NMR characterization of obscurinervine and obscurinervidine using novel computerized analysis techniques

Abstract

The ¹³C and ¹H resonances of the alkaloids, obscurinervine (1) and obscurinervidine (2), are assigned using high-field NMR experiments and computerized data analysis procedures. A 2D INADEQUATE analysis of 26 mg of 2 was performed with a high-sensitivity carbon probe and the data interpreted using the spectral analysis program, CCBOND, to provide unambiguous ¹³C assignments. Although all signals are visually undetectable, CCBOND determined 20 of the 22 carbon–carbon bonds present. Corresponding ¹H chemical shift assignments are made from HETCOR data. Proton–proton couplings are determined from DQF-COSY data using the new analysis program, HHCORR. Since HHCORR models signals as AB spin systems, the determined coupling constants are fairly independent of higher order effects, linewidths and digital resolution. Also a significant sensitivity improvement over visual interpretation of DQF-COSY data is observed. The obtained coupling constants are interpreted through the Karplus relationship to provide conformational details. These novel software analysis techniques allow accurate and more routine analysis of INADEQUATE and DQF-COSY data providing non-specialists access to these powerful experiments. Absolute stereochemistry of 2 is determined by a comparison with the ORD curve of(–)-O-methylaspidolimine. Stereospecific ¹H assignments are obtained from proton–proton couplings and molecular mechanics simulations. The ¹³C and ¹H chemical shift assignments for the related alkaloid, obscurinervine 1, are determined from CCBOND processed 2D INADEQUATE, HHCORR processed DQF-COSY, and HETCOR data. Differences in the rigidity of 1 and 2 in dimethyl sulfoxide (DMSO) are quantified by variable-temperature ¹H NMR spectroscopy. Complete conformations of all ring systems are obtained from molecular mechanics using dihedral angles derived from proton–proton couplings as a check on the quality of the model. All conformational conclusions are independently supported by the X-ray structure of 1.