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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 the cis-cyclohexano-8-crown-3 molecule were detected at 185 K. From relative ¹³C NMR peak area measurements, the conformation with the O— CH2— O unit equatorial was found to be favoured by 4.4 ± 0.4 kJ mol⁻¹. In the spectrum of the minor conformer at low temperature, a 9.6 ppm ¹³C chemical shift difference is present between the two methine carbons. Transannular 1,4 H–H interactions are deemed to be partly responsible for this effect. Merck Molecular Force Field (MMFF) calculations indicate a preference of ca 1.8 kJ mol⁻¹ for the most populated conformation. A boat–chair conformation is present in the 1,3,6-trioxocane ring of the major conformer whereas a twist boat–chair is preferred in the minor form. Two other conformations with populations of ≥ 1% were identified. Copyright © 1999 John Wiley & Sons, Ltd.