Cyclization of ((4- or 5-substituted-2-benzimidazolyl)thio)acetic acids. Isolation and identification of two possible isomers of substituted thiazolo(3,2-a)benzimidazol-3(2H)-one.
作者:KENJIRO TANAKA、MICHIKO INO、YASUOKI MURAKAMI
DOI:10.1248/cpb.29.1876
日期:——
Cyclizations of [(5-substituted-2-benzimidazolyl) thio] acetic acid (2a-e) to the corresponding thiazolo [3, 2-a] benzimidazol-3 (2H)-one (5 and 6) were successfully carried out by heating 2a-e in Dowtherm A or Ac2O/pyridine, and gave the two possible isomers (5a-e and 6a-e) in a ratio of nearly 1 : 1 in all cases. Separation of 5 and 6 was successfully carried out, and their structures (namely, the direction of cyclization) were suggested on the basis of the NMR spectra. Dowtherm A was more effective than Ac2O/pyridine, particularly for the cyclization of [(5-nitro-2-benzimidazolyl) thio] acetic acid (2a), which had been reported not to be cyclized with other reagents. Similar cyclization of [(4-methyl-2-benzimidazolyl) thio] acetic acid (2f) preferentially gave 8-methylthiazolo [3, 2-a]-benzimidazol-3 (2H)-one (6f).
将[(5-取代-2-苯并咪唑基)硫代]乙酸(2a-e)加热环化成相应的噻唑并[3,2-a]苯并咪唑-3(2H)-酮(5和6)的反应已经成功实施,使用Dowtherm A或Ac2O/吡啶作为溶剂,并且在这两种情况下,均以近似1:1的比例得到了两种可能的异构体(5a-e和6a-e)。成功分离了5和6,并根据NMR谱图推断出了它们的结构,即环化的方向。Dowtherm A比Ac2O/吡啶更为有效,尤其是对于[(5-硝基-2-苯并咪唑基)硫代]乙酸(2a),此前有报道指出它不能用其他试剂环化。类似的环化反应中,[(4-甲基-2-苯并咪唑基)硫代]乙酸(2f)优先形成了8-甲基噻唑并[3,2-a]苯并咪唑-3(2H)-酮(6f)。