Convenient synthesis of ethylidenedinaphthazarins by reaction of naphthazarin with ethyl vinyl ether

As a part of our synthetic studies of naphthoquinone derivatives, reaction of naphthazarin (4) with ethyl vinyl ether was conducted. Treatment of 4 with 6 molar amounts of ethyl vinyl ether in acetic acid in the presence of BF3 . OEt(2) at room temperature gave naphthazarin derivative (5) having an ethylidene group linking two naphthoquinone rings. The predominant tautomeric form 5a of the product could be determined by H-1 NMR spectra. In this reaction, treatment using a large excess of ethyl vinyl ether to 4 afforded quinizarin (6) instead of 5a. On the other hand, reaction of dichloronaphthazarin (8) with ethyl vinyl ether gave dichloroquinizarin. In addition, treatment of juglone (10) with ethyl vinyl ether afforded juglone derivative (11) having an ethylidene group.