- Synthese und Photoisomerisierung von sterisch gehinderten 2,6-Dialkylpyridin-N-oxiden
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Synthesis of the new sterically hindered pyridine-1-oxides 2c and 6a-c is described.Copper(II)-catalyzed photolysis of 2 and 6a in aqueous medium provides only small amounts of 2-acylpyrroles 8 and 10a together with by-products.Exclusive formation and better yields of 10 can be received upon irradiation of 6 in none protic solvents at low temperature.Introduction of bulky substituents as in 6 failed to stabilize any of the postulated intermediates in order to be identified during photoreaction.It is assumed that the high rate of polymerization is caused by unstable 1,3-oxazepines like 13.
- Weber, Horst,Rohn, Thomas
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p. 701 - 706
(2007/10/02)
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- PHOTOCHEMISTRY OF PYRIDINE N-OXIDES. TRAPPING OF AN INTERMEDIATE WITH AMINES
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The photochemistry of pyridine N-oxide (1a) and the corresponding 4-benzyl (1b), 4-phenyl (1c), and 2,6-dimethyl (1d) derivatives has been investigated or reinvestigated.Substrates 1a and 1b yield a small amount of the corresponding 2-acylpyrroles (5a,b) as the only isolable product, 1d yields 5d in moderate yield and 1c yields 5c in aprotic and the 2-pyridone 7c in protic solvents.In the presence of diethylamine isomeric 5-diethylaminopentadienenitriles (3,4) are obtained in moderate yield along with deoxygenated pyridines (6).Under this condition no 5 is formed, but the yield of 7 is unchanged.These reactions are most economically accunted for by admitting that nitrene 11 is formed as an intermediate and undergoes rearrangement to pyrroles 5 or polymerization to tars, or it can be trapped by amines (proton abstraction followed by nucleophilic substitution) to yield products 3 and 4.The lactams 7 arise directly from the N-oxide singlet excited state, which is also involved in the deoxygenation by amines.
- Albini, Angelo,Fasani, Elisa,Lohse, Christian
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p. 113 - 124
(2007/10/02)
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- Intramolecular Thermal Oxidoreduction of N-(2-Hydroxypropyl)-β-enaminoesters: Synthesis of N-(Acetonyl)-β-enaminoaldehydes and 2-Acetylpyrroles
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Flow pyrolysis of the β-enaminoesters (1) provides the enaminoaldehydes (2), resulting from intramolecular oxidoreduction, and the acetylpyrroles (3); their relative yields depend on the temperature range.
- Maujean, Alain,Grosdemange-Pale, Catherine,Marcy, Guy,Chuche, Josselin
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p. 1135 - 1136
(2007/10/02)
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