Reaction of 2-hydroxy-1,4-naphthoquinone with aldehydes. Synthesis of 2-hydroxy-3-alk-1-enylnaphthoquinones

In the course of an unsuccessful attempt to prepare 2-(3,7-dimethylocta-1,6-dienyl)-3-hydroxy-1,4-naphthoquinone (1a) by acid catalyzed condensation of 2-hydroxy-1,4-naphthoquinone (2) with citronellal, a tetracyclic quinone (3) was obtained. This quinone was different from the two tetracyclic quinones, (4) and (5), previously obtained from a triethylamine catalyzed reaction. The quinone (1a) is shown to be an intermediate in the formation of (4) and (5). By adjusting the conditions (1a) can be isolated in a reasonable yield. Other 2-hydroxy-3-alk-1-enylnaphthoquinones are prepared similarly in variable yields. Primary and secondary products of this reaction are discussed.