Differentiation of isomeric 1,2-dihydroisoquinolin-4(3H)-ones by proton magnetic resonance of their 4-benzyl derivatives

Reaction 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 the ¹H 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-dimethoxybenzylaminoacetonitriles to be more fully elucidated.