formed with retention of the original symmetry that is conformationally frozen on the NMR timescale.