Microwave-induced Pericyclic Reactions of Aza- and Diazafulvenium Methides
Maria I.L. Soares and Teresa M. V.D. Pinho e Melo
Affiliation: Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal.
The pericyclic reactions of 4,5-dimethoxycarbonyl-substituted aza- and diazafulvenium methides generated under
microwave-induced thermolysis conditions have been described. The 7,7-dimethyl-substituted derivatives undergo allowed
suprafacial sigmatropic [1,8]-hydrogen shifts leading to the efficient synthesis of C-vinylpyrroles and Cvinylpyrazoles.
These intermediates could not be trapped in [8Π+2Π] cycloaddition in the presence of dipolarophiles. Azaand
diazafulvenium methides unsubstituted at C-1 and C-7 behave as 8Π 1,7-dipoles in the presence of dipolarophiles under
microwave irradiation, leading to a range of pyrrolo and pyrazolo annulated heterocycles resulting from addition
across positions 1 and 7. Aza- and diazafulvenium methide derivatives bearing only one methyl group at C-7 are intercepted
in sigmatropic hydrogen shifts to give functionalized pyrroles and pyrazoles, respectively. In the presence of electron-
deficient dipolarophiles, these 7-methyl-substituted derivatives act as 1,7-dipoles in [8Π+2Π] cycloadditions, however,
competitive formation of the corresponding vinyl pyrroles and vinylpyrazoles is also observed.
Keywords: Azafulvenium methides, azomethine imines, azomethine ylides, diazafulvenium methides, 1, 7-dipolar cycloaddition,
dipolarophiles, 1, 7-dipoles, flash vacuum pyrolysis, microwave, pyrazolo[1, 5-c]thiazoles, pyrrolo[1, 2-c]thiazoles, sigmatropic
shift, sulfones, sulfur dioxide, thermolysis, vinylpyrroles, vinylpyrazoles.
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