Carbonyl compounds thioacetalization

Carbonyl compounds react with thiols, RSH, to form hemi-thioacetals and thioacetals, rather more readily than with ROH this reflects the greater nucleophilicity of sulphur compared with similarly situated oxygen. Thioacetals offer, with acetals, differential protection for the C=0 group as they are relatively stable to dilute acid they may, however, be decomposed readily by H20/HgCl2/CdC03. It is possible, using a thioacetal, to reverse the polarity of the carbonyl carbon atom in an aldehyde thereby converting this initially electrophilic centre into a nucleophilic one in the anion (31) ... 

Aggarwal et al.108 reported excellent results with the catalytic asymmetric epoxidation of aldehydes. As shown in Scheme 4-52, a series of thioacetals 137 was prepared from hydroxy thiol 136 and the corresponding carbonyl compound. Among them, compound 138, derived from 136 and acetaldehyde, proved to be the best catalyst for asymmetric epoxidation of aldehydes. 

The electroreduction of disulfides R2S2 (R = Ar, Aik), in the presence of carbonyl compounds and MesSiCl, includes the formation of intermediate thiosilanes and results in trimethylsilyl ethers of hemithioacetals of ketones and aldehydes or in full thioacetals depending on whether a two-compartment (a) or an undivided (b) cell was used (Scheme 48) [218]. 

Reaction of the anion prepared by deprotonation of a ketene thioacetal (799) (LDA or s-butyllithium) with a carbonyl compound has been shown to afford generally the product of y-addition (800) (80JOC2236). Mercury ion-assisted hydrolysis of the 5-hydroxy ketene thioacetal provides access to a y-lactone (801 Scheme 187). The y-selectivity of carbonyl substrates is to be contrasted with the a-selectivity generally exhibited by alkyl halides in... 

Potassium hydroxide, 258 Trimethylsilyl chlorochromate, 327 of carbon-carbon double bonds substituted by heteroatoms m-Chloroperbenzoic acid, 76 of oximes to carbonyl compounds Lithium aluminum hydride-Hexa-methylphosphoric triamide, 159 Titanium(III) chloride-Diisobutylalu-minum hydride, 303 Trimethylsilyl chlorochromate, 327 of protected alcohols Chlorodimethylthexylsilane, 74 Formic acid, 137 p-Methoxyphenol, 181 of thioacetals and -ketals Methoxy(phenylthio)trimethyl-silylmethane, 182... 

When the carbonyl compound is sensitive to both acids and bases, or for other reasons gives poor yields in both the Clemmensen and Wolff-Kishner reductions, a recourse may be reduction of the corresponding thioacetal or thioketal with hydrogen-saturated Raney nickel (Section 11-2B) ... 

Thiols, like alcohols, react readily with carbonyl compounds. The resulting hemithioacetals (102), thioacetals (103) and their cyclic analogues (104) and (105) are of considerable synthetic importance. These compounds are more stable to acid hydrolysis than their oxygen analogues but can be hydrolyzed under mild conditions in the presence of metal ions such as Ag(I) (equation 29). The synthetic importance of the metal ion-promoted hydrolyses has led to the publication... 

Zulfiqar, F. and Kitazume, T. (2000) Lewis acid-catalysed sequential reaction in ionic liquids. Green Chem., 2, 296-297. Kamal, A. and Chouhan, G. (2004) Investigations towards the chemoselective thioacetalization of carbonyl compounds by using ionic liquid [bmim]Br as a recyclable catalytic medium. Adv. Synth. Catal., 346 (5), 579-582. 

Nitration and oxidation. Clay-supported Cu(N03)2, unlike clayfen (12,231), > shelf-stable for months. Like clayfen, it is a convenient source of N02+ and can leave thioacetals or selenoacetals to the carbonyl compound at 25° in high yield. It effects aromatization of 1,4-dihydropyridines in 80-92% yield. In the presence i acetic anhydride, it can effect nitration even of halobenzenes at 25° with marked, - jra-preference, which can be enhanced by use of lower temperatures. 

Dethioacetalization. The combination of phenyl dichlorophosphate, QH5OPOCI2, and Nal converts acetals to the corresponding aldehyde or ketone, but has little, if any, effect on thioacetals. However, the reagent obtained by addition of DMF converts thioketals into the corresponding carbonyl compound at room temperature in 1-17 hours in 70-95% yield. 

Carbonyl compounds as well as their 0,0-acetals undergo thioacetalisation and ram-thioacetalisation in ionic liquids with scandium(III) triflate as catalyst under facile reaction conditions.1681 Both hydrophilic and hydrophobic ionic liquids give good results and the catalyst could be recycled at least three times without change in activity after extraction of the product with diethyl ether. Higher reaction rates were observed relative to the reaction in dichloromethane. The procedure is highly chemoselective in that in the presence of both an aldehyde and a ketone only the aldehyde reacts to form the corresponding thioacetal. 

The acid-catalyzed condensation of 2,2 -thiodiethanethiol with carbonyl compounds under properly chosen conditions leads in good yield to thiocrown ethers containing thioacetal units. The reaction with benzaldehyde has been examined in detail, and the monomer 129, dimer 130, and polymer products have been characterized. The reaction was driven in good yield to any of these products by a proper choice of conditions (Equation 20 <1994J(P1)707>). 

Cleavage of thioacetals Fe(N03)3 or Cu(N03)2 supported on the clay cleaves thioacetals to the carbonyl compound in essentially quantitative yield. They are particularly efficient for cleavage of dithianes and dithiolanes. 

Thioacetalizfltion.23 Aldehydes and ketones react with alkylthiols and alkanedithiols in the presence of catalytic amounts of TiCl4 in CHC13 at 28° to form thioacetals and -ketals in >90% yield. The reaction is satisfactory even with readily enolizable carbonyl compounds. 

Many procedures are available in the literature for the deprotection of 5,S -dialkyl thioacetals to their carbonyl compounds such as clay supported ammonium ion, ferric or cupric nitrates, zirconium sulfonyl phosphonate, oxides of nitrogen, DDQ, Se02/AcOH, DMSO/HCI/H2O, TMSI(Br), LiN(i-C3H7)2/THF, ceric ammonium nitrate in aqueous CH3CN, CuCb/CuO/acetone and reflux, Hg(C104)2/chloroform and m-CPBA/Et3N/Ac20/H20. 

It is also appropriate here to remind the reader that carbonyl compounds or diols can be protected via formation of cyclic acetals or thioacetals. These reactions, as we have already seen, occur under mild conditions owing to the ease of formation of five- or six-membered heterocyclic rings. 

Thioacetals - carbonyl compounds. Hydrolysis of thioacetals to the parent carbonyl compound often presents some difficulty. Fetizon and Jurion have described two new methods for this conversion. In one method, a solution of the thioacetal in moist acetone is heated under reflux for several hours with excess methyl iodide. With... 

The electrochemical deprotection of carbonyl compounds proved to be a useful method especially in cases where alternative chemical reactions are unsuccessful, a-Keto- and a-hydroxythioacetals, when oxidized in MeCN-HoO (9 1 v/v) on a Pt anode, are transformed into the corresponding a-diones and a-ketols [142]. Diethyl dithioacetals of sugars were anodically oxidized in MeCN-H20 (5% H2O) on Pt electrode, and the substrates were successfully deprotected producing the correspondent carbohydrates in 65-85% yield [143]. It is noteworthy that protected hydroxy groups as esters or cyclic acetals were not affected. Selective deprotection to carbonyl compounds electrooxidizing mixtures of thioacetals, like a ketone and an aldehyde thioacetal, the former being preferentially deprotected, was described [144]. 

It has been shown that thioacetal monosulfoxides undergo hydrolysis with greater ease dian thioacetals or dithianes, i.e. the hydrolysis can be carried out with dilute sulfiiiic or perchloric acid. Not only is the hydrolysis of the thioacetal monosulfoxides more facile but the addition of the lithium salt proceeds 1,4 n er than 1,2 to a,p-unsaturated carbonyl compounds. ... 

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