Antitumor activity of shikonin, alkannin and their derivatives. II. X-ray analysis of cyclo-alkannin leucoacetate, tautomerism of alkannin and cyclo-alkannin and antitumor activity of alkannin derivatives.

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Abstract
Cyclo-alkannin and cycloshikonin were formerly considered to be derivatives of hydroanthraquinone. However, chemical and spectral investigations revealed that cyclo-alkannin possesses no secondary hydroxyl group. Thus, the structure of cyclo-alkannin leucoacetate was determined by X-ray analysis, by the direct method; the final R index with hydrogen atoms except for those of methyl groups was 0.065. The cyclization of alkannin and shikonin is a reaction between the hydroxyl group and double bond and does not involve the formation of a carbocyclic ring. In 1H-NMR both alkannin and cyclo-alkannin show 2 singlet signals arising from the protons of the aromatic and quinonic rings. The absence of coupling and lower chemical shift values suggests delocalization of the quinonic ring so that these compounds can be regarded as consisting of tautomeric structures. The results of antitumor tests on alkannin and cyclo-alkannin derivatives are reported. [Naphthoquinones contained in a Chinese medicinal drug, Shikon, were previously reported to exhibit marked antitumor activity against ascites cells of Sarcoma 180 [mouse]. Two kinds of Shikon are known and available in the market: the root of Lithospermum officinale var. erythrorhizon Max. (Boraginaceae) (the source for the alkannin derivatives) and the root of Macrotomia euchroma Pauls. (Boraginaceae) (the source for the Shikonin derivatives).

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