[Ring cleavage of N-arylpyridinium salts by nucleophiles--regioselectivity and stereochemistry of the products. 2].
The reaction of quaternary 3-pyridinecarboxamide compounds 2, 7 and 14 with alkali does not--contrary to literature--result in regiospecific ring opening, but yields mixtures of position isomer penta-2,4-dienal derivatives. With increasing steric hindrance by the 3-substituent, the hydroxide ion prefers the attack at 6-position. Usually products with all trans configuration are obtained. Only the 2-substituted aldehydes from 2 and 14 additionally occur in the 2,3-cis configuration as 5b and 21b, explainable by a hydrogen bond between the formyl and carbamoyl group. Surprisingly the "ring opened" 5-aminopenta-2,4-dienals 5 and 6 resp. 21 and 22 in acetone solution show an equilibrium of the positional isomers in each case, which must be generated by recyclization via the intermediates 24 resp. 25. With hydroxylamine 2, 7 and 14 only react to the 4-substituted pentadienal derivatives, which represent E,Z oxime mixtures. This is proved by dehydration to uniform nitriles and the synthesis from the corresponding aldehydes by oximation.
Möhrle
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H
Institut für Pharmazeutische Chemie, Heinrich-Heine-Universität, Düsseldorf, Germany.
Niessen
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R
D000447:Aldehydes; D003500:Cyclization; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D006898:Hydroxylamines; D007202:Indicators and Reagents; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D011726:Pyridinium Compounds / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
D000447:Aldehydes; D006898:Hydroxylamines; D007202:Indicators and Reagents; D011726:Pyridinium Compounds