A CuI-catalyzed A(3) (amines, aldehydes and alkynes) reaction of tetrahydroisoquinolines (THIQs), aldehydes, and alkynes to give C1-alkynylated THIQ products (endo-yne-THIQs) was developed. This redox neutral C1-alkynylation of THIQs, which was conducted under mild conditions, has a broad scope for the used aldehydes and alkynes. It was proposed that the A(3) reaction first generates in situ exo-iminium ions, which then isomerize to endo-iminium ions and react with copper acetylides to give the endo alkynylated THIQs (endo-yne-THIQs).