AbstractReaction of a 3,3‐dimethyl‐1,2‐dihydroisoquinolin‐4(3H)‐one with a benzyl Grignard reagent gives a 4‐benzyltetrahydroisoquinoline in which the signal of a 6‐methoxy group suffers a large upfield shift in the1H NMR spectrum, whereas a 7‐methoxy group is relatively unaffected. This effect can be used to determine whether an RO‐substituent is in the 6‐ or 7‐position and, in particular, to distinguish isomeric derivatives with different RO‐groups in the 6‐ and 7‐positions. The method will allow the mechanism of cyclization of 3,4‐dimethoxybenzylaminoacetonitrile