Benzylic ketals reduction

The cleavage proceeds by initial reduction of the nitro groups followed by acid-catalyzed cleavage. The DNB group can be cleaved in the presence of allyl, benzyl, tetrahydropyranyl, methoxy ethoxy methyl, methoxymethyl, silyl, trityl, and ketal protective groups. 

Because carbohydrates are so frequently used as substrates in kinetic studies of enzymes and metabolic pathways, we refer the reader to the following topics in Ro-byt s excellent account of chemical reactions used to modify carbohydrates formation of carbohydrate esters, pp. 77-81 sulfonic acid esters, pp. 81-83 ethers [methyl, p. 83 trityl, pp. 83-84 benzyl, pp. 84-85 trialkyl silyl, p. 85] acetals and ketals, pp. 85-92 modifications at C-1 [reduction of aldehydes and ketones, pp. 92-93 reduction of thioacetals, p. 93 oxidation, pp. 93-94 chain elongation, pp. 94-98 chain length reduction, pp. 98-99 substitution at the reducing carbon atom, pp. 99-103 formation of gycosides, pp. 103-105 formation of glycosidic linkages between monosaccharide residues, 105-108] modifications at C-2, pp. 108-113 modifications at C-3, pp. 113-120 modifications at C-4, pp. 121-124 modifications at C-5, pp. 125-128 modifications at C-6 in hexopy-ranoses, pp. 128-134. 

Bamford-Stevens decomposition of tosylhy-drazones, 351 p-Benzoquinone, 308 Benzyl ether hydrogenolysis, 139 Benzyl thioenol ethers, 87 Birch reduction, 11, 49, 50 Birch reduction of estrone methyl ether diethyl ketal, 51... 

Diisobutylaluminum hydride (DIBAL-H) hydrogenolyzes simple ortho esters to acetals at room temperature and reduces acetals and ketals to ethers at 70-80 °C (equation 6) benzyl ethers are cleaved at still higher temperatures. This reagent shows good selectivity and considerable versatility. It has been used to reduce acetals of formaldehyde (equation 7), ° which few other reagents can accomplish. With catechol ketals a single reductive cleavage occurs at room temperature (equation 8). ... 

The piperidine was prepared from 2-(2-cyanoethyl)-butryraldehyde (470) via ketal 471, which was reduced with lithium aluminum hydride and reductively alkylated with benzaldehyde over 10% palladium on charcoal to 472. Acid hydrolysis led to spontaneous cyclization and formation of enamine 473. Treatment of this enamine with methyl bromoacetate and reduction of the iminium species with sodium borohydride gave the 1-benzyl-3,3-disubstituted piperidine 474 which was debenzylated with palladium charcoal under acidic conditions to give the desired piperidine, 466 (216). 

A further report has appeared on the use of trimethylsilyl iodide for ester cleavage/ Although lengthy reaction times are often required, yields of carboxylic acids are excellent. This reagent also offers the possibility of selectivity, as t-butyl and benzyl esters react rapidly at room temperature whereas other alkyl esters (Me, Et, Pr ) require temperatures at 50 °C for reaction to occur. A disadvantage of the reagent is that it will also attack ethers, ketals, and alcohols. An improvement of this method is the use of mixtures of phenyltrimethylsilane and iodine. Benzyl esters can be deprotected by oxidative cleavage with nitrosonium hexafluorophosphate. This method could be valuable as almost all non-hydrolytic methods for the removal of benzyl esters are reductive. 

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