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J. Org. Chem. 1982,47, 545-552
Characterization of an Unusual Tetrahydroprotoberberine Conformer by Carbon-13 Nuclear Magnetic Resonance Spectroscopy Kinuko Iwasa and Mark Cushman* Department of Medicinal Chemistry and Pharmacognosy, School of Pharmacy and Pharmacal Sciences, Purdue University, West Lafayette, Indiana 47907 Received July 28, 1981
The lSCand ‘HNMR spectra of certain 9,lO- and l0,ll-oxygenated tetrahydroprotoberberineswere examined. Both of the two cis-fused as well as the trans-fusedquinolizidine conformers were detected. The chemical shift of the C-5 carbon atom was found to be a reliable indicator of the presence of the unusual B/C cis-fused protoberberine conformer.
It is well-known that the tetrahydroprotoberberinealkaloids may exist as an equilibrium mixture of one B/C trans-quinolizidine (A) and two B/C cis-quinolizidine systems (B and C).l Although the configuration of the
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