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5-Chloro-1H-pyrazolo[4,3-D]pyrimidine is used in preparation of pyrimidine derivatives and related heterocycles as DNA-dependent protein kinase inhibitors for treatment of diseases.

5-Chloro-1H-pyrazolo[4,3-d]pyrimidine is a key heterocyclic chemical building block widely used in the construction of protein kinase inhibitors (such as JAK inhibitors and SYK inhibitors), as well as the heterocyclic core scaffolds of anti-tumour, anti-inflammatory and cardiovascular drugs. The chlorine atom at the 5-position in its structure readily undergoes nucleophilic aromatic substitution reactions, allowing the introduction of diverse side chains through reactions with primary/secondary amines, alcohols/phenols and thiols.

5-Chloro-1H-pyrazolo[4,3-d]pyrimidine is irritating to humans. Exposure may cause respiratory tract irritation, skin irritation and severe eye irritation. It is acutely toxic if ingested.

General procedure for the synthesis of 5-chloro-1H-pyrazolo[4,3-d]pyrimidin-1-yl from 1-(5-chloro-1H-pyrazolo[4,3-d]pyrimidin-1-yl)ethanone: To a stirred solution of 1-(5-chloro-1H-pyrazolo[4,3-d]pyrimidin-1-yl)ethanone (1.5 g, 7.65 mmol) in tetrahydrofuran (THF, 15 mL), was added an 8% hydrochloric acid aqueous solution (14.46 mL). The reaction mixture was refluxed at 50 °C for 30 min. After completion of the reaction, it was cooled to 25 °C and extracted with ethyl acetate (EtOAc, 2 × 50 mL). The organic layers were combined, dried over anhydrous sodium sulfate (Na2SO4), filtered and concentrated under reduced pressure to afford 5-chloro-1H-pyrazolo[4,3-d]pyrimidine as an off-white solid (1.1 g, 93% yield). Mass spectrum (ESI): m/z = 153.0 [M-H]-.

[1] Patent: WO2017/137334, 2017, A1. Location in patent: Page/Page column 123; 124
[2] Patent: CN105906621, 2016, A. Location in patent: Paragraph 0035
[3] Patent: US2012/202785, 2012, A1. Location in patent: Page/Page column 263