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Upon reaction of 1-arylamino-imidazole-2-thiones 1 with dimethyl acetylenedicarboxylate (DMAD) in the presence of 2.2 equiv of sodium hydride, imidazothiazoles 4 were exclusively formed (71–82% yield). However, from the reaction of 1 with DMAD in the absence of base, only the S -substituted products 5 were formed as an E /Z mixture (53–55%), which could also be converted to 4 with 2.0 equiv of sodium hydride (65–68%). Furthermore, 5a - E / Z was converted to arylamino-substituted derivatives 8a upon reaction with 1.1 equiv of sodium hydride in 78% yield. Formation of 8a (75% yield) was also possible by reaction of thione 1a with DMAD in the presence of sodium methoxide in methanol. Substitution on the imidazole 3-NH of thione 1a leading to 6a - Z was observed either by heating 1a with DMAD (91%) or by heating the 5a - E / Z mixture in benzene (90% yield). Finally, when 5a - E reacted with acetic anhydride the acetylated derivative 7a - Z was the only isolated product (58%). Full assignment of all 1 H and 13 C NMR chemical shifts has been unambiguously achieved. |
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