SYNTHESIS OF BENZ[A]AZULENES SUBSTITUTED AT THE BENZO RING

It is shown that the thermal electrocyclic ring-closure reaction of 1,2-di[(E)-prop-1-enyl]benzene to yield 2,3-dimethylnaphthalene (cf. Scheme 1) [10] can successfully be applied also to the synthesis of benz[a]azulenes (cf. Schemes 2 and 3). Starting materials are methyl 4,6,8-trimethylazulen-2-yl ketone (6) and the corresponding 2-carbaldehyde 5, which, in a Horner-Emmons reaction, are transformed into the (azulen-2-yl)-acrylates (E)-8 and (E)-7, respectively. Vilsmeier formylation of these compounds, followed by the Horner-Emmons reaction leads to the formation of the bisacrylates (E,E)-11 and (E,E)-12, respectively. In an alternative reaction, (E)-8, on treatment with dimethyl acetylenedicarboxylate (ADM) in the presence of [RuH2(PPh(3))(4)], can be transformed into the methoxycarbonyl-substituted bisacrylates (E,E)- and (E,Z)-17. All three bisacrylates, on heating at 180-190 degrees in p-cymene, undergo cyclization to yield the corresponding dihydrobenz[a]azulenes 13, 14, and 18, respectively, which could easily be dehydrogenated on heating in the presence of Pd/C. The new benz[a]azulenes 15, 16, and 19 are fully characterized.