A novel synthetic protocol towards trans-NH-tetrahydroisoquinolonic acid esters is based on the Castagnoli–Cushman reaction between aromatic aldehyde oximes and homophthalic anhydride, followed by esterification and TiCl₃-promoted reduction. The scope of the method with respect to the aromatic portion (both electron-rich and electron-deficient) is broader compared to the earlier described approaches, which makes it a suitable synthetic strategy for the structure–activity exploration.