Formation of indoles, isoquinolines, and other fused pyridines from azidoacrylates

Mild thermal decomposition in boiling toluene or xylene of the azidocinnamates (1)–(6), readily prepared from the corresponding aldehyde and ethyl azidoacetate, gives indoles in good yield when there is an unsubstituted ortho position, and dihydroisoquinolines, and hence isoquinolines, when there is an o-methyl or methylene group. In the presence of iodine, which seems to favour a radical type process, the yield of isoquinoline is increased, and isoquinoline formation can compete with the indole-forming cyclisation to a free ortho-position. Iodine also catalyses primary enamine formation by a hydrogen abstraction process. The thiophene (7) and pyrazole (8) are formed and decomposed similarly to give the corresponding c-fused pyridines (28) and (29). The 2,6-dichloro compound (9) thermolyses to the stable 2H-azirine (32) which isomerises to the nitrile (33) on stronger heating. Yields in these azide decompositions are sometimes high, though they can be variable and the reactions, though easily carried out, can be complex.