[CH2:1]([C:3]1[S:11][C:6]2=[N:7][CH:8]=[CH:9][CH:10]=[C:5]2[CH:4]=1)[CH3:2].[Cl:12][S:13](O)(=[O:15])=[O:14].P(Cl)(Cl)(Cl)=O.P(Cl)(Cl)(Cl)(Cl)Cl>C(Cl)Cl>[CH2:1]([C:3]1[S:11][C:6]2=[N:7][CH:8]=[CH:9][CH:10]=[C:5]2[C:4]=1[S:13]([Cl:12])(=[O:15])=[O:14])[CH3:2]
To a solution of 2-ethylthieno[2,3-b]pyridine (0.43 g, 2.6 mmoles) in CH2Cl2 (10 ml) at −78° C. was added chlorosulfonic acid (0.9 ml, 13 mmoles). The reaction was warmed to ambient temperature over a 1 hr. period, then stirred an additional 20 hrs., followed by the addition of phosphorus oxychloride (1.7 ml, 18 mmoles) and phosphorus pentachloride (0.82 g, 4.0 mmoles). One hour later the reaction mixture was added to ice (100 ml) and extracted with EtOAc (75 ml), then washed with sat. NaHCO3 (200 ml). The organic phase was dried (MgSO4), filtered and concentrated to collect a brown oil. Flash chromatography (10% EtOAc/hexanes) provided 0.39 g (56%) of the title compound as a light yellow oil.
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