Synthesis of benzimidazol-2-thiones from dimedone : An unexpected cyclisation into a five-membered ring

When enaminones 9 obtained from dimedone were used instead of furanone 3 and pyranone 4 in the reaction with carbon disulfide in presence of pyridine, a crystalline product was formed whose spectral data and X-Ray structure were inconsistent with the benzodiazepine-thiones 5-6 but agreed with the structure of benzodimidazol-2-thione 11 which is expected to possess a notable pharmacological activities. Theoretical calculations, with DFT/B3LYP method, have been carried out to rationalize the experimental results.