Pyridinium compounds nucleophilic

A proposed explanation of the reactivity of the 4-position versus that of the 2-position in pyridinium compounds has been advanced by Kosower and Klinedinst nucleophiles which are expected to form charge-transfer complexes will tend to substitute at the 4-position. However, it is not clear why this (usually unknown) property should govern the site of substitution, except for a bifunctional nucleophile such as hydrosulfite ion which can form a suitable bridge from the nitrogen to the 4-position. 

Mithramycin shows a completely P-linked chain of D-conflgurated saccharides. This requires a totally different approach for the synthesis which is also done by application of the DBE method. The previously obtained disaccharide 180 is P-glycosylated with the monosaccharide precursor 176 to give the trisaccharide 185. After reductive debromination (Bu3SnH), an acid deformylation deblocked the C-3" position which is oxidized with pyridinium dichromate. Nucleophilic attack at the carbonyl group by methyl lithium affords a 1 1.2 mixture of 186 and 187 none of which is the desired compound [93]. Obviously, the methyl branch is formed exclusively in the axial way. 

Katritzky and coworkers have extensively developed the activation of amines by reaction with pyry-lium salts to provide (V-alkyl (or N-aryl) pyridinium compounds. When buttressing substituents were present to discourage attack on the pyridine ring, the N-alkyl substituent was subject to displacement and elimination processes. In general, primary alkyl substituents reacted with most nucleophiles in a normal 5n2 process as shown in Scheme 12, whereas competition between substitution and elimination took place with the secondary analogs, with elimination dominating the reactions starting from cycloalkyl-amines. 

Pyridinium chlorochromate as oxidant in organic synthesis 82S245. Pyridinium compounds, kinetics and mechanism of nucleophilic sub-... 

The well-known powerful basic catalyst 4-dimethylaminopyridine has been the inspiration for a new transesterification catalyst for a-hydroxy esters (5). The designer compound is 2-formyl-4-pyrrolidinopyridine (6). By the introduction of an aldehyde group into the pyridine ring at the 2-position, the ready formation from (5) of a hemi-acetal (7) makes possible, via a covalent pre-association step, a facile displacement by pyridine nitrogen of the aryloxy group of the a-hydroxy ester (5). The product of this process, a tricyclic pyridinium compound (9), is formed by a nucleophilic mechanism (Scheme 1, pathway b), and interaction with methanol then leads to the methyl ester of the a-hydroxy acid (10) and regeneration of the catalyst. However, a study of the mechanism of this catalytic process discounted the nucleophilic mechanism and provided clear evidence instead for a general base mechanism, which proceeds via a dioxolanone (8) (Scheme 1, pathway a) ... 

More interesting and important than these ring openings are those effected by nucleophilic attack upon a quaternary pyridinium compound or upon a substance formed therefrom. Generally such reactions are of the type... 

Because of the increased importance of inductive electron withdrawal, nucleophilic attack on uncharged azole rings generally occurs under milder conditions than those required for analogous reactions with pyridines or pyridones. Azolium rings are very easily attacked by nucleophilic reagents reactions similar to those of pyridinium and pyrylium compounds are known azolium rings open particularly readily. 

Benzylic quaternary phosphonium and ammonium salts are dealky-lated by mild heating and/or nucleophilic anions, particularly iodide (9) and thiolate (10), but also hydroxide (11). Most N-benzyl-pyridinium or quaternary aryl ammonium compounds are particularly susceptible (12). Decompositions of this sort have seriously limited the usefulness of solid phase-transfer catalysts derived from (chloromethyl)polystyrene (13, 14). 

Activation of a primary alcohol 174 by in situ mesylation and nucleophilic attack of a pyridine nitrogen atom was used in the last steps of a synthesis of cyclohexa[tf]quinolizidines 176. These compounds were obtained by direct NaBH4 reduction of intermediate pyridinium salts 175, and were proposed as tricyclic models containing the ABC-part of 8-azasteroids (Scheme 30) <1999T9269>. 

Cyano, halo, amino, and nitro groups in the 2- or 4-position of pyridinium ions are susceptible to nucleophilic substitution. Treatment of these compounds with aqueous alkali gives the corresponding pyridones. Since this transformation is not the result of oxidation, it will not be further considered here. 

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