Oxidation reactions with iminium ions

The reaction of iminium ions with dihydropyridines is a method, suggested from biosynthetic studies, for the formation of carbon-carbon bonds to these six-membered heterocycles. The 1,4-dihydropyridine (8), a presumed intermediate from the reaction of ammonia with glutaraldehyde, reacts with the cyclic iminium ion (159) to give, after oxidation, nicotine (160) (72CC1091). Another example of this reaction has provided a total synthesis of olivacine (163). The 1,2-dihydropyridine ring system in (161), generated from its chromium tricarbonyl complex, was observed to undergo an intramolecular cyclization... 

Laboratory transformations took advantage of fragmentation of dregamine (30), tabemaemontanine (31), and vobasine (32) as N4 oxide induced by treatment with trifluoroacetic anhydride (Polonovski-Potier reaction) (192) and reductive capture of the intermediate iminium ion (Scheme 1). 

The formation of iminium ions of 20-epipandoline occurred only under Polo-novski-Potier conditions. Thus on treatment of the TV-oxide of 165 with tri-fluoroacetic anhydride followed by an aqueous solution of KCN, the iminium ion 329 was obtained, readily isolated as the corresponding a-amino nitrile 331 (Scheme 17). The reaction was completely regioselective and no traces of the enamine 332 could be obtained. This made the synthesis of spiroketone 333... 

The mercury(II) acetate oxidation of tertiary amines is frequently used to introduce unsaturation into piperidine derivatives. The iminium ion is believed to be an intermediate in this reaction. These ions can give the enamine or react further with nucleophiles to give more complex heterocycles (Scheme 4). 

Another example is provided by the application of the modified Polonovski reaction to A3-piperideines. Treatment of amine oxides (141) with trifluoroacetic anhydride results in the formation of iminium ion (142). These compounds can behave as useful synthetic intermediates, reacting with a number of nucleophiles such as cyanide ion (80JA1064). 

From the observation that N-(mcLhylcnc)morpholinium cations (17) induced the autocatalytic decomposition of tertiary amine W-oxides in combination with the proof that these intermediates were actually present in Lyocell dopes, the question arose why Lyocell solutions were stable at all. The answer is found in the fact that carbenium-iminium ions generated are immediately consumed by reaction with water and carbohydrate structures as the most abundant and most reactive nucleophiles present. Only when the local concentration of 17 increases to such a high level that the consumption by reaction with water or cellulose cannot compensate for its production, does the reaction with NMMO become uncontrollable and lead to an exothermic event. The pulp used in the Lyocell process acts as a quasi-stabilizer for the solvent NMMO, albeit with the drawback of increased chromophore generation. 

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