Imination of N ‐methylpyridinium salts by liquid ammonia‐potassium permanganate . A new synthesis of nudiflorine

Reaction of substituted 1‐methyl(benzyl)pyridinium salts ( 1 ) with liquid ammonia/potassium permanganate leads to introduction of the imino group at the carbon adjacent to the nitrogen. The regiospecificity of the reaction strongly depends on substituent X: at C‐6 for X = H, CONH 2 , C 6 H 5 and at C‐2 for X = CH 3 . 3‐Aminocarbonyl‐1‐ t ‐butylpyridinium iodide ( 5 ) on treatment with liquid ammonia/potassium permanganate exclusively gives the 4‐imino compound 8 ; 1 H nmr spectroscopy shows that 5 in liquid ammonia gives a mixture of the σ‐adducts 4‐amino‐1,4‐dihydro‐ and 6‐amino‐1,6‐dihydro‐3‐pyridinecarbonamide ( 6 and 7 ). Surprisingly, an oxodemethylation reaction is observed on treatment of 3‐aminocarbonyl‐1,6‐dimethylpyridinium iodide ( 13 ) with liquid ammonia/potassium permanganate, 1,6‐dihydro‐1‐methyl‐6‐oxo‐3‐pyridinecarboxamide ( 14 ) being obtained. This compound can easily be converted by phosphorus oxychloride into the alkaloid nudiflorine ( 15 ).