Pummerer reaction of the sulfoxides 5 of N-acyl-N-(aryl)methyl-2-(phenylthio)ethylamines (4) on treatment with trifluoroacetic anhydride (TFAA) effectively caused intramolecular cyclization under a mild condition to give N-acyl-4-phenylthio-1, 2, 3, 4-tetrahydroisoquinolines (TIQs) (7). The reaction of the N-formyl sulfoxide 5c without a methoxy group in the benzene ring using a formyl group for N-protection is particularly efficient. Treatment of the N-formyl sulfoxide 5f with TFAA did not give any TIQ, but a sequential treatment using TFAA and BF₃·Et₂O afforded N-formyl-4-phenylthio-TIQ (7f) in quantitative yield. The efficiency of this method of preparing TIQs was demonstrated in the synthesis of 1, 4-dideuterio-TIQ (10D) and its N-methyl derivative (11D).