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Synlett 2020; 31(04): 363-368
DOI: 10.1055/s-0039-1691567
DOI: 10.1055/s-0039-1691567
letter
Metal-Free Regioselective Alkylation of Imidazo[1,2-a]pyridines with N-Hydroxyphthalimide Esters under Organic Photoredox Catalysis
Bin Sun
a
Collaborative Innovation Center of Yangtze River Delta Region Green Pharmaceuticals, Zhejiang University of Technology, Hangzhou 310014, P. R. of China
c
National Engineering Research Center for Process Development of Active Pharmaceutical Ingredients,
Zhejiang University of Technology, Hangzhou 310014, P. R. of China Email: jincan@zjut.edu.cn
,
Tengwei Xu
b
College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou 310014, P. R. of China Email: jincan@zjut.edu.cn
c
National Engineering Research Center for Process Development of Active Pharmaceutical Ingredients,
Zhejiang University of Technology, Hangzhou 310014, P. R. of China Email: jincan@zjut.edu.cn
,
Liang Zhang
b
College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou 310014, P. R. of China Email: jincan@zjut.edu.cn
c
National Engineering Research Center for Process Development of Active Pharmaceutical Ingredients,
Zhejiang University of Technology, Hangzhou 310014, P. R. of China Email: jincan@zjut.edu.cn
,
Rui Zhu
b
College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou 310014, P. R. of China Email: jincan@zjut.edu.cn
c
National Engineering Research Center for Process Development of Active Pharmaceutical Ingredients,
Zhejiang University of Technology, Hangzhou 310014, P. R. of China Email: jincan@zjut.edu.cn
,
Jin Yang
b
College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou 310014, P. R. of China Email: jincan@zjut.edu.cn
c
National Engineering Research Center for Process Development of Active Pharmaceutical Ingredients,
Zhejiang University of Technology, Hangzhou 310014, P. R. of China Email: jincan@zjut.edu.cn
,
Min Xu
a
Collaborative Innovation Center of Yangtze River Delta Region Green Pharmaceuticals, Zhejiang University of Technology, Hangzhou 310014, P. R. of China
c
National Engineering Research Center for Process Development of Active Pharmaceutical Ingredients,
Zhejiang University of Technology, Hangzhou 310014, P. R. of China Email: jincan@zjut.edu.cn
,
Can Jin∗
a
Collaborative Innovation Center of Yangtze River Delta Region Green Pharmaceuticals, Zhejiang University of Technology, Hangzhou 310014, P. R. of China
b
College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou 310014, P. R. of China Email: jincan@zjut.edu.cn
c
National Engineering Research Center for Process Development of Active Pharmaceutical Ingredients,
Zhejiang University of Technology, Hangzhou 310014, P. R. of China Email: jincan@zjut.edu.cn
› Author AffiliationsThis work was supported by the National Natural Science Foundation of China (21606202) and Natural Science Foundation of Zhejiang Province (LY15B060007). We are also grateful to the College of Pharmaceutical Sciences, Zhejiang University of Technology and Collaborative Innovation Center of Yangtze River Delta Region Green Pharmaceuticals for the financial help.Further Information
Publication History
Received: 14 November 2019
Accepted after revision: 29 December 2019
Publication Date:
14 January 2020 (online)
Abstract
A visible-light-induced direct C–H alkylation of imidazo[1,2-a]pyridines has been developed. It proceeds at room temperature by employing inexpensive Eosin Y as a photocatalyst and alkyl N-hydroxyphthalimide (NHP) esters as alkylation reagents. A variety of NHP esters derived from aliphatic carboxylic acids (primary, secondary, and tertiary) were tolerated in this protocol, giving the corresponding C-5-alkylated products in moderate to excellent yields. Mechanistic studies indicate that a radical decarboxylative coupling pathway was involved in this process.
Supporting Information
- Supporting information for this article is available online at https://doi.org/10.1055/s-0039-1691567.
- Supporting Information
-
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- 17 5-(tert-Butyl)-7-chloro-2-phenylimidazo[1,2-a]pyridine (3aa); Typical Procedure A 10 mL Schlenk-tube was charged with 7-chloro-2-phenylimidazo[1,2-a]pyridine (1a) (114 mg, 0.5 mmol), tert-butyl NHP ester 2a (247 mg, 1.0 mmol), Eosin Y (3 mg, 0.005 mmol), DMSO (2 mL), and TfOH (37 mg, 0.25 mmol). The tube was evacuated and backfilled with N2 (3×). The mixture was then irradiated by 3 W blue LEDs at r.t. for 48 h. The reaction mixture was then quenched with 1 M NaOH aq (15 mL) and extracted with CH2Cl2 (3 × 10 mL). The organic layer was dried with anhydrous Na2SO4 and concentrated under reduced pressure. The crude product was purified by flash chromatography on silica gel (EtOAc/PE, 1:20) to afford 3aa as a white solid. Yield: 116 mg (82%); mp 141.6–142.4 °C. 1H NMR (400 MHz, CDCl3): δ = 8.07 (s, 1 H), 7.96 (d, J = 7.2 Hz, 2 H), 7.58 (s, 1 H), 7.45 (t, J = 7.6 Hz, 2 H), 7.35 (t, J = 7.2 Hz, 1 H), 6.71 (s, 1 H), 1.57 (s, 9 H). 13C NMR (100 MHz, CDCl3): δ = 147.1, 146.5, 145.7, 133.6, 131.5, 128.8, 128.2, 126.2, 114.3, 110.7, 108.9, 35.6, 27.6. HRMS (ESI): m/z [M + H]+ calcd for C17H18ClN2: 285.1153; found: 285.1163.