Flow chemistry: A flow‐microreactor‐mediated synthesis of 1,2,3,4‐tetrahydroisoquinolines (THIQs) is reported (see scheme). Starting from a laterally lithiated aziridine, a tetrahydroisoquinoline lithiated at C4 was generated by thermally induced isomerization. Because the reaction temperature is a crucial parameter, the exquisite thermal control possible in a flow‐microreactor system allowed for fast