Abstract The reaction of naphthazarin with cyclic enol ethers in acetic acid in the presence of BF3·OEt2 gave naphthazarin derivatives such as cycloshikonin, having a cyclic ether substituent, in one step. Cyclic enol ethers did not attack the quinone ring of naphthazarin but the benzene ring, and the corresponding naphthazarin derivatives were produced after tautomerization. This reaction was applicable to juglone to give juglone derivatives bearing a cyclic ether substituent.