N1CCC[C@H]1C(O)=O.[CH3:9][S:10][C:11]1[S:12][C:13]2[CH:19]=[C:18]([CH2:20][CH2:21][CH:22]=[O:23])[CH:17]=[CH:16][C:14]=2[N:15]=1.[Cl:24]N1C(=O)CCC1=O>C(Cl)Cl>[Cl:24][CH:21]([CH2:20][C:18]1[CH:17]=[CH:16][C:14]2[N:15]=[C:11]([S:10][CH3:9])[S:12][C:13]=2[CH:19]=1)[CH:22]=[O:23]L-proline (0.22 g, 1.9 mmol) was added to a solution of 3-(2-(methylthio)benzo[d]thiazol-6-yl)propanal (2.3 g, 9.7 mmol) in DCM (40 mL) at 0° C. followed by addition of N-chlorosuccinimide (1.4 g, 10 mmol). The reaction mixture was slowly warmed to ambient temperature and stirred overnight. The reaction mixture was concentrated under reduced pressure and purified by silica gel chromatography, eluting with 10:1 to 2:1 petroleum ether/EtOAc to afford 2-chloro-3-(2-(methylthio)benzo[d]thiazol-6-yl)propanal as a yellow oil (2.1 g, 79%). 1H NMR (300 MHz, CDCl3) δ 9.56 (dd, J=1.2, 2.1 Hz, 1H), 7.81 (d, J=8.4 Hz, 1H), 7.63 (d, J=0.6 Hz, 1H), 7.28 (d, J=1.5 Hz, 1H), 4.44-4.39 (m, 1H), 3.51-3.44 (m, 1H), 3.20-3.13 (m, 1H), 2.78 (s, 3H). LCMS (ESI) m/z 290 (M+18+H)+.
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