Two series of 5‐furylmethylenehydantoins and 5‐thienylmethylenehydantoins were prepared and their 1H and 13C NMR spectra studied in comparison with those of compounds in the analogous 5‐aryl‐ and 5‐pyridyl‐ series. Differences in the effects of the aromatic, six‐ or five‐membered heteroaromatic rings are discussed. Spectral analysis enables Z/E configurations to be assigned with confidence and conformational preferences to be qualitatively deduced. Some interesting solvent effects were also observed.
The ionization constants of sixteen 5‐furylmethylenehydantoins and 5‐thienylmethylenehydantoins were measured in 80% (w/w) dimethyl sulphoxide–water solvent at 25°C. The effects of the 2‐/3‐furyl and 2‐/‐3‐thienyl rings and the effects of configuration and conformation on acidity are discussed. The very low acidity of (Z)‐5‐(2‐furyl)methylene‐3‐methylhydantoin suggests the possibility of some weak intramolecular interaction between the proton at N‐1 and the 2‐furyl oxygen in the s‐cis conformation.
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