Conformational analysis of the pyrrolidine ring of the N-methylthiohydantoin of 2-methylproline

The conformational analysis of the pyrrolidine ring in 2-methylproline-N-methylthiohydantoin was achieved from the vicinal proton spin-spin coupling constants obtained from the 360 MHz 1H NMR spectrum in hexadeuterobenzene solution. Two descriptions are consistent with the experimental data. One is a large amplitude torsional oscillation centered between a β-endo envelope (C8) and a β-endo-α-exo twist (C2). The other is a 3 : 1 dynamic equilibrium between two fixed states: a (major) β-exo envelope and a minor N-endo envelope with N-endo-δ-exo twist character. The effect of the 2-methyl group on the pyrrolidine ring conformation is evaluated and discussed.