The first example of the Castagnoli-Cushman reaction of homophthalic acid with an amine and an aldehyde conducted in a three-component format is reported. This new protocol employs no dehydrating agents and, unlike the traditional Castagnoli-Cushman reaction of homophthalic anhydride, allows simultaneous mixing of all three reagents and provides good to excellent yields of trans-configured tetrahydroisoquinolonic acids.