Zincke’s salt

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. ... 

Another application of quinolinium and isoquinolinium salts is the formation of chiral salts following the use of Zincke s salt. These products can be treacted with nucleophiles (Grignard, stannane reagents, or even softer nucleophiles) to afford dihydroquinolines or isoquinolines diasteroselective-... 

Zincke s aldehyde (61) and cyanomethyltriphenylphosphonium chloride in acetic anhydride at 100 °C gave the salt (62), isolated as the perchlorate, whereas the same reagents in pyridine gave the phosphorane-phosphonium salt (63), presumably via nucleophilic addition of cyanomethylenephos-phorane to the terminal carbon of (62). 

Dinitrochlorobenzene reacts readily with pyridine, giving l-(2,4-dinitrophenyl) pyridinium chloride. Since Zincke s original observation of this reaction, several activated halogenobenzenes have been used, and activated nitro groups have also been replaced in quaternary salt formation. Examples are given in Table 5.16. 

The configuration of the amine was retained, except in the case of amino acid derivatives, which racemized at the stage of the pyridinium salt product. Control experiments showed that, while the starting amino acid was configurationally stable under the reaction conditions, the pyridinium salt readily underwent deuterium exchange at the rz-position in D2O. In another early example, optically active amino alcohol 73 and amino acetate 74 provided chiral 1,4-dihydronicotinamide precursors 75 and 76, respectively, upon reaction with Zincke salt 8 (Scheme 8.4.24). The 1,4-dihydro forms of 75 and 76 were used in studies on the asymmetric reduction of rz,>S-unsaturated iminium salts. 

---