Reduction acetal/ketal cleavage

Reaction of halides with oxide ion 0-68 Cleavage of oxonium salts 0-79 Reduction of acetals or ketals 0-92 Reaction between Grignard reagents and acetals or ketals dimerization of acetals... 

Olah, G.A., Yamato, T., Iyer, P.S. and Prakash, G.K.S., Catalysis by solid superacids. 20. Nafion-H catalyzed reductive cleavage of acetals and ketals to ethers with triethylsilane, /. Org. Chem., 1986, 51, 2826. 

Removal of the 4-OH was mediated by the Ullmann formation of the phenyl ether, which could be removed reductively by Na/NH3. To facilitate clean removal of the C-4 oxygen function, it was necessary to protect the ketone functions as ketals. A mixture of ketals (123 and 124) was isolated and both could be used in subsequent steps. Cleavage of the C-ring was effected by peroxide oxidation and recyclization of the benzomorphan 6,11-methylcar-boxylates (125) to the C-cyclopentanone (126) occurred in acetic anhydride. 

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). ... 

Methyl acetals and ketals are rapidly reduced to methyl ethers by sodium cyanoborohydride in methanol with dry HCI at ice temperatures. A dioxolane is completely cleaved to a methyl ether, showing intervention by the solvent at some stage (equation 14), but when an inert solvent such as THF is used only single cleavage occurs this reagent shows interesting selectivity in the reduction of benzylidene acetals in the carbohydrate series (see Section 1.9.3.4). 

The rate-controlling step in the acid-mediated reductive cleavage of an acetal or ketal involves the breaking of one of the C— bonds. This step is product-controlling when the two oxygen atoms are dif-... 

At ambient temperature, Zn(BH4)2 in EtjO, in the presence of trimethylsilyl chloride, transforms acetals and ketals into ethers. Under these conditions, esters remain intact, but the double bonds are reduced [KUl] (Figure 2.23). According to the experimental conditions, the regioselectivity of the cleavage can vary [JSl], as shown by the reduction of sugar derivative 2.44 (Figure 2.23). 

Fig. (6). A simple and enantioselective synthesis of (+)-albicanol (66) is desccribed. The hydroxy-ketone (60), prepared from the (-) Wieland-Miescher ketone, undergoes ring cleavage with lead tetra-acetate in methanol. The resulting product on ketalization leads the formation of the product (61) which is converted to oxime (63) by the standard reactions. Intramolecular cyclization followed by reductive hydrolysis and methylenation afforded (+)- albicanol (66).

A review of the preparation, structures, and stereochemistry of cyclic acetals of the aldoses and aldosides has appeared. Pyridinium toluene-p-sulphonate has been reported to be a mild catalyst for the formation and cleavage of dioxolane-type acetals, although no carbohydrate examples were quoted. Acetals and ketals of carbohydrates have been used as co-agents to introduce chirality into the products of sodium borohydride reduction of acetophenone, propiophenone, etc ... 

The reductive cleavage of methyl glycosides has some analogy with the hydrolysis of acetals and ketals [92]. Since glycosides are acetals, then formation of an oxocarbenium ion in the course of both reactions may imply mechanistic similarities. An A-2 mechanism has been suggested for hydrolysis of methyl (or phenyl) D-glycopyranosides [93]. According to this mechanism for the hydrolysis of... 

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