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.

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Keywords 13-crown-4 ethers, Stereochemistry, Tetraoxacyclotridecanes
Journal Magnetic Resonance in Chemistry
Buchanan, G.W, Gerzain, M., & Laister, R.C. (1998). Isomeric cis-cyclohexano-13-crown-4 ethers: A low-temperature 1H and 13C NMR investigation. Magnetic Resonance in Chemistry, 36(10), 687–692.