Reduction, of esters .with

Decamethylene glycol has been prepared by the reduction of dimethyl sebacate and diethyl sebacate with sodium and ethyl alcohol by the reduction of sebacamide with sodium and amyl alcohol and by the reduction of dimethyl sebacate with sodium and liquid ammonia in absolute alcohol. The reduction of esters with sodium and alcohol has also been applied to the preparation of many other glycols. ... 

The direct reduction of esters with diisobuty laluminum hydride in the presence of the Horner-Emmons reagent prepared from ethyl diethylphosphon-ofluoroacetate avoids the necessity to work with sensitive aldehydes in the olefi-nation procedure [72, 75] (equation 62) (Table 23). 

Reductions of Esters with Metals in the Presence of Trimethylchlorosilane 281... 

Other reagents used for reduction are boranes and complex borohydrides. Lithium borohydride whose reducing power lies between that of lithium aluminum hydride and that of sodium borohydride reacts with esters sluggishly and requires refluxing for several hours in ether or tetrahydrofuran (in which it is more soluble) [750]. The reduction of esters with lithium borohydride is strongly catalyzed by boranes such as B-methoxy-9-bora-bicyclo[3.3.1]nonane and some other complex lithium borohydrides such as lithium triethylborohydride and lithium 9-borabicyclo[3.3.1]nonane. Addition of 10mol% of such hydrides shortens the time necessary for complete reduction of esters in ether or tetrahydrofuran from 8 hours to 0.5-1 hour [1060],... 

Although lithium aluminum hydride and boranes are very useful reagents, they are expensive and impractical to employ on a large scale. Other methods of reduction then may be necessary. Of these, the most important are reduction of esters with sodium and ethanol (acids do not react readily),... 

A useful method of forming carbon-carbon bonds involves reduction of esters with sodium metal in aprotic solvents such as ether or benzene and is called the acyloin reaction ... 

Primary alcohols may be conveniently prepared by the reduction of esters with sodium and absolute ethanol (the Bouveault-Blanc reduction, Expt 5.37). 

Finally stoichiometric addition of DIBAH in a nonpolar solvent at low temperature yields the desired aldehyde 11. Overreduction to the corresponding alcohol 33 is prevented under these conditions because of the formation of the relative stable intermediate 34, which decomposes only in the course of aqueous work-up. At higher temperature and in polar solvents such as THF formation of alcohol 33 occurs. Thus, grade of reduction of esters with DIBAH can be controlled by temperature and solvent. 

Figure 19.1 Synthetic scheme for example pheromone components. Abbreviations al and a2 = Wittig-Homer condensations with triethyl 2-phosphonopropionate and triethyl 2-phosphonobutyrate, respectively b = reduction of ester with lithium aluminum hydride c = partial oxidation of alcohol with manganese dioxide to aldehyde dl and d2 = Wittig condensations with ethyltriphenylphosphonium bromide and propyltriphenylphosphonium bromide, respectively e = condensation with dimethylhydrazone phosphonate reagent f = hydrolysis under acidic conditions. Compound numbers are as in Table 19.1.

Table 42 Reduction of esters with sodium gallium hydride. Reproduced with permission from the Korean Chemical Society...

The use of DIBAL-H to reduce nitriles to aldehydes has been added, as has the low-temperature reduction of esters with DIBAL-H to produce aldehydes. Several problems have been added that include these reactions in synthesis. 

Selective reduction of a-hydroxy esters. Reduction of esters with BMS is slow, but can be facilitated by addition of catalytic amounts of sodium borohydride. This method of reduction can be used to effect regioselective reduction of a-hydroxy esters. Thus reduction of dimethyl (S)-( — )-malate (1) results in formation of methyl (3S)-3,4-dihydroxybutanoate (2). 

Reduction of esters. With an alkene hols with this borohydride. Intermediate bi ers are converted to aldehydes on treaime... 

Numerous procedures have been reported for the synthesis of iV-protected-a-amino aldehydes [78]. The A-protected a-amino aldehydes can be prepared either by oxidation of the corresponding A-protected j8-amino alcohols with oxidants such as pyridinium dichromate [79,80] or SOs-pyridine-di-methyl sulfoxide (DMSO) [81] or under Swem conditions [DMSO-oxalyl chloride-A,A-diisopropylethylamine (DIEA)] [81-83], by reduction of esters with diisobutyl aluminum hydride (DIBAL) [84], by reduction of A,A-disubstituted amides [85-87], by reduction of urethane-protected A-carboxy-... 

The mechanism for the reduction of esters with lithium aluminum hydride involves the addition of hydride to the ester carbonyl group to give a tetrahedral intermediate, which undergoes elimination to produce an aldehyde. 

Reduction of wax esters with sodium. The reduction of esters with sodium was first described by Bouveault and Blanc in 1902. Large-scale application of this process was achieved in 1928 (Dehydag). 

Reduction of Esters with Hydride-Reducing Agents... 

FIGURE 18.37 The reduction of esters with the powerful hydride donor lithium aluminum hydride leads to primary alkoxides and then, after acidification, to primary alcohols. 

The usual chemistry of carboxylic acids can also be carried out in proximity of the (T) -diene)iron moiety. The stereocontrolling effect of the iron complex moiety is not exploited in this case as usually no new stereogenic centers are formed in the course of this transformation. Such reactions include reduction of esters with diisobutylaluminum hydride to the corresponding alcohols. The same reagent can be used for a... 

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