Alcohols reduction preferential

The reduction of the bicyclic terpcncs thujone and isothujone has also been studied with various reducing reagents and compared with the traditional Meerwein-Ponndorf-Verley conditions using aluminum isopropoxide in refluxing isopropanol for 4 hours (or alternatively. vec-bu-tanol). Control experiments show essential kinetic control (ca. 3% isomerization of thujones is observed prior to reduction)178. The less stable alcohol is preferentially formed in the Meerwein-Ponndorf-Verley reductions. 

The biocatalytic counterpart for this transformation is done by the alcohol dehydrogenases [ADHs, EC 1.1.1.x., also called ketoreductases (KREDs) or carbonyl reductases (CRs)], which are able to perform stereoselective carbonyl reductions or enantioselective alcohol oxidations [5-8]. These enzymes are probably the most employed oxidoreductases and make use of a nicotinamide cofactor such as NADH or NADPH to transfer electrons into and from the target substrate. Depending on their substrate scope, ADHs can be divided into primary alcohol dehydrogenases, preferentially reducing aldehydes, and secondary alcohol dehydrogenases that have... 

In a similar vein, acylation of the corticoid 50 with furoyl chloride gives the diacyl derivative 51. Reduction with sodium borohydride serves to convert the 11-ketone to the alcohol 52. Hydrolysis under mild acid conditions preferentially removes the acyl group at the less hindered 21 position. The hydroxyl group in that derivative (53) is then converted to the mesylate 54. Replacement by chlorine affords mometasone (55) [12]. 

Most catalyst systems, if they show selectivity at all, favor preferential reduction of the less hindered 4-nitro group hydrogenation of 11 over Pt-on-C in acidic alcohol affords 10 in 70% yield (24). Similarly, Lazer et at. (61), selectively prepared 10 from 27 mmol II, 10 ml of 50% H2SO4, 60 ml HOAc, and 350 mg 5% Pt-on-C at 85 C and 30 psig. The hydrogenation is stopped at... 

By studying the NMR spectra of the products, Jensen and co-workers were able to establish that the alkylation of (the presumed) [Co (DMG)2py] in methanol by cyclohexene oxide and by various substituted cyclohexyl bromides and tosylates occurred primarily with inversion of configuration at carbon i.e., by an 8 2 mechanism. A small amount of a second isomer, which must have been formed by another minor pathway, was observed in one case (95). Both the alkylation of [Co (DMG)2py] by asymmetric epoxides 129, 142) and the reduction of epoxides to alcohols by cobalt cyanide complexes 105, 103) show preferential formation of one isomer. In addition, the ratio of ketone to alcohol obtained in the reaction of epoxides with [Co(CN)5H] increases with pH and this has been ascribed to differing reactions with the hydride (reduction to alcohol) and Co(I) (isomerization to ketone) 103) (see also Section VII,C). 

The transformation of the cyano group could also introduce a new chiral center under diastereoselective control (Figure 5.13). Grignard-transimination-reduction sequences have been employed in a synthesis of heterocyclic analogues of ephedrine [81]. The preferential formation of erythro-/3-amino alcohols may be explained by preferential hydride attack on the less-hindered face of the intermediate imine [82], and hydrocyanation of the imine would also appear to proceed via the same type of transition state. In the case of a,/3-unsaturated systems, reduction- transimination-reduction may be followed by protection of the /3-amino alcohol to an oxazolidinone, ozonolysis with oxidative workup, and alkali hydrolysis to give a-hydroxy-/3-amino acids [83]. This method has been successfully employed in the synthesis L-threo-sphingosine [84]. 

Much emphasis has been placed on the selectivity of quaternary ammonium borohydrides in their reduction of aldehydes and ketones [18-20]. Predictably, steric factors are important, as are mesomeric electronic effects in the case of 4-substituted benzaldehydes. However, comparison of the relative merits of the use of tetraethyl-ammonium, or tetra-n-butylammonium borohydride in dichloromethane, and of sodium borohydride in isopropanol, has shown that, in the competitive reduction of benzaldehyde and acetophenone, each system preferentially reduces the aldehyde and that the ratio of benzyl alcohol to 1-phenylethanol is invariably ca. 4 1 [18-20], Thus, the only advantage in the use of the ammonium salts would appear to facilitate the use of non-hydroxylic solvents. In all reductions, the use of the more lipophilic tetra-n-butylammonium salt is to be preferred and the only advantage in using the tetraethylammonium salt is its ready removal from the reaction mixture by dissolution in water. 

Iridium nanopartides also catalyze the hydrogenation of benzyhnethylketone, with high selectivity in reduction of the aromatic ring (92% selectivity in saturated ketone, 8% in saturated alcohol at 97% benzylmethylketone conversion). This preferential coordination of the aromatic ring can be attributed to steric effects that make carbonyl coordination difficult. Therefore, metallic iridium nanoparticles prepared in ILs may serve as active catalysts for the hydrogenation of carbonyl compounds in both solventless and biphasic conditions. 

It is important that this process results in the preferential formation of a thermodynamically stable alcohol diastereomer. The anion-radicals contain an almost undoubtedly planar C-0 and give rise to pyramidal hydroxy carboradicals. The hydroxy carboradicals form pyramidal hydroxy carbanions, which cannot exist in the presence of ammonium cation for a long time. Therefore, the equilibrium including pyramidal inversion, probably, takes place at the step of carboradical formation, rather than carbanion formation. Transformation of a carboradical into a carbanion obviously proceeds faster than its dimerization or disproportionation. As a consequence, the reduction of an optically active ketone into an alcohol goes without racemization (Rautenstrauch et al. 1981). 

Unsaturated epoxides are reduced preferentially at the double bonds by catalytic hydrogenation. The rate of hydrogenolysis of the epoxides is much lower than that of the addition of hydrogen across the carbon-carbon double bond. In a, -unsaturated epoxides borane attacks the conjugated double bond at -carbon in a cis direction with respect to the epoxide ring and gives allylic alcohols [660], Similar complex reduction of epoxides occurs in a-keto epoxides (p. 126). 

Several reagents reduce aldehydes preferentially to ketones in mixtures of both. Very high selectivity was found in reductions using dehydrated aluminum oxide soaked with isopropyl alcohol and especially diisopropylcarbinol [755], or silica gel and tributylstamane [756]. The best selectivity was achieved with lithium trialkoxyalumimm hydrides at —78°. In the system hexanal/ cyclohexanone the ratio of primary to secondary alcohol was 87 13 at 0° and 91.5 8.5 at —78° with lithium tris(/er/-butoxy)aluminum hydride [752], and 93.6 6.4 at 0° and 99.6 0.4 at —78° with lithium tris(3-ethyl-3-pentyl-oxy)aluminum hydride [752],... 

Since sodium borohydride usually does not reduce the nitrile function it may be used for selective reductions of conjugated double bonds in oc,/l-un-saturated nitriles in fair to good yields [7069,1070]. In addition some special reagents were found effective for reducing carbon-carbon double bonds preferentially copper hydride prepared from cuprous bromide and sodium bis(2-methoxyethoxy)aluminum hydride [7766], magnesium in methanol [7767], zinc and zinc chloride in ethanol or isopropyl alcohol [7765], and triethylam-monium formate in dimethyl formamide [317]. Lithium aluminum hydride reduced 1-cyanocyclohexene at —15° to cyclohexanecarboxaldehyde and under normal conditions to aminomethylcyclohexane, both in 60% yields [777]. 

Mammalian sperm cells metabolize D-fructose preferentially as a source of energy. Fructose is formed in cells of the seminal vesicle from D-glucose via reduction to the sugar alcohol sorbitol using NADPH, followed by oxidation of sorbitol to fructose using NAD+. The fructose concentration... 

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