Structure and NMR properties of 6-substituted- 5,6-dihydrobenzocphenanthridine alkaloids

摘要

We report a preparation of new 6-substituted-5,6-dihydrobenzocphenanthridines by the reaction of azoles with quaternary benzocphenanthridine alkaloids sanguinarine and chelerythrine. The prepared compounds have been characterized by NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. Conformational behaviors of carbazole derivatives in solution have been investigated by low-temperature NMR experiments. Barriers to rotation around newly formed C6–N bonds were determined to be 12–13 kcal/mol. Quantum chemical calculations have been used to reproduce the experimental observations. Large structural effects on several ~1H NMR resonances were observed experimentally, analyzed by Density Functional Theory (DFT) calculations at B3LYP/6-311+G(d,p)/PCM level, and interpreted by ring-current effects of the benzocphenanthridine and carbazole units.