Thioacetals preparation

Thioacetals, prepared from the corresponding ketones, have also been reduced to alkenes using moderately active Raney nickel as the desulfurization agent (Scheme 37). This form of Raney nickel does not reduce the newly formed alkene or react with other isolated carbon-carbon double bonds. Note that under these conditions the saturated hydrocarbon often observed in Raney nickel desulfurizations is not formed. Similar conditions have been used for the conversion of cyclodecanone to cyclodecene. While thioacetals have also served as intermediates in the analogous conversion of enones into dienes, a mixture of diene products is obtained. For instance, dithiane generation from 17p-hydroxy-A -androsten-3-one, followed by desulfurization affords a 4 1 mixture of 17P-hydroxy-A -androstadiene and 17P-hy-droxy-A -androstadiene, respectively. ... 

Access to cyclopropanone thioacetals was provided by dibromocarbene addition to an alkene followed by sequential replacement of the bromine atoms with methylsulfanyl groups. Bi-cyclo[n.l. 0]alkanone thioacetals prepared in this way rearranged in the ring-enlargement mode when treated with formic or trifluoroacetic acid (Table 2). 

Carbanioas derived from the other above mentioned methane derivatives react with alkylboranes, bornic, and boronic esters, providing rich possibihties for the preparation of siagle-carbon iasertion products. Thus 2-aLkyl-l,3,2-dithiaborolanes are converted iato acids or thioacetals by trichloromethyllithium (335). 

The preparation of quinazoline-2(and 4)-thiones follows those of the corresponding pyrimidines (67HC(24-1)270) but there is at least one special primary synthesis for quinazoline-4(3H)-thiones, illustrated by the reaction of o-aminobenzonitrile with thioacetic acid at 110 °C to give 2-methylquinazoline-4(3H)-thione in 90% yield (53JA675). 

RSSiMe3 [R = Me, Et, (-CH2-)3], Zn, Et20, 0-25°, 70-95% yield. This method is satisfactory for a variety of aldehydes and ketones and is also suitable for the preparation of 1,3-dithianes. Methacrolein gives the product of Michael addition rather than the thioacetal. The less hindered of two ketones is readily protected using this methodology. ... 

At low temperature a 1 1 adduct of thioacetic acid and an enamine could be prepared (709). The previously described reaction of aminomethylene ketones with hydrogen peroxide was extended to bisaminomethylene compounds. However, acylated cyclohexenamines led to cyclopentane-carboxamides (770), Trichloromethyl adducts of enamines and the rearranged amine derivatives were described in a further study (777). 

Another example of resort to heteroatoms to obtain both oral potency and a split between androgenic and anabolic activities Ls tiomestrone (99). Trienone, 98, prepared in much the same way as 23, undergoes sequential 1,6 and 1,4 conjugate addition of thioacetic acid under either irradiation or free radical catalysis to afford the compound containing two sulfur atoms. 

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. 

Despite the latent reactivity of ketene thioacetals,3-4 some members of this class such as the title compound have been little studied, perhaps because of preparative inaccessibility. The only previously reported route to 2-methylene-1,3-dlthiolane involves monoacetylation of 1,2-ethanedithiol, cyclization to 2-methyl-1,3-dithiolan-2-yl perchlorate, and exposure of this salt to diisopropylethylamine in acetonitrile 5... 

The parent hexathiaadamantane (185) is obtained preparatively when a solution of formic acid and hydrochloric acid in nitrobenzene is allowed to stand for several weeks in a hydrogen sulfide atmosphere the product which separated is almost insoluble in all common solvents and purification presents a problem. Only large volumes of dimethyl sulfoxide at reflux serve for recrystallization.224 The reaction of thioacetic acid with formic acid in the presence of zinc chloride gives tetramethyl-(186), monomethyl-, dimethyl-and trimethylhexathiaadamantane derivatives (187).225 Other variations include the reaction of thioacetic acid with a /i-diketone,226 and the use of boron trifluoride227 or aluminum chloride as a catalyst.228... 

Figure 9.59 One of the first methods of preparing water-soluble QDs was to use thioacetic acid modification of the nanocrystal surface. This resulted in a negative charge on the surface of each dot that provides like charge repulsion of particles suspended in aqueous solution. The carboxylate group also could be used for conjugation with amine-containing molecules.

Thioacetals have been prepared using essentially a similar technique [33]. The active methylene compounds are adsorbed on KF-alumina, admixed with methanesul-fonothioate and are irradiated in microwave oven to produce thioacetals in good yields (Scheme 6.3). 

Substituted derivatives 219 were prepared from 1,2,3,4-tetrahydroiso-quinoline-l,4-dione 217 and sulfonyl ketene thioacetals 218. The same products were also obtained from 220 and 221 (Scheme 52) (79YZ1234). 

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. 

Preparation of Titanocene-Alkylidenes from Thioacetals and their Application to Olefin Metathesis... 

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]. 

Nitrogen heterocycles can also be prepared by the RCM of unsaturated amines having a diphenyl thioacetal moiety (Table 14.3) [31]. 

Reactions of Titanium Carbene Complexes with Carbon—Carbon Double Bonds Table 14.1. Preparation of carbocyclic compounds from alkenyl thioacetals. 

Table 14.2. Preparation of cyclic ethers and sulfides from alkenyl thioacetals. ...

An interesting application of the cydization of alkenyl thioacetals is the stereoselective preparation of olefmic diols. Thus, oxidative cleavage of the silicon—carbon bond [32] in the ring-closed metathesis products, i.e. cyclic allylsilanes such as 35 and 36, affords (Z)-alk-2-ene-1,5-diols 37 and 38 (Scheme 14.18) [33],... 

An unusual reductive elimination can ensue from titanacyclobutanes possessing an alkenyl group at the carbon a to the titanium atom. Thus, alkenylcarbene complexes 48, prepared by the desulfurization of (fy-unsaturated thioacetals 49 or l,3-bis(phe-nylthio)propene derivatives 50 with a titanocene(II) reagent, react with terminal olefins to produce alkenylcyclopropanes 51 (Scheme 14.22, Table 14.4) [37]. This facile reductive... 

Scheme 14.34. Stereoselective preparation of conjugated dienes by the titanocene(ll)-promoted reaction of thioacetals with alkynes.

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