Computer Assisted Structure Elucidation of Two Biflavonoids from the Leaves of Ochna Mauritiana

As part of a phytochemical investigation of the leaves of O. mauritiana, we have isolated, using a combination of silica gel flash chromatography and preparative layer chromatography (PLC), two biflavonoid compounds (1 and 2) from the organic extract of this plant. Approximately 12.5 mg of 1 and 2.5 mg of 2 were placed in NMR tubes and dissolved with DMSO-d6 in order to elucidate their structures. High-Resolution Mass Spectra were also recorded in order to establish the molecular formulae.

Both compounds were relatively proton deficient resulting in few correlations in the long range 2D NMR experiments. The repeated ring structure made some of the 13C peaks appear very close together which also limited the usefulness of the conventional 2D experiments (Fig. 1). Together with the presence of many exchangeable protons the elucidation of the structures proved to be rather challenging. Here we describe the advanced computation method used for elucidating the structures using a series of 1D and 2D NMR experiments.

Fig. 1: Comparison between the conventional (left) and band-selective (right) 1 H-{13C} HMBC spectrum of compound 1 in the F1 region around 159.5 ppm.

Computer Assisted Structure Elucidation (CASE) together with band-selective versions of the HMBC experiment were employed to assign the NMR data and unequivocally determine the structures, confirming the new molecules. Several closely related structures were ruled out either by the poor agreement between the predicted and experimental 13C chemical shifts or by additional derivatisation experiments.