Diverse Reactivities of Acetylenic Iminium Salts Toward 1,3-Oxazolium-5-olates (Münchnones)

Abstract

The acetylenic iminium salts [(c-C₃H₅)C≡C-C(Ar)=N+Me₂]OTf, Ar = phenyl (1a) or 2- thienyl (1b), both react in different ways with three mesoionic münchnones, namely 3-methyl-1,3-oxazolium-5-olate (2), 3-methyl-2-phenyl-1,3-oxazolium-5-olate (6) and 3-methyl-2-phenyl-4- trifluoroacetyl-1,3-oxazolium-5-olate (9). With 2, a [3+2] cycloaddition reaction followed by extrusion of CO₂ yields pyrroles 5a, b. In the case of 6, the new münchnones 7a, b are obtained which result from an electrophilic substitution at C-4 by the acetylenic iminium cation. In contrast to 1a, b, the 4,4-but-2-yne 1-iminium salt 1c reacts with münchnone 6 to form pyrrole 8. Finally, the reaction of 1a, b and 9 affords (6-oxo-6H-pyran-3-yl)methylene iminium salts 10a, b under microwave heating conditions. The structures of pyrrole 5a, münchnone 7a, and trifluoromethyl-substituted pyranone 10a were established by single-crystal X-ray diffraction analysis.