[Ring cleavage of N-arylpyridinium salts by nucleophiles--regioselectivity and stereochemistry of the products. 1].

The N-arylpyridinium salts 1, 5 und 10 generate with hydroxylamine a ring cleavage to the E/Z-oximes of the (2E,4E)-5-aminopentadienals 2, 6 und 11. With the 3-methylpyridinium derivative 12 the described regiospecifity of the reaction with nucleophiles is disproved. The attack by hydroxylamine is realized to a nearly equal degree at C-2 and C-6. As 2-methyl substituted products the E-oximes of (2E,4E)- and (2Z,4E)-5-aminopentadienal result, while as 4-methyl substituted products the E/Z-oximes of (2E,4E)-5-aminopentadienal are generated. Just so with the hydroxide ion as nucleophile from 12--in contrast to the literature--both position isomer 5-aminopentadienals are formed. They were characterized as 15 and 16. The quaternized product of nicotine 22 shows ring opening with hydroxide ion, not at C-2 as described formerly, but only at 6-position and gives rise to the 4-(N-methylpyrrolidinyl) substituted 5-aminopentadienal 23. By hydroxylamine only the E/Z-oxime mixture of the 4-substituted 5-aminopentadienal 25 is formed, which can be dehydrated to the nitrile 27.