The reactivity pattern of the Castagnoli-Cushman reaction between homophthalic anhydride and imines was completely reproduced by replacing homophthalic anhydride with o-(alkoxycarbonyl)methyl benzoic acids activated by CDI (1,1′-carbonyldiimidazole). This allows obtaining tetrahydroisoquinolonic esters directly and plants the Castagnoli-Cushman chemistry onto a fundamentally new reagent space not requiring the use of cyclic anhydrides.