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Benzoimidazopyrimidine, benzimidazoloquinazolinone, triazolopyrimidine, and triazoloquinazolinone derivatives were synthesized via a one-pot multi-component reaction in the presence of a catalytic amount of oxalic acid dihydrate. The reactions were performed in the mixture of EtOH: H2O or under solvent-free conditions as green media. The advantageous features of these methodologies are inexpensive starting materials and catalyst, high atom economy and good yields, and metal-free synthesis. Moreover, all the products were obtained by simple filtration with no need for column chromatography or tedious separation procedures, which is very important in terms of reducing environmental pollutions.