Reaction with pyridinium salts

The use of microwave has recently been used to provide 5-amino-imidazoles from 2-aminopyridines. At a higher temperature and longer reaction times, dehydration occurs to form the imidazo-pyridinium salts. Reaction with hydrazine adds to the pyrimidinium salt, which then undergoes rearrangement to open the ring. The unstabilized diene undergoes hydrolysis to provide the 2-amino-4,5-di-substituted 1/f-imidazole. Chemically, PPA can be used to form the pyrimidinium salt also. ... 

The nitration of l,2,5-selenadiazolo[3,4-/] quinoline 77 with benzoyl nitrate affords the 8-nitro derivative 78, whereas methylation with methyl iodide or methyl sulfate afforded the corresponding 6-pyridinium methiodide 79 or methosulfate 80, respectively (Scheme 29). The pyridinium salt 80 was submitted to oxidation with potassium hexacyanoferrate and provided 7-oxo-6,7-dihydro derivative 81 or, by reaction of pyridinium salt 79 with phenylmagnesium bromide, the 7-phenyl-6,7-dihydro derivative 82. Nucleophilic substitution of the methiodide 79 with potassium cyanide resulted in the formation of 9-cyano-6,9-dihydroderivative 83, which can be oxidized by iodine to 9-cyano-l,2,5-selenadiazolo [3,4-/]quinoline methiodide 84. All the reactions proceeded in moderate yields (81IJC648). 

Thermolysis of 1-imidoyliminopyridinium N-ylides 109 gave the pyrido-triazines 110 in addition to 111 and 112 (76CL413 77JOC443). Compounds 109 were obtained by the reaction of pyridinium salts 107 with 108. 

The 1-nitroindolizine (113) was formed in a reaction of the pyridinium salt (112) with nitromethane/triethylamine in methanol. Successive treatment of the quinolizinium bromide (114) with piperidine and an a-bromo ketone yielded (115), which was cyclized on an alumina column (Scheme 16) (72TL2607). 

Support-bound alkylating agents have been used to N-alkylate pyridines and dihydropyridines (Entries 7 and 8, Table 15.21). Similarly, resin-bound pyridines can be N-alkylated by treatment with a-halo ketones (DMF, 45 °C, 1 h [267]) or other alkylating agents [246]. Polystyrene-bound l-[(alkoxycarbonyl)methyl]pyridinium salts can be prepared by N-alkylating pyridine with immobilized haloacetates (Entry 8, Table 15.21). These pyridinium salts react with acceptor-substituted alkenes to yield cyclopropanes (Section 5.1.3.6). Pyridinium salts have also been prepared by reaction of resin-bound primary amines with /V-(2,4-dinitrophenyl)pyridinium salts [268,269]. 

The mechanistic aspects of this reaction have been more extensively investigated with pyridine and pyridinium salts than with other... 

Other reactants containing a primary amino gronp will also convert pyryliums into A -substituted nitrogen heterocycles iV-amino azoles are amongst several types of hydrazine derivatives to have been utilised these give iV-(heteroaryl)-pyridinium salts. Reaction of pyryliums with hydroxylamine comparably leads (predominantly) to the formation of pyridine iV-oxides. " ... 

The -quinodimethane (22) generated by reaction of the pyridinium salt (21) with caesium fluoride forms an approximately 1 1 mixture of the B/C cis- and trans-fused tricyclic compounds... 

Reactions of the pyridinium salts (245) with ethyl ethoxymethylenecya-noacetate give the pyrazolo[l,5-a]pyridine (247) via the intermediate (246), Scheme 55, and analogous reactions of the pyridinium salts (248) give the indolizines (249) [80JCR(M)0404, 80JCR(S)18],... 

N-Labelled nicotinamide (27) has been obtained in high yield by the reaction of the pyridinium salt (26) with labelled ammonium chloride (Scheme 13). ... 

The regiospecific reaction of the pyridinium salt (54) with a wide range of nucleophiles gives 4-substituted pyridines (56) after thermolytic or photolytic... 

Af-ylides (prepared from their corresponding pyridinium salts 241) with aromatic aldehydes 242 and malonodinitrile 21 to give tetracychc 1,10-phenanthrohne derivatives 243 (Scheme 13.57) [93]. The reaction is complete within 15min and provides the desired products in high yields over 80%. The 4CR of phenanthridine 245, aromatic aldehydes 244, malonodinitrile 21, and several isonitriles 246 gives tetracychc phenanthridine derivatives 247 in excellent yields over 90% and reaction times between 16 and 48h at rt, as published by Marandi et al. [94], Both reactions follow a Knoevenagel con-densation/Michael addition/cychzation mechanism. 

Zwitterionic salts with Meldrum s acid as the anionic structural element are easily accessible by reacting a CH-acidic A-aUcyl pyridinium salt 434 with 2-alkylidene Meldrum s acid derivatives (Scheme 13.90) [175]. The 2-alkyhdeneMeldrum s acids are generated in situ from Meldrum s acid 112 and an aldehyde 435 under basic conditions. Other heterocycles have been reported to undergo the same reaction [176]. In a 4CR, the pyridinium salts can be formed in situ [177]. The generation of zwitterionic salts from Meldrum s acid, several heterocycles and dimethyl acetylenedicarboxylate has been reported [178]. 

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