Ethers, Acetals, and Ketals

In acetals and ketals, the C-0 stretching vibrations are split into 3, sometimes even 4 to 5 bands. 

Aeetals have an additional band due to a special C-H 8 vibration. 

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The hexitols form the typical derivatives of alcohols and glycols— esters, ethers, acetals and ketals, and metallic complexes. Because of the multiplicity of reactive groups and because of stereoisomerism, the number of theoretically possible derivatives of a given type is enormous. For example, D-mannitol on reacting with a mono reagent such as an etherifying agent or an acid, can form 35 different compounds ... 

Dehydration of alcohols to ethers, acetals, and ketals Deloxan ASP, Amberlyst Selectivity enhancement Gray et al. (1999)... 

Oxidation of Ethers, Acetal and Ketal Derivatives. Haines, A. H.T Methods for the Oxidation of Organic Compounds Academic Press London, 1988 p 183. 

Unsaturated Ethers, Acetals and Ketals, Esters of Unsaturated Alcohols,... 

There are reported numerous examples of the syntheses of dihalocyclo-propanes via addition of PTC-generated dihalocarbenes to unsaturated compounds such as alkenes, unconjugated and conjugated di- and polyenes, allenes, cumulenes functionalized alkenes such as haloalkenes unsaturated ethers, acetals, and ketals esters of imsaturated alcohols alkenes substituted by sulfur, nitrogen, silicon atoms some unsaturated carbonyl compounds, etc (eqs. 108-114). 

The ring-opening process of Equation 8.45 is, of course, simply the reverse of the process by which oxiranes (oxacyclopropanes, epoxides) are formed from halohy-drins (e.g., see item 3,Table 7.6). Further, as written, the processes shown in Schemes 8.90-8.92 are reversible and thus, at least in principle, carbonyl compounds can be converted to enol ethers, acetals (and ketals), and orthoesters. However, while acetals and ketals readily form from alcohols and acids under dehydrating conditions (Chapter 9) and esters undergo exchange reactions with alcohols in the... 

Iron chloride exhibits Lewis acidity to readily cleave C-O bonds of ethers, acetals, and ketals (silyl ethers, later). Anhydrous FeC in dichloromethane can cleave benzyl and p-phenylbenzyl ethers of sugar skeletons, for example (166), in a few minutes at room temperature. Methyl ethers and several acyl groups are not affected under the reaction condition (Scheme 16.46) [53]. 

Methyl Ethers t-Butyl Ethers Allyl Ethers Benzyl Ethers Triarylmethyl Ethers Trimethylsilyl Ethers Acetals and Ketals... 

A Lewis acid is involved in the reaction media when RCu BF3 or R3Al is used to cleave an acetal or ketal framework, and the resulting enol ether contains a E double bond. This is quite reasonable, since the overall reaction proceeds in an anti-SN2 manner. When a v>- -SN2 process is involved, the formation of products containing a Z double bond is observed60 (Table 2). The reaction of alkyllithium reagents with a./J-ethylcnic acetals and ketals proceeds in a. H7i-SN2 manner without assistance of Lewis acids, giving mainly the Z-products61-63. 

Alkyl esters are efficiently dealkylated to trimethylsilyl esters with high concentrations of iodotrimethylsilane either in chloroform or sulfolane solutions at 25-80° or without solvent at 100-110°.Hydrolysis of the trimethylsilyl esters serves to release the carboxylic acid. Amines may be recovered from O-methyl, O-ethyl, and O-benzyl carbamates after reaction with iodotrimethylsilane in chloroform or sulfolane at 50—60° and subsequent methanolysis. The conversion of dimethyl, diethyl, and ethylene acetals and ketals to the parent aldehydes and ketones under aprotic conditions has been accomplished with this reagent. The reactions of alcohols (or the corresponding trimethylsilyl ethers) and aldehydes with iodotrimethylsilane give alkyl iodides and a-iodosilyl ethers,respectively. lodomethyl methyl ether is obtained from cleavage of dimethoxymethane with iodotrimethylsilane. 

The EtsSiH/tetracyanoethylene combination reduces acetals and ketals to the corresponding ethers but the yields are mixed.500 The full reduction of benz-aldehyde acetals to the toluene derivatives is realized by the initial reduction with Et3SiH/SnBr2-AcBr followed by Bu3SnH/AIBN (azobis(isobutyronitrile)) or LiAlH4.479 The overall yields are excellent. 

Diphenylmethylsilyl-protected hemiacetals are reduced upon treatment with Ph2SiH2/Mn(CO)3Ac.295 Et SiH/TiCB reduces tert-butyldimethylsilyl ketals.306 The combination of TBSH/Sn(OTf)2 and a silyl ether converts ethylene glycol acetals and ketals into ethers (Eq. 309).501... 

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. 

In contrast to the esters and ethers, many acetals and ketals are known. It is with these derivatives that most of the interesting structural work has been done. 

A tertiary radical can be formed by elimination during AF of ter/-butyl methyl and ethyl ethers thus, isolation of the respective perfluoro-/erf-butyl ethers, e.g. 1, occurs in only 36 and 42% yield.28 Significant quantities of perfluoro(2-methylpropane) (2) are also isolated. The longer alkyl chains (ethyl and larger) appear to be slightly less prone to scission than the methyl group. Apparently, carbonyl fluoride is more readily eliminated than trifluoroacetyl fluoride, a phenomenon observed during AF of esters.29 Elimination becomes most serious in the special class of polyethers called ortho esters, e.g. 3-5.30 Cyclic ortho esters, acetals and ketals are much less affected than acyclics. 

Homoallylk ethers. lodotrimethylsilane catalyzes the allylation of acetals and ketals by allylsilanes, with transposition of the allylic group (equation I). It does not catalyze allylation of aldehydes and ketones. Note that TiCI can catalyze both of these reactions (7, 370-371). In this respect, ISi(CH3)3 resembles (CH3)3SiOTf (this volume). 

D. A. Home and A. Iordan, An efficient reduction of acetals and ketals to methyl ethers, Tetrahedron Lett 1357 (1978). 

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. 

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