Reductions pyridines, sodium cyanoborohydride

Reductive amination of carbonyl compounds. Sodium cyanoborohydride has been the reductant of choice for this reaction, even though it is highly toxic (4, 448-449). Borane-pyridine is recommended as a nontoxic substitute. The reactions are conducted in acetic acid, in which this borane is fairly stable, and a co-solvent such as CH2CI2 or THE... 

Sodium cyanoborohydride (10) produces mainly 1,4-dihydropyridines (11) in the reduction of 3,5-di-cyano- and 3,5-diethoxycarbonyl-pyridines, diborane produces more of the 1,2-isomer. With NBH, mixtures of 1,2- and 1,4-dihydro adducts are produced, the latter predominating when carried out in pyridine solution. Nicotinamide (13) in ethanol can be reduced to (8) in moderate yield at 140 C in diglyme the tetrahydropyridine (8) was isolated in admixture with the piperidine (14), presumably via dehydration of the amide. 3-Nitropyridine affords 3-nitropiperidine in moderate yield when reduced in ethanol. The carboxylic acid and halo derivatives of pyridine are generally not reactive toward NBH. 

Secondly, selective chemical modification may be performed because the carbohydrate residues have a distinct low reactivity. This can be done by periodate oxidation which cleaves C-C bonds bearing adjacent hydroxyl groups and converts them to aldehydes [25,26]. The generated dialdehyde can react with a variety of nucleophiles - usually primary amino groups on the surface of carrier materials. The resulting Schiff bases can be further stabilized by sodium boro-hydride, sodium cyanoborohydride or pyridine-borane reduction [27]. 

Selective reduction of either the pyridine or the benzene rings in quinolines and isoquinoline can be achieved the heterocyclic ring is reduced to the tetrahydro level by sodium cyanoborohydride in acid solution,by sodium borohydride in the presence of nickel(II) chloride, by zinc borohydride," or, traditionally, by room temperature and room pressure catalytic hydrogenation in methanol. However, in strong acid solution it is the benzene ring which is selectively saturated " longer reaction times can then lead to decahydro-derivatives. 

Reductive amination of aldoses with chiral l-(—)-a-methylbenzylamine in the presence of sodium cyanoborohydride, carried out overnight at room temperature, followed by acetylation with acetic anhydride in pyridine (100°C, Ih) gives diastereoiso-meric l-(N-acetyl-a-methylbenzylamino)-l-deoxyalditol acetates. LC of these derivatives on silica gel permits resolution of enantiomeric pairs, which are detected by UV photometry at 230 nm. 

Reduction. Quinoline may be reduced rather selectively, depending on the reaction conditions. Raney nickel at 70—100°C and 6—7 MPa (60—70 atm) results in a 70% yield of 1,2,3,4-tetrahydroquinoline (32). Temperatures of 210—270°C produce only a slightly lower yield of decahydroquinoline [2051-28-7]. Catalytic reduction with platinum oxide in strongly acidic solution at ambient temperature and moderate pressure also gives a 70% yield of 5,6,7,8-tetrahydroquinoline [10500-57-9] (33). Further reduction of this material with sodium—ethanol produces 90% of /ra/ j -decahydroquinoline [767-92-0] (34). Reductions of the quinoline heterocycHc ring accompanied by alkylation have been reported (35). Yields vary widely sodium borohydride—acetic acid gives 17% of l,2,3,4-tetrahydro-l-(trifluoromethyl)quinoline [57928-03-7] and 79% of 1,2,3,4-tetrahydro-l-isopropylquinoline [21863-25-2]. This latter compound is obtained in the presence of acetone the use of cyanoborohydride reduces the pyridine ring without alkylation. 

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