Nevv Reactions of Vinylindoles as Heterocyclic Dienes with 4-Phenyl-1,2,4-triazoline-3,5-dione: Non-Concerted versus Concerted Processes

The reactions of some 3-vinylindoles and one 2-vinylindole with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) were investigated. In dependence on the structure of the vinylindole, the experimental results revealed the occurrence in some cases of a non-concerted step to furnish Michael-type adducts and in other cases of a probably concerted Diels-Alder reaction to furnish novel pyridazino[b]indoles, respectively. In one particular case, PTAD-catalysed dimerisation of the 3-vinylindole took place.