Pyridinium derivatives

To the best of our knowledge, the hrst paper which mentioned an A-(l-haloalkyl)pyridinium compound appeared 66 years ago in the Chemische Berichte (Krohnke 33CB1386). Tlie author described the reaction of phenacyl pyridinium derivatives 1 with bromine in acetic acid to give the halides 2 (36CB2006 37CB864). Tire addition of bromine to the double bonds of A-vinylpyridinium salts 3 and 4 giving the adducts 5 and 6 has also been reported (51CB399) (Scheme 1). 

Treatment of triphenylpyrylium perchlorate with a-methylheterocycles, (e.g., 23), yields fused pyridinium derivatives (e.g., 24) and low yields of aryl derivatives 25 <96MI99>. 

Brief photocyclization of pyridinium derivative 55 afforded 2,4-diphenyl-7-substituted 5,6-dihydropyrido[2,l-tf]-phthalazinium salts 56 (Equation 10) <1995IZV296>. 

In the case of quaternary derivatives made from the non-planar aliphatic amines 7.64, 7.65 and 7.66, steric strains further destabilise the C-N+ bond so that reaction with cellulose occurs under alkaline conditions at 30 °C, whereas temperatures of about 40-50 °C are required for the pyridinium derivatives 7.67. The quaternisation approach appeared to offer the opportunity to prepare dyes yielding reactivity levels intermediate between those of aminochloro- and dichlorotriazine dyes without loss of the desirable stability of the dye-fibre bond to acidic conditions that is characteristic of aminohalotriazine dyes. Unfortunately, this ideal was not attainable because of the objectionable odours of the tertiary amines liberated by the fixation reaction and the sensitivity of the reactivity behaviour of the quaternised derivatives to the nature of the chromogen attached to the triazine ring, making it difficult to select compatible combinations of dyes. 

The resonance mechanism shown in Fig. 8.15 accounts for the greater stability of dihydropyridine pro-prodrugs vs. their pyridinium metabolites in base-catalyzed and enzymatic reactions of hydrolysis, but it also suggests a decreased stability of the former in acid-catalyzed hydrolysis. Indeed, the carbonyl O-atom is deduced from Fig. 8.15 to be more nucleophilic in dihydropyridine (A) than in pyridinium derivatives (B). The stability of dihydropyridine pro-prodrugs under the acidic conditions of the stomach and small intestine should, therefore, be examined further. 

The electrochemical reduction of the corresponding nitrophenylpyridine or pyridinium derivatives only produces cyclic hydroxamic acids. 

Hemicyanines are polymethines with an ammonium salt at one end of the chain. Typically these can be pyridinium (3.30), indolenium (3.31a, X = C(CHj)2) and ben-zothiazolium (3.31b, X = S) derivatives. The pyridinium derivatives fluoresce in the long wavelength red region, whilst the others are in the near-infrared, which is a very useful property for certain laser dye outlets and in fluorescent imaging in medical diagnosis and biological probes. 

At the 145th ACS meeting in New York, September 1963, Dr. Fraser and coworkers reported observing an increase in rate with increasing pH for the iV-methyl derivative as well as for the pyridine-4-carboxylato complex as shown in Figure C. We recently have found similar behavior for the pyridine-2-carboxylato complex and its iV-methyl derivative. In view of the acid dependency found for the N-alkyl pyridinium derivatives of these complexes, it does not seem likely that the acid dependency of the reaction can be explained merely by acidic and basic forms of the complex since the iV-alkyl pyridinium derivatives do not possess such a basic form. 

SCHEME 64. Reaction of zinc organometallics with pyridinium derivatives... 

The substituted 1,4-dihydropyridines 156 in dry acetonitrile, containing Bu4NC104, are oxidized in a one-electron step leading presumably to a radical-cation (157).235 The final product obtained in dry acetonitrile is a substituted pyridine (158) or pyridinium derivative (159), a two-electron product. This indicates a disproportionation of the initially formed radical-cation. By adding water to act as a base, the wave doubled in height indicating a deprotonation of the radical cation 157 to a radical that is oxidizable... 

In 1982 a number of young people in California injected themselves with an illegally manufactured opiate drug that was subsequently found contaminated with N-methyl-4-phenyltetrahydropyridine (MPTP). Within a few days they developed irreversible symptoms of Parkinson disease. Subsequent investigation revealed that MPTP itself is not toxic but that it is oxidized by monoamine oxidase B (MAO-B) to the corresponding pyridinium derivative MPP+ (Eq. 30-4). It is this pyridinium derivative, or perhaps... 

Similarly, in the PF6 salt of the pyridinium derivative (163), the two rings bonded to the C=C carbons are nearly coplanar with the C3 ring266. The single bonds in the ring, C—C (mean) 1.462A, are shorter than in 155 other bonds have their usual lengths, Cl =C21.295 (14), Cl—N 1.374(8), C3—N (mean) 1.475 A. 

The two target molecules, which illustrate cyclisation reactions involving substituted 1,5-dicarbonyl compounds in the presence of appropriate reagents, are diethyl 2,6-dimethylpyridine-3,5-dicarboxylate (85) and 2,4,6-triphenylpyrylium fluoroborate (86). The latter is of specific interest since it provides a simple example to illustrate a general procedure for the conversion of pyrylium salts into pyridinium derivatives these latter compounds are important reagents for a variety of functional group interconversions (see Sections 5.5.6, p. 574, and 5.15.3, p. 768). 

Host-guest complexes involving pyridines and their benzene derivatives can be divided into categories depending upon the interactions involved. In the largest group, pyridinium derivatives act as Jt-acceptors and their interaction with electron rich 7i-donors stabilizes the complexes. Within this group, there is still much diversity. 

The triphenylpyrylium perchlorate 58 undergoes reaction with a-methyl heterocycles to give fused pyridinium derivatives 59 and low yields of the aryl by-product 60 (Equation 194). 

A-(nitrobenzimidazol-2-yl)pyridinium derivatives have been synthesized in order to study their antiprotozoal activity [693-695], A H and 13C NMR study has been carried out for structural determination of these compounds (S13C in Table 3.25). Quantitative structure-activity relationships (QSAR) between the in vitro antileish-manial activity of /V- ben zazolylpyridinium salts and their 13C NMR chemical shifts have been studied in order to determine the influence of benzimidazole substituents upon antileishmanial activity [695],... 

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