Deprotection of acetals

Scheme 31. Attempted deprotection of acetals 135 d,e with alkyllithiums [125]...

This electrochemical oxidation mediated by NHPI was applicable to benzylic carbons, allylic carbons, deprotection of acetals, oxidative cleavage of cyclic acetals and amide to afford benzoylated compounds, enones ", carbonyl compounds, -hydroxyethyl esters and imides, respectively (equations 31-35). 

Kametani T, Kondoh H, Honda T, Ishizone H, Suzuki Y, Mori W (1989) Simple and chemo-selective deprotection of acetals using aqueous dimethyl sulfoxide. Chem Lett 5 901-904... 

Eash KJ, Pulia MS, Wieland LC, Mohan RS (2000) A simple chemoselective method for the deprotection of acetals and ketals using bismuth nitrate pentahydrate. J Org Chem 65 8399-8401... 

Sen SE, Roach SL, Boggs JK, Ewing GJ, Magrath J (1997) Ferric chloride hexahydrate a mild hydrolytic agent for the deprotection of acetals. J Org Chem 62 6684—6686... 

Kaur G, Trehan A, Trehan S (1998) Highly efficient deprotection of acetals and ketals under neutral and anhydrous conditions using (trimethylsilyl)bis(fluorosulfuryl)imide. J Org Chem... 

Marko IE, Ates A, Gautier A, Leroy B, Plancher J-M, Quesnel Y, Vanherck J-C (1999) Cerium(IV)-catalyzed deprotection of acetals and ketals under mildly basic conditions. Angew Chem Int Ed 38 3207-3209... 

Dalpozzo R, De Nino A, Maiuolo L, Procopio A, Tagarelli A, Sindona G, Bartoli G (2002) Simple and efficient chemoselective mild deprotection of acetals and ketals using cerium(III) triflate. J Org Chem 67 9093-9095... 

Bailey AD, Chemey SM, Anzalone PW, Anderson ED, Emat JJ, Mohan RS (2006) A convenient method for in situ generation of I2 using CuS04/NaI and its applications to the deprotection of acetals, etherifications and iodolactonizations. Synlett 2 215-218... 

Pluth, M.D., Bergman, R.G. and Raymond, KN. (2007) Catalytic deprotection of acetals in basic solution using a self-assembled supramolecular nanozyme . Angew. Chem., Int. Ed., 119, 8741-8743. 

In this Section, hydrolysis, acetolysis, and isomerization of acetals are considered. Selective deprotection of acetals may also be achieved through halogenation, hydrogenolysis, ozonolysis, and photolysis, but these topics, are covered in an accompanying article in this Series, on the reactivity of cyclic acetals of aldoses and aldosides,3 and will not be discussed here. 

Deprotection of acetals.1 Dialkyl acetals are cleaved to the carbonyl compound when refluxed in aqueous DMSO. Deprotection of cyclic acetals (1,3-dioxolanes, 1,3-dioxanes) requires more drastic conditions, such as refluxing aqueous DMSO in combination with a cosolvent such as ethanol or 2-butanone. 

Acid-catalysed hydrolysis is the most common method for the deprotection of acetals and a mechanism for the reaction was given in section 1.2.2. Extensive kinetic and thermodynamic studies on cyclic acetal hydrolysis (see reviews in section 2.9.4) allow several conclusions to be drawn12-15 ... 

The solvent dependence of the rate of solvolysis of TBS ethers has useful implications for selective deprotection of acetal-type protecting groups. Thus, dilute... 

Deprotection of acetals. Acetals, ketals, and dioxolanes are converted to the carbonyl compounds by TiClj in ether, usually in yields of 75-90%. 

The selective deprotection of acetal to a carbonyl group can be achieved with lithium tetrafluoroborate LiBp4 in THE... 

The mild deprotection of acetal 15 with CUSO4 in acetone was accomplished in high yield [12]. The mildness of copper Lewis acids for the hydrolysis of hydra-zones [13] is advantageous for acid-sensitive substrates (Sch. 5). The /3,a-unsaturated hydrazone 17 was effectively hydrolyzed without conjugation of the double bond to afford 18 in good yield (Sch. 5) [14]. SAMP-Hydrazones can be hydrolyzed without racemization [15]. The two-step alkylation and hydrolysis of 19 provided 20 in 84 % overall yield and 94 % ee. 

Selective deprotection of acetals—determined by the relative rate of hydrolysis as influenced by steric, inductive, and stereoelectronic effects... 

Deacetalization The deprotection of acetals is performed in high yields (9... 

DDQ, a reagent typically used to deprotect p-methoxybenzyl ethers (see section 4.3.4), has been used for the deprotection of acetals and thioacetals. The use of 0.1-0.4 equivalents of DDQ in acetonitrile-H20 (9 1) cleaves isopropylidene groups at room temperature to 80 C without affecting p-toluenesulfonyl, benzoyl, benzyl, or acetate groups. Monosubstituted dioxolanes [Scheme 3.17] are more readily hydrolysed than bicyclic, spirocyclic, and disubstituted systems and 1,3-dioxanes are more labile than 1,3-dioxolanes. Removal of dithioacetals requires 2 equivalents of DDQ at 80 C. 

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