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The two positionally isomeric cyclohexanotetraoxacyclotridecanes (13-crown-4 ethers) were synthesized and studied via low-temperature NMR methods. For the 1,4,8,11-tetraoxa isomer, the cyclohexane ring inversion is a degenerate process, whereas for the 1,4,7,11-tetraoxa isomer, a preference of 1.4 kJ mol^-1 for the form in which the propyleneoxy group is equatorial was determined. Molecular mechanics calculations using MM⁺ indicated a preference of 0.6 kJ mol^-1 for this conformer. Resonance assignment was facilitated by the synthesis of a selectively deuterated derivative and by COSY, HMQC and HMBC experiments. The results were compared with those for the related 10-crown-3 system and ¹³C chemical shift trends are discussed in terms of MM⁺ calculated geometries. © John Wiley & Sons Ltd.

Two conformations of cis-cyclohexyl-10-crown-3 were detected in solution below 250 K using ¹H NMR at 400 MHz and ¹³C 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 ¹³C chemical shift difference between conformers (ca. 10 ppm) was found for one of the methylene carbons bearing oxygen. Stereochemical factors contributing to the observed ¹³C shielding differences between conformers are discussed. © 1997 by John Wiley & Sons, Ltd.