Conformational analysis of crown ether analogs in solution: cis-cyclohexyl-10-crown-3 as studied via low-temperature H-1 and C-13 NMR spectroscopy

Two conformations of cis-cyclohexyl-10-crown-3 were detected in solution below 250 K using H-1 NMR at 400 MHz and C-13 NMR at 100 MHz. Chemical shift assignments were facilitated by spectra of two dideuterio derivatives. From relative peak area measurements, the conformation with the O-CH2-CH2-CH2-O unit equatorial was found to be favored by 2.8 +/- 0.3 kJ mol(-1) at 210 K. Results of a molecular mechanics calculation were compared with the experimental findings. A remarkably large C-13 chemical shift difference between conformers (ca. 10 ppm) was found for one of the methylene carbons bearing oxygen. Stereochemical factors contributing to the observed C-13 Shielding differences between conformers are discussed. (C) 1997 by John Wiley & Sons, Ltd.