Oxidation and reduction of heterocycles

Oxidation and reduction can initiate changes leading to heterocycle-heterocycle conversions. The reaction of tetraphenylfuran with singlet oxygen (Scheme 34) (B-73MI50303) and that of isoxazoles with LAH (Scheme 35) are examples. 

Desulfonation reaction (reductive and oxidative desulfonation, nucleophilic substitution, elimination, and SO2 extrusion) in synthesis and transformations of heterocycles 99T10547. 

A Reductions of Heterocyclic N-Oxides and Aromatic Nitro Groups 1165... 

Reductions of Heterocyclic JV-Oxides and Aromatic Nitro Croups... 

Detailed investigations of the chemical reactivity of the diketiminato-stabilized phosphenium cations like 28 (Scheme 17) are to date rare and include only two reports dealing with the substitution and reduction of P-halogen-derivatives. Thus, reaction of 28 (X=Br) with sodium hydroxide in toluene was reported to proceed with displacement of the halide substituent at phosphorus and conservation of the heterocyclic ring to give a mixture of bromide and triflate salts containing a P-hydroxy-substituted cation, both of which were isolated in small yields [89], The products are remarkable as they represent one of very few examples of a stable phosphinous acid which does not rearrange to the tautomeric secondary phosphine oxide. Potassium reduction of the P-chloro-substituted derivative 34 produced the... 

The catalytic effect of tetra-n-butylammonium fluoride in the homogeneous reduction of heterocyclic A-oxides and nitroarenes by hexamethyldisilane in tetra-hydrofuran can occur with EXPLOSIVE violence, but can be controlled by the slow addition of the disilane to the A-oxide (or nitroarene) and tetra-n-butylammonium fluoride to yield the parent heterocycle (>70%) (or azobenzene 84%). In a similar manner, azoxybenzene is converted into azobenzene (95%), and 4-nitropyridine-l-oxide, is reduced to azoxypyridine-l,l -dioxide (78%), with minor amounts of azopyridine-1, l -dioxide and azopyridine-1-oxide [5,6]. 

The discovery of the metal-like properties of conducting polymers has once again focused attention on the oxidation and reduction characteristics of aromatic systems. It turns out that most of these conducting materials consist of chainlike connected carbocyclic or heterocyclic aromatics [94-97]. 

It is doubtful whether these should be considered as nucleosides. However, they are readily prepared from C polyhydroxyalkyl derivatives (type 1.5) by periodate oxidation and reduction. They are also prepared by hy-droxymethylation of the respective heterocycles (86MI4). 

Deoxygenation of N-oxides,4 TiCl4 -NaBH4 (1 2) in DME reduces the N-oxide of pyridine and mcthylpyridines (picolines) to the corresponding heterocycles in high yield. However the N-oxide of quinolines and isoquinolines is reduced further to dihydro derivatives of the hetcrocyclcs. Pyridine, quinoline, and isoquinoline themselves are not reduced by this low-valent titanium species. Reduction of heterocyclic N-oxides with TiCI, has been reported (6, 588). 

This section deals with transformations of heterocyclic rings and includes not only addition of carbenes and nitrenes, but also rearrangements frequently following these additions. Also included are topics such as the synthesis of Reissert compounds as well as miscellaneous oxidations and reductions. 

LOSS Of the /V-oxide group. Reduction of A-oxidcs affords the parent heterocycle and can be achieved... 

The 26 chapters, written by experts from 14 countries, cover a wide spectrum of topics related to the chemistry of enamines, including theory, structural chemistry, spectral properties, formation, reactions and stereochemistry, acidities, rearrangements, oxidation and reduction, as well as other topics. In two chapters, the material related to enamines was meagre. Hence, the chapter on radiation chemistry also deals with compounds with non-conjugated C=C and amino groups, and the chapter on synthesis and uses of isotopically labelled enamines includes enamines in which the nitrogen is part of a heterocyclic system. 

Although oligomers of type D can be made by such chemical conversions from the corresponding type C compounds (as in the indole series), more commonly used methods are oxidative and reductive condensation of heterocyclic monomers (cf. D2 oligomers). 

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