AbstractThe reaction between 1‐aryl‐3,4‐dihydroisoquinolines and monocyclic anhydrides was studied for the first time. The method is the first one‐step route to previously unknown angularly‐aryl substituted racemic [1,4]thiazino[3,4‐a]isoquinoline derivatives with trans configuration. It was found that electron‐withdrawing substituents in the p‐position of the aryl group in the starting 3,4‐dihydroisoquinoline led to higher yields and shorter reaction times, which is contrary to previous observations for the reactions of homophthalic anhydride and similar acyclic N‐arylmethanimines.