Abstract
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 [RuH~2~(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° 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.