STRUCTURAL AND CONFORMATIONAL STUDY OF MACROCYCLIC POLYETHERS BY NMR-SPECTROSCOPY AND MOLECULAR MODELING

Two macrocyclic polyether isomers were obtained by radical cyclization of )-8-iodo-3,6-dioxaoctyl-3-ethoxycarbonylpropenoate and separated b y liquid-solid chromatography over silica gel. They were characterized , for the first time, by two-dimensional NMR; notably, the HMBC experi ment gave access to the assignment of the two isomers through coupling via the carbonyl oxygen belonging to the macroheterocycle. The determ ination of the (3)J(HH) coupling, associated with Monte Carlo simulati on, gave information on the conformational space of these two isomers and showed that 85 conformers for one isomer and 29 conformers for the other were in equilibrium in the solution mixture. These results are discussed with regard to the complexing properties of these macrocycli c polyethers. (C) 1997 John Wiley & Sons, Ltd.