NMR CHARACTERIZATION OF OBSCURINERVINE AND OBSCURINERVIDINE USING NOVEL COMPUTERIZED ANALYSIS TECHNIQUES

The C-13 and H-1 resonances of the alkaloids, obscurinervine (1) and o bscurinervidine (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 da ta interpreted using the spectral analysis program, CCBOND, to provide unambiguous C-13 assignments. Although all signals are visually undet ectable, CCBOND determined 20 of the 22 carbon-carbon bonds present. C orresponding H-1 chemical shift assignments are made from HETCOR data. Proton-proton couplings are determined from DQF-COSY data using the n ew analysis program, HHCORR. Since HHCORR models signals as AB spin sy stems, the determined coupling constants are fairly independent of hig her order effects, linewidths and digital resolution. Also a significa nt 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 nove l software analysis techniques allow accurate and more routine analysi s of INADEQUATE and DQF-COSY data providing nonspecialists access to t hese powerful experiments. Absolute stereochemistry of 2 is determined by a comparison with the ORD curve of (-)-O-methylaspidolimine. Stere ospecific H-1 assignments are obtained from proton-proton couplings an d molecular mechanics simulations. The C-13 and H-1 chemical shift ass ignments for the related alkaloid, obscurinervine 1, are determined fr om CCBOND processed 2D INADEQUATE, HHCORR processed DQF-COSY, and HETC OR data. Differences in the rigidity of 1 and 2 in dimethyl sulfoxide (DMSO) are quantified by variable-temperature H-1 NMR spectroscopy. Co mplete conformations of all ring systems are obtained from molecular m echanics using dihedral angles derived from proton-proton couplings as a check on the quality of the model. All conformational conclusions a re independently supported by the X-ray structure of 1.