Reactions of substituted 1H-imidazole-2-thiols with pent-2-en-4-yn-1-ones.

The base-catalyzed reaction between 4-phenyl-1H-imidazole-2-thiol and 1,5-diaryl-substituted (E)-pent-2-en-4-yn-1-ones proceeded with the involvement of the C≡CCH=CH moiety of the substrate and HS, HN groups of the reagents to give 5H-imidazo[2,1-b]-[1,3]thiazine derivatives (6-endo-trig cyclization). Under the same conditions, the reactions of these linearly conjugated enynones with 4,5-diphenyl-1H-imidazole-2-thiol proceeded with the furan ring closure (5-exo-dig cyclization). The difference in the reaction pathways involving 4-phenyl- and 4,5-diphenyl-substituted 1H-imidazole-2-thiols was explained by the steric effects arising in the transition state. [ABSTRACT FROM AUTHOR]