ISOMERIC CIS-CYCLOHEXANO-13-CROWN-4 ETHERS - A LOW-TEMPERATURE H-1 AND C-13 NMR INVESTIGATION

The two positionally isomeric cydohexanotetraoxacyclotridecanes (13-cr own-4 ethers) were synthesized and studied via low-temperature NMR met hods. 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 gr oup is equatorial was determined. Molecular mechanics calculations usi ng MM+ indicated a preference of 0.6 kJ mol(-1) for this conformer. Re sonance assignment was facilitated by the synthesis of a selectively d euterated derivative and by COSY, HMQC and HMBC experiments. The resul ts were compared with those for the related 10-crown-3 system and C-13 chemical shift trends are discussed in terms of MM+ calculated geomet ries. (C) John Wiley & Sons Ltd.