Reduction of Heterocyclic Aromatics

Phenanthrene is converted to partially and totally reduced phenanthrene by catalytic hydrogenation [415, 416, 417], Partial reduction to 1,2,3,4-tetra-hydrophenanthrene can also be achieved by sodium [418.  

Reduction of furan and its homologs is readily achieved by catalytic hydrogenation using palladium [419, nickel [420] or Raney nickel [421 as catalysts. Too energetic conditions are to be avoided since hydrogenated furan rings are easily hydrogenolyzed [422,423.  

Benzofuran was reduced over Raney nickel at 90° and 85 atm to 2,3-dihydrobenzofuran, at 190° and 119 atm to octahydrobenzofuran, and further hydrogenation over copper chromite at 200-300° caused hydrogeno-lysis to 2-ethylcyclohexanol [422], 

4-Dihydrobenzopyran (chromane) and its methyl homologs were reduced by lithium in ethylamine and dimethylamine to 3,4,5,6,7,8-hexahydrobenzo-pyrans in 84.5% yields [424.  

Catalytic hydrogenation of thiophene poses a problem since noble metal catalysts are poisoned, and Raney nickel causes desulfurization. Best catalysts proved to be cobalt polysulfide [425], dicobalt octacarbonyl [426], rhenium heptasulfide [5i] and rhenium heptaselenide [54]. The last two require high temperatures (230-260°, 250°) and high pressures (140, 322 atm) and give 70% and 100% of tetrahydrothiophene (thiophane, thiolene), respectively. 

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Highly enantioselective reduction of heterocyclic aromatic ketones with borane and the spiroborate ester derived from (S)-diphenyl prolinol... 

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

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

Oxidation of heterocyclic aromatic amines, N-oxide reduction... 

For the purpose of this review the term heteroaromatic is applied to 7r-radical species when they may be regarded as arising from aromatic heterocyclic molecules or ions by addition or removal of an odd number (usually one) of electrons. Thus, entities such as the anion- and cation-radicals of pyridine (1 and 2) are clearly heteroaromatic. The dilemma whether or not to regard thianthrene (3) as aromatic (its central ring possessing eight electrons) does not arise for the thianthrene cation radical (4) it is heteroaromatic on the grounds that it may formally arise by one-electron reduction of the aromatic thianthrenium dication (5). 

Other conditions for the reduction of the aromatic nucleus are discussed in method 4. The hydrogenation of heterocyclic nuclei is treated in method 554. 

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. 

Reduction of heterocyclic or aromatic ketones containing reactive halogen atoms are accompanied by exchange of these halogens nitro compounds, which are stabilized as aci-nitro compounds in alcoholic solution, are not reduced and in certain circumstances ether groups may be cleaved. 

The irradiation of aliphatic amines in the presence of aromatic compounds is known to yield products of acyclic 1,2- and 1,4-addition and reduction of the aromatic compound.10 The reaction has now been examined with pyridinium and pyridine derivatives and diethylamine.56 The former heterocyclic compounds undergo a thermal reaction with the amine whereas the pyridine derivatives give the reduction products (37) and (38) but no adducts. Light-induced reduction of... 

Aromatic hydrocarbons and heterocycles are important in many natural products and reduction of the aromatic ring is very important from a synthetic viewpoint. Clearly, the most simple example is reduction of... 

Mammalian aldehyde oxidases (AOs) catalyze the oxidation of aldehydes to carboxylic acids and play important roles in the metabolism of N-heterocyclic compounds, the reduction of nitro-aromatic compounds and the... 

This chapter formally discusses heterocycles, which are aromatic compounds that contain heteroatoms. Several important compounds, often identified as heterocyclic compounds, result from reduction of heterocycles. Most of these ring systems... 

Seven heterocyclic nitrogen pyridine derivatives which consist of a nitrogen atom substituted into a six-member aromatic ring are also discussed in this chapter. The pyridine derivatives include the nonsubstituted pyridine, three monomethyl-substituted pyridines, and two ethyl-substituted pyridine derivatives. Reduction of the aromatic pyridine ring with hydrogen yields the piperidine molecule which is a secondary amine that has a hydrogen atom attached to the nitrogen atom. The pyridine derivatives serve both as true solvents and as chemical intermediates for numerous chemical products. 

Abstract Compounds with a 1,2-, 1,4- and 1,6-dioxygen pattern and related bifunctional structures are presented. Disconnection of the internal bonds results in illogical synthons because of the mismatch of charges in the patterns with an even number of C atoms between the functional groups. Three-membered heterocyclic rings are presented as an important class of illogical nucleophiles in the retrosyn-thesis of the 1,2-difunctional pattern. Retrosynthesis of the 1,6-dicarbonyl pattern by reconnection and rctro-Birch reduction of the aromatic building block is related to chemoselective Birch reduction and ozonolysis in the synthetic route. The retrosynthesis and synthesis of salbutamol and asymmetric synthesis of —)-frontalin are presented. 

Melo A, egas O, Ega R, Petisca C, Pinho O, Ferreira IMPLVO (2008) Extraction, detection, and quantification of heterocyclic aromatic amines in Portuguese meat dishes by HPLC/Diode array. J Liq Chromatogr Relat Technol 31 772-787. doi 10.1080/10826070701855987 Mendes S, Pereira L, Batista C, Martins LO (2011) Molecular determinants of azo reduction activity in the strain Pseudomonas putida MET94. Appl Microbiol Biotechnol 92 393-405. doi 10.1007/s00253-011-3366-4... 

The preparation of enamines by reduction of aromatic heterocyclic bases and their quaternary salts or of lactams is not the most useful approach (97). The lithium aluminum hydride reduction of N-acyl enamines has been used with both fruitful and unsuccessful results. A series of 3-N-acetyl -d -cholestenes (104) has been prepared by desulfurization of the appropriate thiazolidine (105) (98,99). Lithium aluminum hydride reduction of the... 

The reduction of iminium salts can be achieved by a variety of methods. Some of the methods have been studied primarily on quaternary salts of aromatic bases, but the results can be extrapolated to simple iminium salts in most cases. The reagents available for reduction of iminium salts are sodium amalgam (52), sodium hydrosulfite (5i), potassium borohydride (54,55), sodium borohydride (56,57), lithium aluminum hydride (5 ), formic acid (59-63), H, and platinum oxide (47). The scope and mechanism of reduction of nitrogen heterocycles with complex metal hydrides has been recently reviewed (5,64), and will be presented here only briefly. 

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