Total Syntheses of Multi-substituted Carbazole Alkaloids and Phenolic Related Compounds, and Evaluation of Their Antioxidant Activities.
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.