Monothioacetals

Zn, (CH30)2CH2, BrCH2C02Et, 80-82% yield. Formation of the meth-oxymethyl thioether with dimethoxymethane avoids the use of the carcinogen chloromethyl methyl ether. The reaction forms an intermediate zinc thiolate, which then forms the monothioacetal. 

The related 5-benzyloxymethyl monothioacetal (BOM) has been used for cysteine protection. ... 

T AHBr, AcOH, rapid. The 5-isobutoxymethyl monothioacetal is stable lo 2 N hydrochloric acid and to 50% acetic acid some decomposition occurs m2 N sodium hydroxide. ... 

The monothioacetal is also stable to 12 N hydrochloric acid in acetone (used to remove an TV-triphenylmethyl group) and to hydrazine hydrate in refluxing ethanol (used to cleave an A -phthaloyl group). It is cleaved by boron trifluoride etherate in acetic acid, silver nitrate in ethanol, and tiifluoroacetic acid. The monothioacetal is oxidized to a disulfide by thiocyanogen, (SCN)2- ... 

An S-2-tetrahydropyranyl monothioacetal is oxidized to a disulfide by iodine or thiocyanogen, (SCN)2- ... 

M (marginal) indicates that the stability of the protected functionality is marginal, and depends on the exact parameters of the reaction. The protective group may be stable, may be cleaved slowly, or may be unstable to the conditions. Relative rates are always important, as illustrated in the following example (in which a monothioacetal is cleaved in the presence of a dithiane), and may have to be determined experimentally. 

An. S-tetrahydropyranyl monothioacetal is stable to 4 A HCI/CH3OH, 0° and to reduction with Na/NH3. (An O-tetrahydropyranyl acetal is cleaved by 0.1 AHCl,... 

Searching for a method of synthesis of enantiopure lamivudine 1, the compound having a monothioacetal stereogenic centre, Rayner et al. investigated a lipase-catalysed hydrolysis of various racemic a-acetoxysulfides 2. They found out that the reaction was both chemoselective (only the acetate group was hydrolysed with no detectable hydrolysis of the other ester moieties) and stereoselective. As a result of the kinetic resolution, enantiomerically enriched unreacted starting compounds were obtained. However, the hydrolysis products 3 were lost due to decomposition." In this way, the product yields could not exceed 50% (Equation 1). The product 2 (R = CH2CH(OEt)2) was finally transformed into lamivudine 1 and its 4-epimer. ... 

Examples of nucleophilic attack at the saturated 2-carbon of dihydro-l,4-thiazines, which may be assisted by the neighboring sulfur atom, are shown below. The nucleophiles include water, which was used in the acid-catalyzed hydrolysis of the ketal in 236 (Equation 21) <1982S424>, methanol in the conversion of 214 into a monothioacetal (Equation 22) <1982JHC131>, ethanol and dimethylaniline, which both reacted with 237 (Scheme 29) <1982TL4963>, and triethyl phosphite that was used to convert 238 into the phosphonate 239 required for Wadsworth-Emmons reaction (Equation 23) <2004BML1477>. Compound 240 reacted with both methanol and methanethiol (Equation 24) <1990JME1898>. 

The C-H bond-dissociation energies Z>c-h of the acetals 130 and 131, monothioacetal 132, and the dithioacetal 133 (Figure 7) were calculated from experimental reaction kinetic data <2005MI379>. The C-H dissociation energies of the 1,3-dithiane derivative 133 proved to be lowest, followed by the 1,3-oxathiane derivatives 132,... 

The replacement of a phenylsulfanyl group in a saccharide monothioacetal by fluorine using DAST in the presence of /V-bromosuccinamidc (NBS) has been reported to occur at temperatures in the range 0-20°C (see Table 8).64... 

Only a few reports deal with the reaction of thionoesters with organometallic reagents, and carbophilic addition was observed as the usual mode. The reaction of O-ethyl thioesters with alkyllithiums afforded, after methylation, good yields of monothioacetals, which can be hydrolysed to ketones [353]. 

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