Collect. Czech. Chem. Commun. 1988, 53, 2503-2510

Proton magnetic resonance study of the conformation of the pyrrolidine ring in some proline-thiohydantoins

Jef J. M. Sleeckx, Marc J. O. Anteunis and Frans A. M. Borremans

Laboratory for Organic Chemistry, NMR Spectroscopic Unit, Rijksuniversiteit-Gent, Krijgslaan 281 (S-4), B-9000 Gent, Belgium


The conformational behaviour of a series of N-substituted thiohydantoins of proline was studied by proton NMR. The pseudorotational parameters of the proline moiety were calculated from the ten vicinal proton-proton coupling constants assuming a two state equilibrium (N ⇋ S). The effect of the different substituents (methyl, phenyl, para-nitrophenyl) at the thiohydantoin nitrogen on the conformation of the pyrrolidine rings is discussed. The spectra of proline-N-methylthiohydantoin were recorded at different temperatures in octadeuterotoluene solution. The conformational analysis of these spectra showed that the ratio of the populations of the N and S forms changes considerably with temperature, while the conformational identity of both the N and S forms remains strictly preserved. These facts provide an additional experimental justification of the two-state assumption, at least so for the present model compounds.