[CH3:1][N:2]1[C:6]([SH:7])=[N:5][N:4]=[N:3]1.C([Li])CCC.[C:13](OCC)(=[O:19])[C:14]([O:16][CH2:17][CH3:18])=[O:15].Cl>O1CCCC1>[C:13]([CH2:1][N:2]1[C:6]([SH:7])=[N:5][N:4]=[N:3]1)([C:14]([O:16][CH2:17][CH3:18])=[O:15])=[O:19]A solution of 1-methyl-1H-tetrazole-5-thiol (23 g) in dry tetrahydrofuran (80 ml) was added over 20 minutes with stirring under dry nitrogen atmosphere to a n-butyllithium (250 ml, 15% in hexane) solution in dry tetrahydrofuran (250 ml) precooled to -15° to -10° C. After stirring at the same temperature for 40 minutes, the mixture was cooled to -60°±5° C. and thereto was added dropwise a solution of diethyl oxalate (28.95 g) in dry tetrahydrofuran (80 ml) over 30 min. The reaction mixture was stirred at -60°±5° C. for 1.5 hours. 10% Aqueous hydrochloric acid (72 ml) was added with stirring and the separated aqueous layer (pH 7.5) was adjusted to pH 2.5 with 10% hydrochloric acid and extracted with ethyl acetate (200 ml×3). The combined extracts were dried over magnesium sulfate and evaporated to give oily residue of 1-ethoxalylmethyl-1H-tetrazole-5-thiol (34.3 g).
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