Amongst heterocyclic organoselenium cores, 1,3-selenazole skeleton has received much attention from medicinal chemists. The worldwide high profile of 1,3-selenazoles is not only because of the presence of the selenazole moiety in many pharmacologically active substances such as selenazofurin and amselamine, but also biological properties include antioxidant [1], anticancer [2], human carbonic anhydrase IX inhibitory [3], antimicrobial and anticonvulsant [4] activities. Based on the broad spectrum biological activities of selenazoles and aiming to synthesize novel fused heterocycles containing Se in their scaffold, in the present protocol, 2-(pyrrolidin-1-yl)-[1,3]selenazolo[4,5-d]pyrimidin-7(6H)-one (1) was synthesized as a starting material and subsequently treated with POCl3 and hydrazine hydrate to give 7-hydrazinated selenazolo[4,5-d]pyrimidines (2). The heterocyclization of compound (2) with CS2 via pyridine base-catalyzed Dimroth rearrangement yielded a novel tricyclic [1,3]selenazolo[5,4-e][1,2,4]triazolo[1,5-c]pyrimidine framework which was subsequently converted to the desired S-alkylated derivatives (3a-f) on treatment with various alkyl halides in the presence of K2CO3/DMF in good yields.

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