1,2,3-Triazole-Mediated Synthesis of 1-Methyleneisoquinolines: A Three-Step Synthesis of Papaverine and Analogues.

A metal-free three-step synthesis toward functionalized 1-methyleneisoquinolines from readily available substrates is reported. First, acetal-containing 1,2,3-triazoles were prepared via a high-yielding triazolization reaction and quantitatively converted into triazolo[5,1-a]isoquinolines. Next, the acid-promoted ring opening of these fused triazoles was studied in order to obtain coupling to a diverse scope of nucleophiles, including carbon nucleophiles such as veratrole. By means of non-nucleophilic strong acids under anhydrous conditions, a series of unprecedented isoquinolines and imidazo[5,1-a]isoquinolines was synthesized.