Tandem Reaction of Aminoalkynes Catalyzed by Copper Salts for Azaheterocycles: Product Selectivity by Temperature and X-ray Structure of an Iminium Ion.

The tandem intramolecular hydroamination-Povarov-type reaction of 1,4-aminoalkynes at 50 or 75 °C gave the spiro-dipyrroloquinolines (2) in 61-91% yields. When the temperature of the reaction was increased to 110 or 135 °C, yet another homocoupled monopyrroloquinoline (4) was isolated in 24-86% yields along with 2 as a minor product. From the reaction of the terminal 1,3-aminoalkyne in methanol at 110 °C, one of the diastereomers of an octahydro-dipyrroloquinoline was isolated as a solid in a 48% yield. Conversely, the long-chain 1,5-aminoalkyne afforded its methyl ketone product, instead of a quinoline derivative. The iminium ion, one of the initial products of the tandem reaction, was isolated as crystals, and its structure was determined by the X-ray method. Subsequent reactions of the iminium ion with aminoalkynes, the formation of deuterated 2, the conversion of 2 to 4, and its reverse reaction led us to propose a mechanism that explains the observed results.