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Fifty natural and synthetic hydroxy-, aminohydroxynaphthazarins and related quinones were tested for their antiradical activities using 2,2'-azinobis(3ethylbenzothiazoline-6-sulfonate) (ABTS) scavenging assay. The main features of structure-antiradical activity relationship of amino-and hydroxyquinones were determined for the first time. It was found that naphthazarins (5,8-dihydroxy-1,4-naphthoquinones) are much more active against ABTS-radical than related 1,4-naphthoquinones and 5-hydroxy-1,4-naphthoquinones. Natural aminonaphthazarins (spinamine E, echinamine A) and polyhydroxylated naphthazarins (spinazarin, echinochrome, spinochrome D and E) revealed high antiradical properties. In most cases, the presence of long chained O-alkoxy substituent in benzenoid part of naphthazarin core enhanced their antiradical activity.

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A series of 6,7-disubstituted 2-hydroxy-3-nitronaphthazarins were prepared by treatment of 2,3-dichloronaphthazarins with sodium nitrite. Acid-catalyzed hydrolysis of a mixture of two isomeric 6(7)-ethoxy-7(6)-ethyl-substituted 2-hydroxy-3-nitronaphthazarins followed by chromatographic separation led to the individually precursors of echinamines A and B. Further reduction of nitroquinones using various reducing agents gave echinamines and related 3-amino-2-hydroxynaphthazarins in good yields.

New convenient two-step synthesis of natural pigment echinamine A by the condensation of 2,3-dichloro-6-ethyl-7-hydroxy-1,4naphthoquinone with sodium nitrite and subsequent reduction of intermediate 7-ethyl-2,5,6,8-tetrahydroxy-3-nitro-1,4-naphthoquinone has been developed.

Twenty five hydroxy-, chloro-and methoxy derivatives of natural and synthetic naphthazarins and their acetylated O-glycosides were synthesized. Targeted compounds were screened as cytotoxic agents on mouse Ehrlich ascites carcinoma cells using MTT test. Chloro-and methoxy-substituted naphthoquinones as well as naphthoquinone O-acetylglucosides were the most potent with IC 50 in low micromolar concentration range. Glucosidation of hydroxynaphthoquinones was shown to enhance cytotoxicity, whereas methoxylation yielded both more active and less active derivatives depending on the number and position of methoxy groups. Evaluation using a phenotypic sea urchin embryo assay suggested that naphthazarins exerted their cytotoxic effects through tubulin-unrelated mechanism.