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

H-1 and C-13 NMR assignments for 1a-4a and 1b-4b 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 (T-2(1)) and envelope-twist (E-1-T-1(2)) conformations. In chloroform-d(1), the H-1 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 (E-1-T-1(2)) conformation populations. Copyright (C) 2002 John Wiley Sons, Ltd.