Conformational studies on indole alkaloids from Flustra foliacea by NMR and molecular modeling

1H and 13C NMR assignments for 1a4a and 1b4b were obtained using HSQC, HMBC and NOESY techniques. Differences and ambiguities from literature assignments are reconciled. For the pyrrolidine C-ring, the combined use of NMR spectroscopy and molecular mechanics calculations revealed that this ring exists in a dynamic conformational equilibrium between twist (2T1) and envelope-twist (1E1T2) conformations. In chloroform-d1, the 1H NMR coupling constants indicate that the pyrrolidine ring is biased in favor of the envelope-twist conformation. Steric requirements of the N-prenyl group enhanced the envelope-twist (1E1T2) conformation populations.