Preferential reduction groups

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... 

Selective reduction of ketones.1 This reagent can be used to effect selective reduction of the more hindered of two ketones by DIBAH or dibromoalane. Thus treatment of a 1 1 mixture of two ketones with 1-2 equiv. of 1 results in preferential complexation of the less hindered ketone with 1 reduction of this mixture of free and complexed ketones results in preferential reduction of the free, originally more hindered, ketone. An electronic effect of substituents on a phenyl group can also play a role in the complexation. This method is not effective for discrimination between aldehydes and ketones, because MAD-complexes are easily reduced by hydrides. MAD can also serve as a protecting group for the more reactive carbonyl group of a diketone. The selectivity can be enhanced by use of a more bulky aluminum reagent such as methylaluminum bis(2-f-butyl-6-( 1,1-diethylpropyl)-4-methylphenoxide). 

Preferential reduction of a nitro group in the presence of a carbonyl group in 4-nitroacetophenone ISD, intramolecular rearrangements of o-nitro-benzanilides 32) intramolecular cyclizations of o-nitro-ferf-anilines to benzimidazol-1-oxides 153,154) cyclizations of acylated 2-nitrodiphenylamines to phenazine-l-oxides i ), intramolecular additions of nitro groups to double bonds 156) remarkably ef-... 

There are a number of cases where a less reactive enone group remains intact while a more reactive saturated ketone present in the same substrate is selectively reduced, but there are a number of examples of polyfunctional natural products where simultaneous reduction of both saturated and unsaturated ketones or of preferential reduction of the unsaturated one is achieved with LiAlH(OBu )3.2 2 ... 

When ethyl 4-methyl-2-methylthiopyrimidine-5-carboxylate or ethyl 2,4-dimethylpyrimidine-5-carboxylate was treated with lithium aluminum hydride at — 70°C, preferential reduction of the ester group occurred, yielding the 5-hydroxymethyl derivative rather than ring reduction.181... 

Preferential reduction of an a,)3-unsaturated ketones C=C bond is more easily accomplished than preferential reduction of the C=0 bond. However, a keto group can be hydrogenated in preference to a C=C bond after the appropriate choice of the catalyst, usually Pt or Ru . Additives such as FeCl2 (a carbonyl reduction promoter) and zinc acetate (a double bond saturation inhibitor) may be used in conjunction with platinum oxide . The less hindered ketone in 6 is selectively hydrogenated over Pt02 [equation... 

Oxidation of ADP using periodate, followed by partial reduction with boro-hydride, affords a mixture of the fully reduced bis(hydroxymethyl) product and the semialdehydic reduction intermediate. By dint of comparison with model compounds using n.m.r. spectrometry, it has been shown that the first reduction step takes place at C-3, to give (107) (which will prefer to cyclize to the hemiacetal). It is not clear why preferential reduction at C-3 occurs either the aldehydic group at this position is more reactive per se, or it is more accessible to borohydride. It seems likely that (107) and its analogues could prove to be useful new affinity labels. 

Reduction of benzene derivatives carrying oxygen or nitrogen functions in ben-zylic positions is complicated by the easy hydrogenolysis of such groups, particularly over palladium catalysts. Preferential reduction of the benzene ring in these compounds is best achieved with ruthenium or rhodium catalysts, which can be used under mild conditions. For example, mandelic acid is readily converted into the cyclohexyl derivative 29 over rhodium-alumina, whereas with palladium, hydrogenolysis to phenylacetic acid is the main reaction (7.18)... 

The preference of the tin radical for the olefin can be due to the low solubility of oxygen in organic solvents. The oxygen concentration is probably kept to a constant and sufficient level by the improved contact between the gas and the sonicated solution (nebulization effect). The carbon radical which reacts with oxygen gives a peroxyl radical. From alkynes, the vinyl radical formed by addition of the trialkyltin group is more reactive towards the tin hydride, and preferential reduction occurs without any hydroxystannylation. 

Prim, alcohols from aldehydes Preferential reduction of aldehyde groups with retention of keto-groups... 

Novel preferential reduction. Diaryl ketones are much more reactive toward Li-N-dihydropyridylaluminum hydride (formula s. 944) than are either dialkyl or arylalkyl ketones. This sequence is in contrast to that found with NaBH4 in isopropanol and appears to be without precedent in nucleophilic addition to carbonyl groups. E. and limitation s. P. T. Lansbury and J. O. Peterson, Am. Soc. 54, 1756 (1962). 

Pyridine s. under H2S Zinc/acetic acid Preferential reduction of hindered carbonyl groups Phthalimidines from phthalimides... 

The Birch reduction of D-homo derivatives of type (218) with a 17a-methoxy group can take place in two directions with the formation of the enol ethers (219) and (221) giving the corresponding ketones (220) and (222) on hydrolysis In all such reactions, the ketones (222) with a A -bond predominated in the reaction products, which shows preferential reduction via the ether (221) (Schemes 96, 99, 102, 103). 

The inhibition of DPNH oxidation by ascorbate is probably due to the rapid preferential reduction of the postulated quinone by ascorbate. Alternatively, the enzyme-catalyzed oxidation of ascorbate may be so rapid as to preclude any participation of the catechol grouping as cofactor in side chain hydroxylation when ascorbate is present, so that under these conditions none of the DPNH-reactive quinone is formed. In either case, the stoichiometry of reaction (14) would be maintained. 

However, treatment of cortisone 3,20-bissemicarbazone with acetic anhydride and pyridine removes the 20-semicarbazone group preferentially. Selective removal of a protecting group can be also achieved by a selective reaction to give a new intermediate which can be converted into the desired product ketone. Thus progesterone 20-monoenol acetate (42) is prepared from the 3,20-bisenol acetate (40) via selective electrophilic attack of iodine at C-6 followed by reductive dehalogenation of (41). ... 

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