TiCl4/t-BuNH2-promoted hydroamination/annulation of delta-keto-acetylenes : synthesis of novel pyrrolo[1,2-a]indol-2-carbaldehydes

An original TiCl4/t-Bu-NH2 mediated hydroamination/annulation domino reaction of delta-keto-acetylenes is described. The synthesis of pyrrolo[1,2-a]indole-2-carbaldehydes, starting from 2-carbonyl-1-propargyl-1H-indoles run under mild reaction conditions. A conceivable mechanism is also discussed. TiCl4 has proved to be an effective multi-activity reagent: catalyst / Lewis acid / water scavenger. Some unpublished 2-carbonyl-1-propargyl-1H-indoles are prepared by means of Suzuki- and Negishi-type reactions.