Synthese von Pyrimido[1,2-a]benzimidazolen durch Umsetzung von 2-Aminobenzimidazol mit Acetylendicarbonsäure-dimethylester und ihre Überführung in Imidazo[1,2-a]benzimidazole
✍ Scribed by Franz Troxler; Hans Peter Weber
- Publisher
- John Wiley and Sons
- Year
- 1974
- Tongue
- German
- Weight
- 509 KB
- Volume
- 57
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
The main product of the reaction between 2‐aminobenzimidazole and dimethyl acetylenedicarboxylate is shown to be methyl 1, 2‐dihydro‐2‐oxo‐pyrimido[1, 2‐a]benzimidazole‐4‐carboxylate (6), which can be methylated at position 1 to give 7. Catalytic hydrogenation of 7 leads to the 1,2,3,4‐tetrahydro derivative 8, whereas NaBH~4~ reduces the ester and, to some extent, the double bond to yield a mixture of 9 and 10. When a 1‐substituted 1, 2‐dihydro‐4‐hydroxymethyl‐pyrimido[1, 2‐a]benzimidazol‐2‐one (for example 11) is catalytically hydrogenated, the double bond is unaffected, but hydrogenolysis of the alcohol group occurs instead to give 13. The lactam group is less stable in the tetrahydro series than in the dihydro compounds. For example, the lactam is cleaved when 8 is treated with amines containing a small amount of water, and the monoamides 16 and 17 are formed. Similarly, sodium hydroxyde cleaves the lactam under mild conditions to the dicarboxylic acid 19, which can be converted to 2, 3‐dihydro‐1‐methyl‐2‐oxo [1__H__]imidazo[1, 2‐a]benzimidazole‐3‐acetic acid (4‐methyl)piperazide (20) with thionyl chloride and N‐methylpiperazine. However, when 7 is treated with methylamine at low temperature, the amide 22 is formed, whilst at room temperature the amine attacks both the ester and the double bond to give 23. The structure of 8 was confirmed by X‐ray analysis.