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Synthesis of 6-isomer of 5,8- O -dimethyl ether of shikonin ( 13 ), a promising anticancer scaffold, always remains a huge challenge. Herein a key intermediate for  13 , 2-(1-hydroxyl-4-methyl-3-pentenyl)-1,4,5,8-tetramethoxynaphthalene ( 10 ), was obtained on the large-scale synthesis. A ring-closing/ring-opening strategy was applied to avoid the undesired reactivity posed by the side chain and racemization of the chiral centre. Incorporation of bulky substituent 4-(( tert butoxycarbonyl)amino)phenyl to hydroxyl group in the side chain redistributed electron density of naphthalene core ( 10 ), overwhelmingly favoring the generation of  13  when oxidized by cerium(IV) ammonium nitrate followed by hydrolysis. As a result, three 6-isomers ( 14a – 14c ) with very potent antitumor activity were easily synthesized. This study opened an novel avenue to selectively prepare 6-isomers of 5,8-dimethoxy1-1,4-naphthaquinones, bearing the synthetically challenging side chain such as 2-hydroxyl-5-methylpentenyl group.  Graphical abstract   A novel semi-synthesis of 6-isomer of 5,8- O -dimethyl ether of shikonin ( 13 ) was developed in a threestep rounte using a ring-closing/ring-opening strategy and a bulky substituent-mediated oxidative demethylation oneElectronic supplementary material  The online version of this article (doi:10.1186/s13065-017-0306-0) contains supplementary material, which is available to authorized users.

chemistry central journal

Regioselective semi-synthesis of 6-isomers of 5,8-O-dimethyl ether of shikonin derivatives via an ‘intramolecular ring-closing/ring-opening’ strategy as potent anticancer agents