N([O-])=O.[Na+].N[C:6]1[S:7][C:8]([CH2:15][CH3:16])=[C:9]([C:11]([O:13][CH3:14])=[O:12])[N:10]=1.[Na+].[Cl-:18].OS(O)(=O)=O>O.[Cl-].[Na+].O.Cl[Cu]>[Cl:18][C:6]1[S:7][C:8]([CH2:15][CH3:16])=[C:9]([C:11]([O:13][CH3:14])=[O:12])[N:10]=1
The following diazotization step was adapted from Barton, A. et al., J.C.S. Perkin Trans I, 159-164 (1982): A solution of NaNO2 (166 mg, 2.4 mmol) in water (0.6 mL) was added slowly to a stirred, cold (0° C.) solution of methyl 2-amino-5-ethyl-1,3-thiazole-4-carboxylate (186 mg, 1.0 mmol), CuSO4.5H2O (330 mg, 1.32 mmol), NaCl (260 mg, 4.45 mmol) and H2SO4 (5.5 mL) in water (7.5 mL). The mixture was stirred at 0° C. for 45 min and allowed to warm up to room temperature where it stirred further for 1 h before CuCl (118 mg) was added. This mixture was stirred further at room temperature for 16 h before it was diluted with brine and extracted with ether twice. The organic layers were combined, dried over MgSO4 and concentrated to give methyl 2-chloro-5-ethylthiazole-4-carboxylate (i.e., Cap-172, Step a) (175 mg, 85%) as an orange oil (80% pure) which was used directly in the next reaction. Rt=1.99 min (Cond.-MD1); LC-MS: Anal. Calcd. for [M+H]+ C7H9ClNO2S: 206.01; found: 206.05.
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