Pyridines pyridiniums

N, P, S, or C, is thermodynamically and kinetically favored, yielding pyridine, pyridinium, phosphaben-zene, thiopyrylium, or benzenoid rings. Ring-opening occurs with alkali cyanides ring contraction to... 

The acetate ion is too weak a nucleophile to attack pyridines, pyridinium salts or their analogues at a ring carbon. IV-Oxides require coordination at oxygen before they can react. 

The pyridine/pyridinium buffers were mixed in such a manner that the pH was high enough to avoid the Soft Nef-Reaction... 

Though there are no drugs yet on the market from this particular system, the pyridine-pyridinium redox CDS has been widely exploited experimentally to deliver hormones and drugs to the brain (Bodor and Farag, 1984 Shek ef al., 1987). By using a variety of groups, R, in 27 from, for example, methyl through to octyl, the lipophilicity can be adjusted to provide optimum properties for passive transport. 

Hydrogen Fluoride/Pyridine [Pyridinium Poly(hydrogen fluoride)], (70% Hydrogen Fluoride by Weight) 31... 

Fused benzene rings aid nucleophilic attack on pyridines, pyridinium and pyrylium ions, and pyrones the loss of aromaticity involved in the formation of the initial adduct is less in monobenzo derivatives and still less in linear dibenzo derivatives than in monocyclic compounds. For the same reason, the tendency for this initial adduct to re-aromatize is less for benzopyridines. Fused benzene rings also influence the point of attack by nucleophilic reagents attack rarely occurs on a carbon atom shared with a benzene ring. Thus, in linear dibenzo derivatives, nucleophilic attack is at the y-position (157). 

Keywords aromaticity, arsabenzene, boratabenzene, germabenzene, heterocycles, metallabenzene, phosphabenzene, pyridine, pyridinium, pyrylium, silabenzene, thiopyrylium... 

J Pyrylium Thiopyrylium Selenopyrylium Pyridine Pyridinium Pyridine- A-oxide... 

The foregoing brief discussion clearly indicates that, although some features of the substitution reactions of pyridine, pyridinium salts and... 

Different charge types in rapid equilibrium. There may just be two, (e.g., pyridine/pyridinium cation), but there may also be more than two (e.g., 2,6-diaminopyridine/monocation/dication). 

When 2-chloro-6-phenyl-phenolmethylether is transfered into the Grignard compound and then reacted with cyclohexanone, dehydrated and finally dehydrogenated, 2,6-diphenylphenolmethylether is formed61). The synthesis is versatile since a variety of cyclohexanone-derivatives can be used. This and other phenolethers can then easily be demethylated. One of the mildest demethylations is to react the methylether with borontribromide at low temperatures °21. Hydroiodine acid, Lewis acids, such as aluminium-chloride 63 , pyridine/pyridinium chloride, or sodium in pyridine 64) are also valuable tools in ether demethylations. 

The reaction of 2//-azirines (e.g.. 1) with hydrogen fluoride in pyridine [pyridinium poly(hy-drogen fluoride). Olah s reagent] gives /f,/f-difluoro amines (e.g., 7) or a-fluoro ketones (e.g., 5) (Table 10). In some cases, pyrazines (e.g.. 9) are the main reaction products (Table IQj 31-35 2//-Azirines arc more reactive than aziridines towards pyridinium poly(hydrogcn... 

Oxidizing agents such as CrOg 2 pyridine, pyridinium chlorochromate (PCC), and Mn02... 

One Nitrogen Atom (it is self-subdivided into Pyridines, Pyridinium Compounds, Ylides, Pyridine N-Oxides, Applications of Pyridines, Bipyridines and Related Systems, Hydropyridines, Biologically Active Pyridines and Hydropyridines, Pyridines Annulated with Carbocycles, Pyridines Annulated with Heterocycles). 

Electrophilic nitration and bromination of pyridine iV-oxides can be controlled to give 4-substituted products by way of attack on the free A-oxide. ° Under conditions where the A-oxide is 0-protonated, substitution follows the typical pyridine/pyridinium reactivity pattern thus, in fuming sulfuric acid, bromination shows P-regioselectivity. Mercuration takes place at the a-position, however mercuric-catalysed sulfonation produces the 3-sulfonic acid. ... 

Mannich-type cyclizations of vinylsilanes have found considerable application in the area of alkaloid total synthesis. Cyclizations that occur in the exocyclic mode with respect to the vinylsilane nucleophile have been widely employed to assemble 3-alkylidenepiperidine substructures with high stereocontrol. Overman and coworkers have made extensive use of the acid-promoted conversion of bicyclic oxazolidines to alkylideneindolizidines in their total syntheses of pumiliotoxin A alkaloids (Scheme 36). - " - An illustration of the mild nature of iminium ion-vinylsilane cyclizations is provided in the conversion of (101) to (102), the penultimate precursor of (-i-)-pumiliotoxin A. This conversion was accomplished in 71% yield by heating (101) at 80 C in a methanolic pyridine-pyridinium tosylate buffer (pH 4.5). More strongly acidic conditions had to be avoided since they led to competitive solvolysis of the allylic benzyl ether functionality of the pumiliotoxin A side chain. To the limits of detection by high... 

Although pyridoxal phosphate is involved in a wide variety of reactions the processes are all related mechanistically by the electron withdrawal toward the cationic or hydrogen-bonded imine nitrogen and into the electron sink of the pyridine/pyridinium ring. 

Hydrogen fluoride-Pyridine (Pyridinium polyhydrogen fluoride) 285... 

DEHYDROGENATION t-Butyl hypochlorite. 2,3-Dichloro-5,6-dicyanobenzoquinone. Pyridine. Pyridinium hydrobromide. Sodium bis-(2-methoxyethoxy)aluminum hydride. Sulfur. Trityl tetrafluoroborate. Trityl trifluoroacetate. 

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