A new series of mono- and dipiperidine, oxazoline, and oxazolidine crown ethers. Stability constants, ion transport rates, and nuclear magnetic resonance studies

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Abstract
A series of monopiperidine, 1, dipiperidine 2, 3 (syn and and), as well as the corresponding oxazoline and oxazolidine crown ethers have been prepared and their ionophoric properties evaluated. The syn-dipiperidine 2c turned out to be a particularly efficient Ca2+ ligand (log K = 9.7). 13C and 23Na nuclear magnetic resonance spectroscopy was used to evaluate the nature of the complex between the monopiperidine 1c, the syn-dipiperidine 2c, and Ca2+ and Na+ ions. In methanol–water solution, the dimeric salt (2c)2Ca exhibits both a tight or asymmetric and a symmetric or loose ion pair between the cation and the carboxylate groups. Sodium-23 chemical shifts and line-width variation both support a 1:1 complex with 1c and 2c. The structural results correlate with observations from ion transport experiments.

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