Nucleophilic Addition Followed by Ring Opening

There are many examples of pyridinium salts, particularly, but not exclusively, those with powerful electron-withdrawing A-substituents, adding a nucleophile at C-2 and then undergoing a ring opening. The classic example is addition of hydroxide to the pyridine sulfur trioxide complex, which produces the sodium salt of glutaconaldehyde, as shown below. 

Another intriguing example is a synthesis of azulene that utilises the bis(dimethylamine) derivative of glutaconaldehyde produced with loss of 2,4-dinitroaniline from l-(2,4-dinitrophenyl)pyridinium chloride (Zincke s salt).  

The reaction of Zincke s salt with primary amines, inclnding a-amino acid esters, is a nsefnl synthesis of varionsly iV-substitnted pyridininm salts the ititrogen of the final prodnct is the nitrogen of the primary amine reactant.  

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Furoxancarboxylic acids are characterized by a more or less ready decomposition, which is probably invariably accompanied by ring cleavage. The derivatives are much more stable. Scheme 12 illustrates a number of the reactions of furoxandicarboxylic acid derivatives. The tendency to nucleophilic addition, followed by ring opening with deacylation, is marked in the diacyl series some of these reactions were covered in Sections V,C,1 and 2. 

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