[NH:1]1[C:9]2[C:4](=[CH:5][CH:6]=[CH:7][CH:8]=2)[C:3]([CH2:10][CH2:11][SH:12])=[CH:2]1.[C:13]([O:19][CH3:20])(=[O:18])[CH2:14][C:15]([CH3:17])=O.C1C=CC=CC=1.C1(C)C=CC(S(O)(=O)=O)=CC=1>O>[CH3:20][O:19][C:13](=[O:18])[CH2:14][C:15]1([CH3:17])[C:2]2[NH:1][C:9]3[C:4]([C:3]=2[CH2:10][CH2:11][S:12]1)=[CH:5][CH:6]=[CH:7][CH:8]=3Indole-3-ethanethiol (1.5 g.) and methyl acetoacetate are mixed with 50 ml. of benzene and the solution heated for 30 min. (bath temperature 70° - 80° C.). p-Toluenesulfonic acid (0.15 g.) is added and the reaction mixture is subjected to reflux and stirring for 12 hours. Water formed in the reaction mixture during this period is collected by a water separator. After cooling the benzene solution is washed with 10% solution of sodium bicarbonate, water, saturated brine and dried over sodium sulfate. Evaporation of the benzene solution yields the ester, 1-methyl-1,3,4,9-tetrahydrothiopyrano[3,4-b]indole-1-acetic acid methyl ester as a semi-solid, γ maxCHCl 3 1715 cm-1.
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