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

Attempted O-alkylation of the flexible macrocycle 1 with alkyl bromoacetate s in the presence of NaH or Cs2CO3 gave only one pure stereoisomer, 3a-c, w hile other possible isomers were not observed. In contrast, a significant a mount 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, althou gh the extractabilities are somewhat lower than that for the corresponding calix[6]arene tetraethyl ester 6. A high Rb+ selectivity was observed for t etraethyl ester 3b. However, no significant high ion selectivity was observ ed with tetramethyl ester 3a. H-1 NMR titration of tetra-tert-butyl ester 3 c with KSCN clearly demonstrates that a 1 : 1 complex is formed with retent ion of the original symmetry that is conformationally frozen on the NMR tim escale.