20165-96-2Relevant articles and documents
Ring cleavage of N-arylpyridinium salts by nucleophiles - Regioselectivity and stereochemistry of the products - Part 2
Moehrle,Niessen
, p. 448 - 457 (2007/10/03)
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 2lb, 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.
