Carbazomadurin A

Antioxidant activities of 3-oxygenated and 3,4-dioxygenated carbazole alkaloids and their related carbazoles were comprehensively evaluated. In all assay systems, the 3,8-dihydroxycarbazoles carbazomadurin A (2) and B (3), and their synthetic precursors 2a and 3a exhibited higher antioxidant activities than the 3-monohydroxycarbazoles carazostatin (1), and the synthetic precursors 4a and 4b of carquinostatin A (4). In particular, 2a and 3a exhibited strong scavenging activities due to the reducing ability of formyl group at the C-5 position of carbazoles. The results suggest that these compounds could serve as useful clues for designing and developing novel antioxidants.

This review describes the syntheses and antioxidant activities of carbazole alkaloids carquinostatin A, carbazomadurin A and carbazomadurin B and their related carbazoles. The key step was an allene-mediated electrocyclic reaction involving an indole [b]-bond for the construction of a highly substituted carbazole ring. Antioxidant activities of 3-oxygenated and 3,4-dioxygenated carbazole alkaloids and their related carbazoles were comprehensively evaluated by 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS)+ radical scavenging assays, and cupper reducing power. Furthemore, the antioxidant activities of simple phenolic carbazoles were evaluated by DPPH and ABTS+ radical scavenging assays. After this, bond dissociation energies (BDE) and highest occupied molecule orbital energy levels (EHOMO) of a series of phenolic carbazoles, including phenolic carbazole alkaloids, were also calculated and then examined for correlation with their antioxidant activities. The phenolic carbazole core possessing a hydroxyl group at the 1-, 3-, 6-, or 8-positions could play an important role in the antioxidant activity of carbazole alkaloids. The results suggest that these compounds could serve as useful clues for designing and developing novel antioxidants.

The highly oxygenated neuronal cell protecting carbazole alkaloid carbazomadurin A was synthesized in nine steps and 11% overall yield from isovanillic acid.