Regioselective C(sp2)-H halogenation of pyrazolo[1,5-a]pyrimidines facilitated by hypervalent iodine(iii) under aqueous and ambient conditions.

An efficient and mild approach has been developed for the regio-selective direct C3 halogenation of pyrazolo[1,5-a]pyrimidines employing readily available potassium halide salts and a hypervalent iodine(iii) reagent at ambient temperature. The protocol is both practical and environmentally friendly, utilizing water as a green solvent, potassium halides as an inexpensive and bench stable halogen source and PIDA as a non-toxic reagent, enabling clean and efficient halogenation at room temperature. The procedure yields a range of C3 halogenated pyrazolo[1,5-a]pyrimidines in good to excellent yields. Mechanistic studies suggest the involvement of electrophilic substitution mechanism in the halogenation process.

Visible Light Assisted Direct C3-H Arylation of Pyrido[1,2-a]pyrimidin-4-ones and Thiazolo[3,2-a]pyrimidin-5-ones.

Metal-catalyst-free, organic dye-catalyzed C3-H arylation of pyrido[1,2-a]pyrimidin-4-ones using visible light irradiation was developed under mild reaction conditions. This operationally simple and direct C-H functionalization approach effectively produced biologically significant C3 arylated pyrido[1,2-a]pyrimidin-4-one and thiazolo[3,2-a]pyrimidin-5-one derivatives, including medicinally important endothelial cell dysfunction inhibitor and anti-inflammatory agents in good to excellent yields with good functional group tolerance. The present photoinduced direct C3-H arylation approach was suitable for scale-up synthesis.

Organic-Dye-Catalyzed Visible-Light-Mediated Regioselective C-3 Alkoxycarbonylation of Imidazopyridines by Carbazates.

A mild and eco-friendly visible-light-mediated regioselective C-H alkoxycarbonylation of imidazo[1,2-a]pyridine heterocycles using rose bengal as a photoredox catalyst at room temperature has been developed. Biologically important alkoxycarboxylated imidazo[1,2-a]pyridines at the C-3 position as well as coumarins and quinoxalin-2(1H)-ones have been prepared. The present approach has the advantage of having a user- and eco-friendly catalyst, a carbonyl source, as well as extremely mild conditions for direct and regioselective C-H alkoxycarbonylation mediated by visible light as a green energy source.