Acetals thioacetals

Figure 20 1 shows the structures of various derivatives of acetic acid (acetyl chlo ride acetic anhydride ethyl thioacetate ethyl acetate and acetamide) arranged m order... 

PhSH, BF3 Et20, CHCI3, 0°, 10 min, 86% yield. ZnCl2 and MgBr2 have also been used as catalysts. With MgBr2, acetals can be converted to thioacetals in the presence of ketones. 

HS(CH2) SH, CICH2CH2CI, TeCl4, rt, 80-99% yield."" This method is also effective for converting dimethyl acetals to the thioacetal and for selectively protecting an aldehyde in the presence of a ketone. 

The mechanisms for hydrolysis of 0,5-acetals have been reviewed. The following acid-catalyzed cleavage rates show that the 0,5-acetals have a stability that lies between thioacetals and acetals ... 

NaSMe, MeOH, 23°, 81-95% yield. This procedure is chemoselective for removal of a thioacetate in the presence of an acetate. 

The hetero Diels-Alder [4+2] cycloaddition (HDA reaction) is a very efficient methodology to perform pyrimidine-to-pyridine transformations. Normal (NHDA) and Inverse (IHDA) cycloaddition reactions, intramolecular as well as intermolecular, are reported, although the IHDA cycloadditions are more frequently observed. The NHDA reactions require an electron-rich heterocycle, which reacts with an electron-poor dienophile, while in the IHDA cycloadditions a n-electron-deficient heterocycle reacts with electron-rich dienophiles, such as 0,0- and 0,S-ketene acetals, S,S-ketene thioacetals, N,N-ketene acetals, enamines, enol ethers, ynamines, etc. 

Aldehydes and ketones react with thiols to yield thioacetals just as they react with alcohols to yield acetals. Predict the product of the following reaction, and propose a mechanism ... 

Thioesters are named like the corresponding esters. If the related ester has a common name, the prefix thio- is added to the name of the carboxylate acetate becomes thioacetate, for instance. If the related ester has a systematic name, the -oate or -carboxylate ending is replaced by -thioate or -carbothioate butanoate becomes butanelhioate and cyclohexanecarboxylate becomes cyclohexane-carbothioate, for instance. 

Considerable efforts have been devoted to the stereoselective introduction of a /(-methyl function in intermediates for the synthesis of 1 jS-methylcarbapenems. While the trimethylsilyl trifluoromethanesulfonate catalyzed reaction of a 4-acetoxyazetidinone derivative with ketene acetals shows no selectivity, ketene thioacetals lead to stereoselective formation of the a-methyl isomer108. The zirconium enolate, however, shows high /(-methyl selectivity. 

The reaction of crotonaldehyde and methyl vinyl ketone with thiophenol in the presence of anhydrous hydrogen chloride effects conjugate addition of thiophenol as well as acetal formation. The resulting j3-phenylthio thioacetals are converted to 1-phenylthio-and 2-phenylthio-1,3-butadiene, respectively, upon reaction with 2 equivalents of copper(I) trifluoromethanesulfonate (Table I). The copper(I)-induced heterolysis of carbon-sulfur bonds has also been used to effect pinacol-type rearrangements of bis(phenyl-thio)methyl carbinols. Thus the addition of bis(phenyl-thio)methyllithium to ketones and aldehydes followed by copper(I)-induced rearrangement results in a one-carbon ring expansion or chain-insertion transformation which gives a-phenylthio ketones. Monothioketals of 1,4-diketones are cyclized to 2,5-disubstituted furans by the action of copper(I) trifluoromethanesulfonate. ... 

Our group has exploited 4-phenylthio-l,3-dioxanes as convenient precursors to 4-lithio-l,3-dioxanes [45,65-69]. 4-Phenylthio-l,3-dioxanes 184 were originally prepared from -silyloxy aldehydes 183 [65] (Eq. 28). Lewis acid-promoted addition of phenylthiotrimethylsilane gave an unstable thioacetal intermediate, which could be converted in situ to the corresponding 1,3-dioxane. Yields for this process are variable, as the product is unstable under the conditions of its formation. The reaction slowly evolves to a mixture of the desired product, the phenylthio acetal of 183, the phenylthio acetal of acetone, and a variety of other unidentified products. 

The molecular structures of several [TpBut]ZnX derivatives have been determined by x-ray diffraction. For example, x-ray diffraction studies confirm that the acetate ligand in [TpBut]Zn(r)1-02CMe) is bound to zinc in a unidentate mode, similar to that proposed for [TpBut]Mg(7j1-02CMe), but in contrast to the bidentate coordination proposed for the copper analogue [TpBut]Cu(T)2-02CMe) (86,87). Such a change in coordination mode for copper and zinc derivatives is to be anticipated on the basis of structural studies on the nitrate derivatives [TpBut]M(N03) (M = Co, Ni, Cu, Zn), as described in Section V,B,2,e. The thioacetate [TpPh]-Zn V-SC(0)Me (81), and cyanoacetate [Tp lZnlr -C CCH N) (88) derivatives also exhibit unidentate coordination. 

Several of the lower molecular weight aliphatic compounds, in a mixture, are part of the roasted coffee aroma. A nine-compound mixture with roasted coffee aroma contained isopentane, n-hexane, acetaldehyde, dimethyl sulfide, propanal, isobutanal, isopentanal, methanol, and 2-methylfuran.20 In addition, the freshness of aroma and taste has been correlated with 2-methylpropanal and diacetyl. When the concentration of these falls off, so does the taste.21 Other aliphatic compounds that are steadily lost from ground roasted coffee, unless it is vacuum packaged, include methyl formate, methyl acetate, methyl thioacetate, and acetone.22 The concentrations in roast coffee for four compounds whose contribution to the fresh flavor have long been known are dimethyl sulfide (4 ppm), methyl formate (12 ppm), isobutanal (20 ppm), and diacetyl (40 ppm). The taste thresholds are 0.1, 0.5, 0.5, and 1.0 ppm, respectively, in the brew made with 5 g coffee per 100 ml water.15... 

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) ... 

C. Moreau, J. Lecomte, S. Mseddi, and N. Zmimita, Stereoelectronic effects in hydrolysis and hydrogenolysis of acetals and thioacetals in the presence of heterogeneous catalysts, J. Mol. Catal. A Chem., 125 (1997) 143-149. 

In soft cheeses, such as Brie, Camembert, and Limburger, the following sulfur compounds were implicated 3-(methylthio)propanol, MT, DMS, DMDS, DMTS, dimethyl tetrasulfide, methyl ethyl disulfide, diethyl disulfide, 2,4-dithiapentane, 3-methylthio-2,4-dithiapentane, methional, 2,4,5-trithiahex-ane, 1,1-fe-methylmercaptodisulfide, methyl thioacetate (=methanethiol acetate), benzothiazole, methylthiobenzothiazole, methyl ethyl sulfonate, methyl methane thiosulfonate, thiophene 2-aldehyde, and H2S.34 Many of these were only present in small amounts Limburger cheese was notable for 13.2% of DMDS, 0.5% of methyl thioacetate, and 0.8% of DMTS. 

The efficiency of catalysts 86-89 for the asymmetric aldol reaction of a series of nucleophiles toward benzyloxyacetaldehyde was studied. For example, compound 89c was found to be an excellent catalyst for the asymmetric aldol reaction of silylketene acetal derivatives of t-butyl thioacetate, ethyl thioacetate, and ethyl acetate with benzyloxyacetaldehyde. In the presence of 0.5 mol% of the catalyst, the asymmetric aldol reaction took place at —78°C in CH2C12, affording the respective /i-hydroxy esters with excellent enantioselectivity (Scheme 3-32). 

Fig. 7.10. Approximate charge distribution in methyl acetate and S-methyl thioacetate, as calculated by the INDO quantum-mechanical method [144]...

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