We report the design, synthetic route, and cytotoxicity of a library of 49 newly synthesized tricyclic oxazolo[5,4-d]pyrimidines. The condensed pyrimidinones were constructed from ethyl 5-aminooxazole-4-carboxylate building blocks. A tricyclic ring system was built using the naturally occurring mackinazolinone alkaloid with a focus on the molecular diversity at position C-2 of the oxazole ring. Synthesized