Diverse synthesis of pyrimido[1,2-a]benzimidazoles and imidazo[2,1-b]benzothiazoles via CuI-catalyzed decarboxylic multicomponent reactions of heterocyclic azoles,aldehydes and alkynecarboxylic acids

We have developed a straightforward approach to diverse synthesis of 2,3-, 2,4-disubstituted pyrimido 1,2-a]benzimidazoles, 2,4,10-trisubstituted 2,10-dihydropyrimido 1,2-a]benzimidazoles and 2,3-disubstituted imidazo 2,1-b]benzothiazoles via multicomponent reactions (MCRs) of heterocyclic azoles, aldehydes with easily storable and handling alkynecarboxylic acids. In the presence of a catalytic amount of CuI and K₂CO₃, the pyrimido 1,2-a]benzimidazole or imidazo 2,1-b]benzothiazole scaffold could be rapidly constructed through a 6-endo-dig or 5-exo-dig cyclization, respectively. The preliminary mechanistic study suggested that the formation of 2,3- disubstituted pyrimido 1,2-a]benzimidazoles, which completes the assembly of the scaffold and its C-3 position functionalization in one pot, undergoes a novel cascade process involving a decarboxylation, A 3] coupling, 6-endo-dig cyclization, nucleophilic addition and dehydration.