Antitumor activity of shikonin and their derivatives. II. X-ray analysis of cyclo-alkannin leucoacetate, tautomerism of alkannin and cyclo-alkannin and antitumor activity of alkannin derivatives - Sankawa,U.; Otsuka,H.; Kataoka,Y.; Iitaka,Y.; Hoshi,A.; Kuretani,K. (Chem Pharm Bull, 1981)