CONVENIENT SYNTHESIS OF ETHYLIDENEDINAPHT HAZARINS BY REACTION OF NAPHTHAZARIN WITH ETHYL VINYL ETHER

As a part of our synthetic studies of naphthoquinone derivatives, reac tion of naphthazarin (4) with ethyl vinyl ether was conducted. Treatme nt of 4 with 6 molar amounts of ethyl vinyl ether in acetic acid in th e presence of BF3 . OEt(2) at room temperature gave naphthazarin deriv ative (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 th e other hand, reaction of dichloronaphthazarin (8) with ethyl vinyl et her gave dichloroquinizarin. In addition, treatment of juglone (10) wi th ethyl vinyl ether afforded juglone derivative (11) having an ethyli dene group.