Report on an unusual cascade reaction between azulenes and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (=4,5-dichloro-3,6-dioxocyclohexa-1,4- diene-1,2-dicarbonitrile; DDQ)

Abstract

The oxidation of 1-(3,8-dimethylazulen-1-yl)alkan-1-ones 1 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (=4,5-dichloro-3,6-dioxocyclohexa-1,4- diene-1,2-dicarbonitrile; DDQ) in acetone/H2O mixtures at room temperature does not only lead to the corresponding azulene-1-carboxaldehydes 2 but also, in small amounts, to three further products (Tables 1 and 2). The structures of the additional products 3-5 were solved spectroscopically, and that of 3a also by an X-ray crystal-structure analysis (Fig. 1). It is demonstrated that the bis(azulenylmethyl)-substituted DDQ derivatives 5 yield on methanolysis or hydrolysis precursors, which in a cascade of reactions rearrange under loss of HCl into the pentacyclic compounds 3 (Schemes 4 and 7). The found 1,1′-[carbonylbis(8-methylazulene-3,1-diyl)]bis[ethanones] 4 are the result of further oxidation of the azulene-1-carboxaldehydes 2 to the corresponding azulene-1-carboxylic acids (Schemes 9 and 10). © 2010 Verlag Helvetica Chimica Acta AG.