Synthesis of some N-methyl-1,2,3,4-tetrahydroisoquinolines by Friedel–Crafts cyclisation using benzotriazole auxiliary

Abstract

1-Hydroxymethylbenzotriazole reacts with phenylethylamines to give the respective N,N-bis(benzotriazol-1-ylmethyl)phenylethylamines, which are then subject to an intramolecular Friedel–Crafts cyclisation at room temperature to yield N-benzotriazol-1-ylmethyl-1,2,3,4-tetrahydroisoquinolines. These crystalline UV- and oxygen-stable products can be reduced at room temperature to the corresponding N-methyl-1,2,3,4-tetrahydroisoquinolines using NaBH₄. The method offers an elegant approach to a wide range of N-methylated 1,2,3,4-tetrahydroisoquinolines since it can be applied not only for the synthesis of 1,2,3,4-tetrahydroisoquinolines with electron-donating substituents on the aromatic moiety, but also for deactivated derivatives. All steps involved work under very mild conditions in high to excellent yields.