New Synthesis of 2-Aminopyrimidines by Amination of Biginelli Reaction Products - Oxadiazocines

Previously, we synthesized methanobenzo[g][1,3,5]oxadiazocines and methanonaphtho[2,3 -g][1,3,5]oxadiazocines via the Biginelli reaction using thiourea with acetoacetic ester or acetylacetone and salicylaldehyde or 2-hydroxy-1-naphthaldehyde under acidic catalysis and microwave irradiation. These compounds were then subjected to alkylation reaction with ethyl bromide in the presence of potassium carbonate in dimethylformamide. Under these mild reaction conditions, possible thione-thiol tautomerism of the oxadiazocines, as well as their structural analogs of cyclic thioureas, predominantly yielded in S-alkylation products with minor amounts of N-alkylation products. Further nucleophilic substitution reactions of S-ethylmethanobenzo[g][1,3,5]oxadiazocines and S-ethyl-methanonaphtho[2,3-g][1,3,5]oxadiazocines revealed that alongside nucleophilic substitution by morpholine, elimination of phenol or naphthol occurred, followed by aromatization of the dihydropyrimidinone ring into an aromatic pyrimidine ring, resulting in the formation of 2-morpholinyl-substituted pyrimidines. Only a few examples and multi-step syntheses for obtaining such 2amino-substituted pyrimidines are presented in the literature. Therefore, our newly discovered method for obtaining widely demanded derivatives of 2-aminopyrimidines is presented both relevant and promising.