4-Oxo-4H-pyridino[1,2-a]pyrimidine-3-diazonium tetrafluoroborate and related heteroaryl diazonium salts are useful reagents in organic synthesis, which are readily available in three steps from alkyl 2-acylamino-3-(dimethylamino)propenoates. In the course of this study on photoredox arylations of title diazonium salts, a novel synthetic method for the preparation of 3-(hetero)aryl-substituted quinolizinones and related fused pyrimidinones was developed. 2-Aminopyridine, 2-aminothiazole and 2-(pyridyl)acetonitrile were reacted in acetic acid with 2-acylamino-3-(dimethylamino)propenoate reagents to yield 3-benzoylamino-4H-pyridino[1,2-a]pyrimidin-4-one, 6-(benzyloxycarbonyl)amino-5H-thiazolo[3,2-a]pyrimidin-4-one and 3-(benzyloxycarbonyl)amino-1-cyano-4H-quinolizin-4-one. The benzoyl protecting group in the case of pyridino[1,2-a]pyrimidinone was removed with hydrochloric acid via acid hydrolysis and benzyloxycarbonyl protecting group in the case of thiazolo[3,2-a]pyrimidinone and 1-cyanoquinolizinone was removed with hydrobromic acid via acidolysis. The obtained amines were then converted into aryl diazonium tetrafluoroborates by the addition of tetrafluoroboric acid and sodium nitrite. After optimizing the reaction conditions, diazonium tetrafluoroborates were successfully arylated with (hetero)arenes, such as furan, thiophene, N-Boc-pyrrole, 2-methylfuran, ferrocene, benzene, benzo[b]thiophene and benzo[b]furan. The arylations were facilitated by the irradiation of the reaction mixture with green light in the presence of eosin Y disodium salt as a photocatalyst. The reactions were performed in a mixture of acetonitrile and deionized water, allowing easy isolation of the final products by column chromatography. Furthermore, while the scope of the reaction was investigated, the mechanism of arylation of diazonium tetrafluoroborates was also studied. Based on the obtained results, radical mechanism of photoredox arylations was proposed.
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