Thioacetals General

The reactions of titanium-alkylidenes prepared from thioacetals with unsymmetrical olefins generally produce complex mixtures of olefins. This complexity arises, at least in part, from the concomitant formation of the two isomeric titanacyclobutane intermediates. However, the regiochemistry of the titanacyclobutane formation is controlled when an olefin bearing a specific substituent is employed. Reactions of titanocene-alkylidenes generated from thioacetals with trialkylallylsilanes 30 afford y-substituted allylsilanes 31, along with small amounts of homoallylsilanes 32 (Scheme 14.16) [28]. 

Several Me2SO adducts of rhodium(II) of the general type [Rh2(OOCR)4(Me2SO)2] have been synthesized (284,388) and their thermal decomposition via a two-step loss of sulfoxide studied (52). In addition, Me2SO adducts of rhodium(II) thioacetate (46) and thiobenzoate (45, 47) complexes have been reported. Coordination of the sulfoxide moieties via sulfur is reported on the basis of spectroscopic data. 

In order to ascertain whether sufficient nickel to complete a given reaction has been used, the liquid phase may be tested for starting material before an attempt is made to isolate a product. In general the sodium fusion test for sulfur is satisfactory for this purpose but in certain individual cases a specific test may be more convenient. Thus, in the desulfurization of thioacetals (mercaptals), unreacted material... 

Tricyclic sulfur heterocycles 341 were prepared utilizing an intramolecular [4 + 2] cycloaddition. Heating of allenyl sulfides 340 to 110 °C leads to Diels-Alder products 341 in reasonable yields (Scheme 8.93) [163], Unfortunately, this method does not allow general access to these heterocycles, since a particular substitution pattern of the substrate is required. No reaction occurred with substrates lacking the thioacetal moiety. 

Low basicity, even in combination with a reasonably stable carbonium ion intermediate, will not permit general acid catalysis if the leaving group is not sufficiently good. Thus, hydrolysis of benzaldehyde methyl S-phenyl thioacetals [67] is not subject to general acid catalysis (Fife and Anderson, 1970) even though... 

DetkioketaKzation. Various thioacetals and thioketals are readily hydrolyzed by pyridinium bromide perbromide (1 equivalent) under phase-transfer conditions. Tetrabutylammonium bromide is used as catalyst and aqueous methylene chloride as solvent. The reaction is more efficient in the presence of pyridine as buffer. Yields are generally 75-90%.1... 

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... 

A more general and straightforward method for the synthesis of the three classical thienothiophenes (la)-(lc) involves the condensation of an aldehyde function with an appropriate thioacetic acid substituent, in an ortho bifunctional thiophene derivative. 

The reaction of thioacetals with two equivalents of peroxy acids generally affords epimeric gem-disulfoxides, the ratio of epimers being subject to kinetic and/or equilibrium control (70). However, Poje, Sikirica, Vickovic, and Bruvo (71) have reported the first example of a stereospecific oxidation in the gem-disulfide series. They found that 2,2-bis(methylthio)-l,3-diphe-nylpropane is smoothly oxidized to the corresponding meso-disulfoxide with m-chloroperbenzoic acid. The study of conformational properties in solution by nmr indicated that the meso-disulfoxide exists in conformation 119 which... 

Peptide aldehydes constitute a rather general example of protease inhibitors. The electrophilic carbonyl group is attacked reversibly by the cleaving nucleophile, forming a covalent acetal or thioacetal intermediate. With cysteine proteases the preferred inhibitors are strong electrophiles, for example ketones, chloromethyl ketones, epoxides, or vinyl sulfones. Many cysteine protease inhibitors form an enzyme-inhibitor complex irreversibly these are therefore denoted suicide-inhibitors . 

In contrast to other methods, functional groups such as esters, amides, nitriles, alkenes, alkynes, etc. were unaffected, under these conditions. In pure methanol, ethanol or ethylene glycol, thioacetals were converted to acetals. The use of [bis(trichloroacetoxy)iodo]benzene gave equally good results [44]. Generally, since trichloroacetic acid is 3 times cheaper than trifluoroacetic acid on a mole basis, [bis(trichloroacetoxy)iodo]benzene may be an alternative to BTI. 

In general, thioacetals can be made in a similar way to normal (oxygen-based) acetals-by treatment of an aldehyde or a ketone with a thiol and an acid catalyst—though a Lewis acid such as BF3 is usually needed rather than a pro tic acid. The most easily made, most stable toward hydrolysis, and most reactive towards alkylation are cyclic thioacetals derived from 1,3-propanedithiol, known as dithianes. 

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