[CH:1]([C:3]1[S:7][C:6]([C:8]([OH:10])=[O:9])=[CH:5][C:4]=1[CH3:11])=O.[CH2:12]([NH:14][CH2:15][CH3:16])[CH3:13].C(O)(=O)C.C(O[BH-](OC(=O)C)OC(=O)C)(=O)C.[Na+]>C1COCC1>[CH2:12]([N:14]([CH2:1][C:3]1[S:7][C:6]([C:8]([OH:10])=[O:9])=[CH:5][C:4]=1[CH3:11])[CH2:15][CH3:16])[CH3:13]
To a solution of 5-formyl-4-methyl-thiophene-2-carboxylic acid (6.90 g, 40.5 mmol) in THF (150 mL), diethylamine (5.93 g, 81.1 mmol) and acetic acid (10 mL) followed by sodium triacetoxyborohydride (14.32 g, 60.8 mmol) is added. The mixture becomes slightly warm. The reaction mixture is stirred at rt for 16 h before it is concentrated and separated by CC on silica gel eluting with EA containing 0-50% of methanol to give the title compound (4.95 g) containing large amount of diethylammonium salts. The material (4.94 g, 21.8 mmol) is therefore dissolved in methanol (70 mL) and treated with chloro trimethylsilane (23.7 g, 218 mmol). The mixture is stirred at 50° C. for 18 h before it is concentrated, dissolved in EA and washed with sat. aq. NaHCO3 solution. The organic extract is dried over Na2SO4, filtered, concentrated and dried to give 5-diethylaminomethyl-4-methyl-thiophene-2-carboxylic acid methyl ester (4.05 g) as a yellow oil; 1H NMR (CDCl3): δ 1.08 (t, J=7.0 Hz, 6H), 2.19 (s, 3H), 2.59 (q, J=7.0 Hz, 4H), 3.67 (s, 2H), 3.87 (s, 3H), 7.28 (s, 1H), 7.52 (s, 1H). This material is dissolved in 1 N aq. HCl (80 mL) and stirred at 65° C. for 214 h. The solvent is evaporated and the residue is dried under high vacuum to give the title compound (3.56 g) as a pale beige solid; LC-MS: tR=0.26 min; [M+1]+=228.20; 1H NMR (D6-DMSO): δ 1.28 (t, J=7.3 Hz, 6H), 2.31 (s, 3H), 3.12 (q, J=6.8 Hz, 4H), 4.48 (s, 2H), 7.59 (s, 1H).
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