The synthesis and thermal and photochemical transformations of 1, 3-dipolar cycloadducts derived from 1, 2,3-triazolium-N-imides
Byrne, Ciaran (1996) The synthesis and thermal and photochemical transformations of 1, 3-dipolar cycloadducts derived from 1, 2,3-triazolium-N-imides. PhD thesis, Dublin City University.
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The cycloadducts formed on addition of triazolium-l-imides to a variety of dipolarophiles were originally assigned as pyrazolotriazoles. Subsequent re-examination in the 1980’s led to the discovery of a tandem cycloaddition - sigmatropic rearrangement mechanism leading to pyrrolotriazoles. These pyrrolotriazoles contain an azimine function as part of the ring system and the thermal and photochemical transformations of these compounds are of interest because of the wide structural variety found among the products.
In the current work on cycloaddition reactions leading to pyrrolotriazoles a number of side products, including triazoles and substituted anilines are identified The acid catalyzed epimerization of the cycloadduct was also observed. A range of new cycloadducts have been synthesized.
The thermal reaction of hexahydropyrrolo[2,3-i/]triazoles led to fragmentation to substituted triazoles The tetrahydro-analogues, however underwent a 1,3-sigmatropic rearrangement followed by electrocyclic ring expansion to substituted 2,5-dihydro-l,2,3- triazines in high yield, and the structure was confirmed by x-ray crystallography. The importance of the unsaturation was confirmed as hexahydro derivatives were oxidized and found to produce the triazines in high yield.
The photochemical rearrangements of these species were found to depend on both the degree of unsaturation and the bridgehead substituents. For the 3a,6a-diaryl hexahydroderivatives, irradiation led to a disrotatory ring expansion to the new 2,5,6,7-tetrahydro- 188.8.131.52-tetrazocines, the structure of which was confirmed by x-ray crystallographic determination. The tetrahydro-analogues, on ring opening to 2,5-dihydro-l,2,3,5- tetrazocine underwent a transannular ring contraction followed by a 1,4-sigmatropic rearrangement to imidazo[4,5-c]pyrazoles The proposed intermediacy of 2,5-dihydro- 184.108.40.206-tetrazocine in the rearrangement of 3a,6a,-diaryl tetrahydropyrrolotriazoles was confirmed.
Irradiation of 3a,6a-dimethyl hexahydropyrrolo[2,3-i/] 1,2,3-triazole led to pyrrolo[3,2- 6 ]indole via a complex series of sequential transformations. The pathway was investigated by isolation of the intermediates and was found to consist of an initial epimerization, a 1,2-phenyl migration and an eliminative rearrangement followed by 5-exotrig closure to pyrrolo[3,2-6]indole. Two intermediates were characterized by x-ray crystallographic analyses.
Irradiation of the tetrahydro-analogues led to the formation of substituted pyrroles and azobenzene.
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