Selective reduction of carbonyl group in

Selective reduction of carbonyl group in /S.tunsaturated oc-ketoesters... 

SELECTIVE REDUCTION OF CARBONYL GROUP IN p,y-UNSATURATED a-KETOESTERS BY TRANSFER HYDROGENATION WITH Ru(P-CYMENE)(TsDPEN)... 

Selective reduction of carbonyl groups in the presence of an ester group. Selective reduction of various keto esters to hydroxy esters is possible in fair yield with LiAlH4 in the presence of a small amount of silica gel (equation I).1... 

Some selected results are collected in Table 13.1. Entries 1 and 2 refer to the selective reduction of carbonyl groups in the presence of unconjugated C=C double bonds. Both the ketones can be converted into the corresponding alcohols with excellent yields, leaving the other functionalities unaffected. 

In this chapter, we will cover both aspects, the aldol strategy and the selective reduction of carbonyl groups in the context of 1,3-diol formation. Additionally, nonaldol approaches will be covered as weU. 

The TEMPO moiety (2,2,6,6-tetramethylpiperidine-l-oxyl) has been incorporated into acetoacetic derivatives to achieve E-selective Knoevenagel condensations, exploiting the steric hindrance that it causes.269 In contrast, acylacetoamides (including Weinreb amides) produce Z-adducts. Downstream reductions of carbonyl groups in the products allow access to a variety of useful materials. 

The nature of the cation is unimportant in aqueous or other highly polar solutions of borohydrides, but influences the rate of reaction in isopropanol or pyridine, where the reagent exists mainly as associated ion-pairs [44]. Lithium borohydride is more reactive than the sodium compound in these solvents Li+ can probably associate more closely than Na+ with the carbonyl oxygen, promoting polarisation of the C=0 group, and so aiding hydride transfer from the anion. Other cations [e.g. Ca +j and solvents e.g. dimethylformamide) provide variations in reactivity which can have valuable uses for selective reduction of carbonyl groups [42]. 

Aldehydes and ketones are reduced to primary and secondary alcohols respectively, often in high yield. The reaction owes its usefulness to the fact that carbon-carbon double bonds and many other unsaturated groups are unaffected, thus allowing selective reduction of carbonyl groups. For example, cinnamaldehyde is converted into cinnamyl alcohol, o-nitrobenzaldehyde gives o-nitrobenzyl alcohol and phenacyl bromide gives the alcohol 76 (7.57). 

In the late 1970s, Jean-Louis Luche and co-workers began to evaluate the role of lanthanide salts in selective organic reductions. Even with the extensive arsenal of available reagents, including sodium cyanoborohydride, diisobutylaluminum hydride and 9-borabicyclononane (9-BBN), there has remained room for improvement in yield, selectivity and reaction conditions in reductions of carbonyl groups in complex organic molecules. 

In summary, 1,3-diols are prominent motives in polyketides, and they can be constmcted by various advanced methods. A practical access to 1,3-diols starts often with an aldol reaction. This aldol reaction can already construct a 1,3-diol if one hydroxyl group is already present in the starting material. Alternatively, syn- or anti-selective reductions of carbonyl groups offer an easy to predict stereoselective way to 1,3-diols. As we have seen in particular in the synthesis of roxaticin, direct reduction of diketones or p-keto esters proves an efficient entry into 1,3-diol segments. 

The exo reactivity of 2-norbomanone 25 in nucleophilic addition (such as reduction with hydride) is a classical example of the facial selectivity of carbonyl groups in bicyclic systems [80]. 

Reduction of carbon-carbon double bond Microalgae easily reduce carbon-carbon double bonds in enone. Usually, the reduction of carbonyl group and carbon-carbon double bond proceeds concomitantly to afford the mixture of corresponding saturated ketone, saturated alcohol, and unsaturated alcohol because a whole cell of microalgae has two types of reductases to reduce carbonyl and olefinic groups. The use of isolated reductase, which reduces carbon-carbon double bond chemoselectively, can produce saturated ketones selectively. 

Carbonyl groups may be hydrogenated catalytically, as carbon-carbon unsaturated linkages were (p. 191). It is, however, normally more difficult to effect the catalytic reduction of C=0 than of C=C, C=C, C=N, or C N, so that selective reduction of the former—in the presence of any of the latter—cannot normally be achieved catalytically. This can, however, be done with various, usually complex, metal hydrides. 

For the reduction of carbonyl groups or the oxidation of alcohols in the presence of C-C double and triple bonds, MPVO catalysts seem to be the best choice with respect to selectivity for the carbonyl group, as reductions with com-... 

BINAP (40a) was first reported as a ligand in an enantioselective hydrogenation in 1980 [172], and provides good selectivity for the reductions of dehydroamino acid derivatives [173], enamides, allylic alcohols and amines, and a,p-unsaturated acids [4, 9, 11, 12, 174, 175]. The fame of the ligand system really came with the reduction of carbonyl groups with ruthenium as the metal [11, 176]. The Rh-BINAP systems is best known for the enantioselective isomerizations... 

Reduction of carbonyl groups.1 This reagent effects highly selective 1,2-reduction of a,0-enones in THF. 

Hydrostannation of carbonyl compounds with tributyltin hydride is promoted by radical initiation and Lewis or protic acid catalysis.The activation of the carbonyl group by the acidic species allows the weakly nucleophilic tin hydride to react via a polar mechanism. Silica gel was a suitable catalyst allowing chemoselective reduction of carbonyl groups under conditions that left many functional groups unchanged. Tributyltin triflate generated in situ from the tin hydride and triflic acid was a particularly efficient catalyst for the reduction of aldehydes and ketones with tributyltin hydride in benzene or 1,2-di-chloromethane at room temperature. Esters and ketals were not affected under these conditions and certain aldehydes were reduced selectively in preference to ketones. 

In the previous section we have seen that enzyme-mediated reductions of carbonyl groups proceed with complete facial selectivity. Enzymes are complicated chiral molecules and synthetic organic chemistry is challenged to come consistently close to matching their selectivity. What we do here is merely point to some of the general directions that are followed in order to achieve selectivity. 

A number of hydrogen-transfer reactions involving carbonyl groups are known in organic chemistry however, these are for the most part limited to liquid-phase, homogeneously catalyzed systems. In this paper there is described a vapor-phase surface-catalyzed reaction which like the liquid-phase aluminum alkoxide catalyzed reductions of Meerwein-Ponndorf-Verley (J) will selectively reduce a carbonylic group in conjugation with a carbon-carbon double bond. 

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