Theoretical and experimental NMR chemical shifts of norsanguinarine and norchelerythrine

Norchelerythrine and norsanguinarine, tertiary benzo[c]phenanthridine alkaloids, were examined by gradient-selected 2D NMR spectroscopy and the later also by extensive theoretical calculations. H-1, C-13 and N-15 chemical shifts assignments of the title isoquinoline alkaloids based on NOE and multiple-bond chemical-shift correlation experiments (GSQMBC) are reported. Various methods were used for the NMR chemical shifts calculations. Molecular mechanics (MM3 forcefield), AM1 method and Ab initio methods were used for optimizing the geometry. Chemical shielding constants were computed by density functional theory, GIAO and IGLO approaches were used. Chemical shifts calculated by all methods display good qualitative agreement with experimentally determined values. The best overall agreement was achieved when geometry was optimized by RHF/6-31G** method and chemical shielding constants were calculated by B3LYP/6-311G** method, GIAO approach. (C) 2003 Elsevier B.V. All rights reserved.