Solution conformation, ion binding, and stereochemistry of griseochelin as studied by high-field nuclear magnetic resonance spectroscopy

Abstract

The carboxylic acid ionophore antibiotic griseochelin and its Cd²⁺ salt have been studied by ¹³C (50 MHz), ¹H (200 and 400 MHz), and ¹¹³Cd (88.73 MHz) n.m.r. spectroscopic methods. Carbon-13 spin–lattice relaxation times H–D exchange rates of hydroxy protons, and cadmium–proton (metal–ligand) spin–spin couplings have shown that, in non-polar solvents, the essentially open-chain metabolite and its complex salt exist in a conformationally stable pseudocyclic form. This has made it possible to interpret the proton–proton couplings and nuclear Overhauser enhancement data in terms of backbone stereochemistry and to define the relative configurations at all but one of the 13 chiral centres of the molecule, rel-(2S)-2-{(2S,5S6S;9E,13E)-6-[(1R,2S,3S,4S,5S,6R,11R,12R)-2,4,6,12-tetra-hydroxy-1,3,5,11,13,15-hexamethyloctadeca-9,13-dienyl]-5-methyltetrahydropyran-2-yl}propionic acid or its enantiomer.