STEREOCHEMISTRY OF CROWN ETHERS .4. H-1/C-13 NMR SPECTROSCOPIC STUDY OF THE SOLUTION STRUCTURE OF SOME BIS(BENZOCROWN ETHERS)

The H-1 and C-13 NMR spectra of some bis(benzocrown ethers) were recorded and assigned by means of H-1/C-13 HETCOR spectroscopy, heteronuclear selective decoupling experiments and the refocused INEPT experiment optimized for long-range H-1, C-13 coupling constants. It is shown that the efficiency of the so-called 'biscrown effect' is dependent on the radii of the alkali metal ions. Long-range H-1, C-13 coupling constants, high-field shifts within gamma-fragments in the C-13 NMR spectra and NOEs from 2D-NOESY spectra are discussed with respect to both the configuration and the preferred conformation of the studied compounds.