Synthesis of Novel Diastereomer Pyrimido[1,2-a]Benzimidazoles by Using Silica Sulfuric Acid/Ethylene Glycol

The scope of the reaction of 2-aminobenzimidazole, aldehydes and diethyl malonate in the presence of silica sulfuric acid/ethylene glycol was investigated. The reaction gave two unexpected products, 2-hydroxyethyl 3-hydroxy-4-aryl-2- oxo-tetrahydropyrimidobenzimidazole-3-carboxylates 2 and 4-aryl-hydropyrimido[1,2-a]- benzimidazol-2(1H)-ones 3 without isolation of the expected ethyl 4-aryl-2-oxo-1,2,3,4-1,2,3,4-tetrahydropyrimido[1,2-a]benzimidazole-3-carboxylates 1. NMR spectroscopic data and X-Ray analysis were used for assignment and confirmation the structures of the unexpected products. 2-Hydroxyethyl carboxylate derivatives 2 may be obtained via formation of 1 as intermediates, which were subjected to trans-esterification by ethylene glycol and oxidation of the proton on pyrimidine ring at position-3 to afford products 2. The unexpected products 3 may be obtained via formation of compounds 1 that was subjected to deesterification followed by decarboxylation to afford compounds 3.