Reduction of heterocycles

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

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

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 presence of an electron-withdrawing substituent can favor reductions of heterocycles. In this way, the electroreduction of an activated thiopyran affords a dihy-drothiopyran with elimination of dimethy-lamine (Scheme 14) [19]. 

The cathodic reduction of heterocycles generally affords nucleophilic species that can react with electrophilic reagents. 

The electrochemical reduction of heterocyclic enammonium salts can give... 

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). 

Reduction of Heterocyclic Compounds by Metal-Ammonia Solutions and Related Reagents A. J. Birch and J. Slobbe, Heterocycles, 1976, 5, 905-944. 

The reduction of heterocyclic compounds is a versatile process of great use in the synthesis of alkaloids185. The choice of catalyst is crucial however, as illustrated by the indolizidine alkaloid synthesis shown in Scheme 34185a. The reduction of 37 to 38 proceeds with a very high degree of stereocontrol which sets up the correct framework for intramolecular cyclization to complete a short synthesis of y-lycorane 39186. 

On the other hand, for the reduction of heterocyclic nitriles, the electrochemical reduction can compete with catalytic methods. For example, Reilly 562) produces amino-methylpyridines by electrochemical reduction of the corresponding nitriles ... 

IX. Reduction of Heterocycles Containing Three Nitrogen Atoms 89... 

The reduction of heterocycle (41) via its methiodide to give compound (42) has been noted in Section 7.15.5.1. 

The reduction of heterocyclic thiocyanates has not been reported the reaction would be expected to be a reduction to cyanide ion and a mercapto compound. 

Reduction of heterocyclic compounds parallels reduction of aromatic compounds with the added factor that fission of the ring may occur. In most instances, catalytic hydrogenation is preferred to chemical reduction, purer products and more consistent results being obtained. 

Inevitably there is some overlap with other chapters, particularly those dealing with reduction of C=N bonded groups (Chapters 1.2-1.8, this volume) and that on the reduction of heterocycles (Chapter 3.8, this volume). For completeness, most of the important reduction methods for the appropriate groups are also referred to here. An attempt has been made to cover the literature to late 1989 a few later references are also included. 

Partial and Complete Reduction of Heterocycles Containing More Than One Heteroatom... 

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