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 [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.