Reduction of Esters and Lactones

The combination of (EtO)3SiH/CsF (or KF) provides a convenient reagent for the reduction of esters to alcohols.76,80,83 The yields are in the 70% range. Potassium tetraethoxyhydridosilicate also reduces esters in moderate yields.288 The combination of PM HS/CpiTiCE/n-BuLi reduces esters in high yields even in the presence of an epoxide and a trisubstituted olefin (Eq. 140).289 The reagent combination can reduce a methyl ester in the presence of a tert-butyl ester (Eq. 141).290 

Cp2Ti(OC6H4Cl-4)2 TBAF/alumina, MeQdH, rt 

The reaction of lactones of benzyl alcohols with Et3SiH/TFA results in complete reduction of the alcohol part of the lactone to the methylene group while preserving the carboxylate function (Eq. 148).305 

The ft -hydroxy ester resulting from the reaction of the tert-butyldimethylsilyl ketene acetal of ethyl acetate with a lactone under acid conditions can be reduced to the /3-alkoxy ester.306 The overall yields are excellent (Eq. 149). 

The reaction of tert-butyl esters with Et3SiH/TFA results in the reductive deprotection of the ester and formation of isobutane. The yields of the isobutane are not recorded, but the acids are obtained nearly quantitatively (Eq. 150).307 In a similar manner, the lactone shown in Eq. 151 is converted into the acid in good yield.308 In like manner, the reductive deprotection of allyl esters provides the carboxylic acids in high yields.270 

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The most widely used reagent for partial reduction of esters and lactones at the present time is diisobutylaluminum hydride (DiBAlH).83 By use of a controlled amount of the reagent at low temperature, partial reduction can be reliably achieved. The selectivity results from the relative stability of the hemiacetal intermediate that is formed. The aldehyde is not liberated until the hydrolytic workup and is therefore not... 

Table 10. organosilane reduction of Esters and Lactones (continued)... 

Supplemental References for Table 10. Organosilane Reduction of Esters and Lactones... 

The scope and mechanism of the reductions of esters and lactones to ethers with diborane has been reported in detail cf. ref. 279). Initial co-ordination of BH3 with the alkoxy-oxygen is considered to be the essential step in the reduction to alcohols. Ether formation seems to occur as an alternative pathway when the... 

Probably the most widely used reagent for the partial reduction of esters and lactones at the present time is diisobutylaluminum hydride. By use of a controlled... 

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