Method development was completed for a strategy to access a novel pyrimidine-fused heterocyclic scaffold. The key step for this synthetic route entails an intramolecular inverse electron demand hetero-Diels-Alder reaction of imines or iminiums formed in situ from allylaminopyrimidinealdehydes 3 and anilines. The reactions provided exclusively cis-configuration products 6. Products 6 were readily precipitated in the reaction solution in good to excellent yields. Further transformations of the phenylthio group were demonstrated by an oxidation and subsequent nucleophilic substitution sequence. The synthetic strategy provides an efficient way to access libraries of the tetracyclic pyrimidine-fused heterocycles that can be explored for potential pharmaceutical or biological activities.