Isomeric cis-cyclohexano-13-crown-4 ethers: a low-temperature <SUP>1</SUP>H and <SUP>13</SUP>C NMR investigation<FNR HREF="f1"></FNR> <FN ID="f1">Taken in part from the PhD Thesis of M. Gerzain, Carleton University, February 1997. </FN>

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.