Preparation methods and Acylation reaction of 2-Aminopyridine

2-Aminopyridine is a pyridine derivative that typically appears as a light beige to pale yellow crystalline solid. 2-Aminopyridine exhibits remarkable basicity and excellent chemical stability, which make it a valuable compound in various chemical applications. Although 2-Aminopyridine is insoluble in water, it is readily soluble in alcoholic organic solvents and in dimethyl sulfoxide (DMSO). Owing to its solubility profile and stable nature, 2-Aminopyridine is commonly employed as an organic synthesis intermediate. By taking advantage of the chemical transformability of its amino group, 2-Aminopyridine can be utilized in the synthesis of pyridine-based pharmaceutical molecules. For instance, it has been reported in the literature that 2-Aminopyridine can be used in the synthesis of the analgesic and anti-inflammatory drugs piroxicam and lornoxicam, among others.

Basic introduction

2-Aminopyridine is a structurally simple, low‑molecular‑weight, and well‑functionalized compound that is widely recognized for its utility in the synthesis of various biologically active molecules. Across the globe, numerous pharmaceutical companies strive to prepare low‑molecular‑weight entities that can serve as pharmacophores directed against a range of biological targets. In the synthesis of such molecules and in guiding them toward their respective pharmacological objectives, 2-Aminopyridine can act as an ideal "driving unit" or locomotive. A major advantage of 2-Aminopyridine lies in its uncomplicated design, which allows for the generation of single products with minimal side reactions. Furthermore, because the synthesized compounds derived from 2-Aminopyridine have precisely defined and low molecular weights, this feature greatly facilitates the straightforward identification of metabolites responsible for toxicity during drug discovery programs.[1]

Preparation methods

Figure1: Synthesis of 2-Aminopyridine

Under a nitrogen atmosphere and with continuous magnetic stirring, the nitro-compound (0.7 mmol) and N,N-diisopropylethylamine (DIPEA, 3.5 mmol) were dissolved in dry acetonitrile (5 mL). Separately, a solution of freshly distilled trichlorosilane (HSiCl₃, 2.5 mmol) was prepared in 2 mL of dry acetonitrile. This resulting solution was then added dropwise to the first solution over a period of 10 minutes while maintaining the temperature at 0 °C. After the addition was complete, the reaction mixture was stirred for 18 hours, during which time the temperature was allowed to rise gradually from 0 °C to room temperature. Upon completion of the reaction, 5 mL of a saturated aqueous sodium bicarbonate (NaHCO₃) solution was added dropwise to the mixture. The resulting biphasic mixture was then stirred for 30 minutes at room temperature to ensure thorough quenching. The crude mixture was subsequently extracted with ethyl acetate, and the combined organic layers were dried over anhydrous sodium sulfate (Na₂SO₄). After filtration to remove the drying agent, the solvent was removed under reduced pressure to afford the crude product. Finally, the crude product was purified by flash column chromatography using mixtures of hexane and ethyl acetate (AcOEt) as the eluent to give the product 2-Aminopyridine. [2]

Acylation reaction

Figure2: Acylation reaction of 2-Aminopyridine

2-Aminopyridine (5.0 g, 1 equivalent) was dissolved in dichloromethane (DCM) while the solution was cooled in an ice bath. To this cooled mixture were then added potassium carbonate (K₂CO₃, 22.0 g, 3 equivalents) and 2-chloroacetyl chloride (6.3 mL, 1.5 equivalents), and the resulting reaction mixture was subsequently stirred at room temperature. The progress of the reaction was monitored by thin-layer chromatography (TLC) to ensure that the conversion had proceeded as desired. Upon completion, the reaction was quenched by the addition of a saturated aqueous solution of sodium bicarbonate (NaHCO₃). The quenched mixture was then extracted three times with dichloromethane (DCM), and the combined organic phases were dried over anhydrous magnesium sulfate (MgSO₄). After filtration to remove the drying agent, the solvent was removed under reduced pressure, and the resulting residue was purified by column chromatography on silica gel. The elution was carried out using a gradient that started with pure petroleum ether and gradually increased in polarity to a mixture of 10% ethyl acetate in petroleum ether (i.e., petroleum ether to 10% ethyl acetate in petroleum ether), which afforded the desired product, 2-chloro-N-(pyridin-2-yl)acetamide. [3]

Reference

[1] Rao R N, Chanda K. 2-Aminopyridine–an unsung hero in drug discovery[J]. Chemical Communications, 2022, 58(3): 343-382.

[2] Orlandi, M.; et al, Metal-Free Reduction of Aromatic and Aliphatic Nitro Compounds to Amines: A HSiCl3-Mediated Reaction of Wide General Applicability, Organic Letters (2015), 17(16), 3941-3943.

[3] Miao, Zhuang; et al, A Novel Bifunctional μOR Agonist and σ1R Antagonist with Potent Analgesia Responses and Reduced Adverse Effects, Journal of Medicinal Chemistry (2023), 66(23), 16257-16275.