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Iminium-functionalized 2,3-dihydroisoxazoles (4-isoxazolines) were prepared in high yields by [3+2] cycloaddition of N-methyl-C-phenylnitrone and acetylenic iminium salts. Conversion of the very hygroscopic cycloadducts into neutral compounds was achieved by hydrolysis or hydride reduction of the iminium functional group. By a thermal rearrangement, the cycloadducts were converted into 2,3-dihydro-1H-imidazolium salts. With C,N-diphenylnitrone as the reaction partner, the expected cycloadduct, a [(2,3-dihydroisoxazol-4-yl)-1-(phenyl)methylene]-N,N-dimethylammonium triflate, could be prepared at −75°C and was characterized by NMR. This product was found to be thermally unstable above −30°C, being converted into an inseparable product mixture. Basic hydrolysis of this mixture afforded a structurally interesting acyclic enaminone/half-aminal, which is likely derived from a 2,3-dihydro-1,3-oxazole.

Acetylenic iminium triflates with the general formula [R–C≡ C–C(Ar)=N+R2 TfO−] were found to be excellent dipolarophiles in [3+ 2] cycloaddition reactions with diazoacetates leading to (1H-pyrazol-3(5)-yl)methanaminium triflates in high yields. The terminal acetylenic iminium salt (propyne iminium salt) [HC≡C–C(Ph)=N+Me2 TfO−] reacted with an equimolar amount of methyl diazoacetate instantaneously at 20°C to form the expected pyrazole in almost quantitative yield. When a 2:1 stoichiometry was applied, subsequent Michael addition of the pyrazole at the alkyne occurred and the bis(iminium) ditriflate 4 was obtained in high yield. By hydride reduction or hydrolysis of the iminium group, some of the highly hygroscopic pyrazole iminium salts were converted into neutral, twofold functionalized, di- and tri-C-substitued 1H-pyrazoles.