Towards novel difluorinated sugar mimetrics; syntheses and conformational analyses of highly-functionalised difluorinated cyclooctenones

Highly-functionalised difluorinated cyclooctenones were synthesised from trifluoroethanol using either metallated difluoroenol acetal or carbamate chemistry, followed by a [2,3]-Wittig rearrangement or aldol reaction. Efficient RCM reactions afforded the title compounds which showed rather restricted fluxional behaviour by VT 19F NMR. Topological characterisation by molecular modelling and NOESY/ROESY experiments offered a number of challenges, but allowed the identification of two favoured boat-chair conformers which interconverted by pseudorotation with relatively large activation barriers

Specificity of interproton nuclear Overhauser effects in gramicidin-S dissolved in deuterated ethylene glycol.

The 250-MHz high-resolution proton magnetic resonance spectra of gramicidin-S in solution in deuterated methanol, deuterated ethylene glycol, and binary mixtures of these solvents have been recorded. Starting from previously published partial assignments for deuterated methanol solution, the solvent transition yields partial assignments in deuterated ethylene glycol solution. In the latter the rotational correlation time for the peptide backbone, tauc, is calculated to be 14 ns at 25 degrees C. The long tauc leads to proton spin relaxation behavior that mimics that of moderate-sized proteins in water, and yields negative nuclear Overhauser effects, which have been measured for the protons of the phenylalanine ring. The results suggest that there is rapid and efficient spin-diffusion within closely-connected "islands" of protons, and less efficient spin-diffusion between islands. The results are compatible with the accepted solution conformation of gramicidin-S