2-Azapyrylium salts from aluminum halide .sigma. complexes of cyclobutadienes and nitrosyl chloride. On the possible intermediacy of a heteroatom-substituted pyramidal cation

The Application of Cyclobutane Derivatives in Organic Synthesis

“…In extension of the well-known cycloaddition of cyclobutadiene radical cations to alkynes as well as nitriles giving benzenes and pyridines, respectively, Hogeveen et al developed the synthesis of 2-azapyrylium salts using nitrosyl chloride, thus broadening the scope of this methodology. The tetramethylazapyrylium salt 410 was obtained in 95% yield from the corresponding cyclobutyl cation 409 143 …”

“…The tetramethylazapyrylium salt 410 was obtained in 95% yield from the corresponding cyclobutyl cation 409. 192 Saalfrank et al reported a convenient access to 3,4dihydro-2,4-dioxo-2H-pyrans and 2-pyrones providing synthetic equivalents of 1,3-dianions. In an interesting reaction without solvent, a mixture of the spirocyclic 411 and aluminum trichloride was heated at 130-140 °C to give the tetrahydropyran-annelated R-pyrone 412 in 80% yield.…”

Interaction in olefin–NO+ complexes: structure and dynamics of the NO+–2,3-dimethyl-2-butene complex

“…Olefin-NO + π-complexes have often been postulated as intermediates in reactions of olefins with nitrosyl halides. 1,2 These reactions play an important role in the syntheses of various organic compounds such as nitroso and nitro halides, [1][2][3][4] aza heterocycles, 5 etc. However, available experimental data on these π-complexes are few in number (cf.…”