Abstract
Abstract In the presence of selenium dioxide, guaiazulene and 4,6,8-trimethylazulene condensed with the active methylene compounds, ethyl acetoacetate and acetylacetone, to give ethyl α-(3-guaiazulenyl)acetoacetate (3a), α-(3-guaiazulenyl)acetylacetone, ethyl α-(4,6,8-trimethyl-1-azulenyl)acetoacetate (5a), and α-(4,6,8-trimethyl-1-azulenyl) acetylacetone. These α-substituted β-diketones exist exclusively in their corresponding enol forms, as evidenced by their NMR spectra. In the NMR spectra of 3a and 5a, the ethyl protons of the ester group show an ABX3 pattern; the observed nonequivalence of the methylene protons is attributable to the restricted rotation between azulene and the substituent carbon atoms. From an inspection of the chemical shifts of the methyl group of metal chelate compounds of azulene derivatives, it is now reasonably concluded that there is no aromaticity associated with the chelate ring, providing further support for Musso’s proposal.
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