Thioacetal exchange

Thioacetyl derivatives (155) are obtained by direct heterocyclization reactions (365. 378, 563) and by a sulfur-oxygen exchange" reaction involving thioacetic acid and A-2-oxazoline-5-one (154) or A-2-thiazoline-5-one (156) (Scheme 81) (365, 378, 379). Ra-Ni reduction of 155 affords the 5-unsubstituted thiazole (379). 

A convenient two step transformation of aUcyl halides or tosylates to thiols has also been reported (Scheme 2.26) [47]. The initial displacement of the leaving group with an thioacetate immobilized on a ion exchange resin afforded the inter-... 

Potassium nitrosodisulfonate, 258 Trimethylsilyl chlorochromate, 327 By hydrolysis of acetals or thioacetals Amberlyst ion-exchange resin, 152 Methylthiomethyl p-tolyl sulfone, 192 By isomerization of allylic alcohols N-Lithioethylenediamine, 157 By oxidation of aromatic side chains Trimethylsilyl chlorochromate, 327 From oxidative cleavage of alkenes [Bis(salicylidene-7-iminopropyl)-methylamine]cobalt(II)... 

The hydroxyl group of (+)-32 was protected as a methoxymethyl ether to give ether (+)-63 in 80% yield (Scheme 12). To reduce the carbonyl group of (+)-63 to a methylene moiety of (+)-66 (Scheme 13), we tried the thioacetal reduction method. However, the attempt was unsuccessful because of the exchange of the acetal group at the 1-position by a thioacetal group. Although we also tried the... 

It is unnecessary to furnish examples for such double exchange. We may also bypass the further development of views concerning decomposition or combination through the introduction of monatomic in place of polyatomic radicals or vice versa, since these views have been extended by Gerhardt to all classes of compounds in the manner in which I reported them in connection with thioacetic acid. 

Corey s method20 relies on metal exchange with the bromocyclopropane 69 prepared by carbene addition. The extra stabilisation of cyclopropyl anions (chapter 8) makes both this lithium derivative and the ylid 63 more easily handled. Addition to aldehydes or ketones gives mixtures of adducts 70 [it turns out that none of the stereochemistry of 69 or 70 matters] which fragment under Lewis acid catalysis to give the thioacetal 71. Careful hydrolysis releases the 3,4-enal -72, the product of a homoaldol reaction with an aldehyde homoenolate and RCHO and a difficult compound to make as the double bond moves into conjugation very easily. 

Abstract This chapter emphasises on the important aspects of steric and stereo-electronic effects and their control on the conformational and reactivity profiles. The conformational effects in ethane, butane, cyclohexane, variously substituted cyclohexanes, and cis- and tra/ ,v-decalin systems allow a thorough understanding. Application of these effects to E2 and ElcB reactions followed by anomeric effect and mutarotation is discussed. The conformational effects in acetal-forming processes and their reactivity profile, carbonyl oxygen exchange in esters, and hydrolysis of orthoesters have been discussed. The application of anomeric effect in 1,4-elimination reactions, including the preservation of the geometry of the newly created double bond, is elaborated. Finally, a brief discussion on the conformational profile of thioacetals and azaacetals is presented. 

Deacetalization and dethioacetalization. Dimethyl acetals, 4-p-methoxy-phenyl-l,3-dioxolanes are cleaved withDDQ in the presence of water. For deprotection of thioacetals under mild conditions photochemical assistance seems advantageous. The same reaction principle of deacetalization can be extended to ether exchange. Thus alcohol protection is possible by mixing with 2,2-dimethoxypropane in the presence of DDQ, and proximal diols are converted to acetonides. Replacement of anomeric arylmethoxyl groups by this method complements other glycosylation procedures. 

Through a similar ligand-exchange reaction between Sr(CH3COO)2 and thioacetic acid, the acetato/thioacetato Sr complex 13 is obtained [16]. In the presence of 15-crown-5 ether, the reaction of SrC03 with thioacetic acid yields a polyether Sr complex with both acetato and thioacetato ligands [194]. 

Condensation of 2-mercaptoethanol or 3-mercaptopropanoI with ketones is usually achieved with the aid of an acid catalyst. Hydrogen chloride has been used but more common agents are boron trifluoride , freshly fused zinc chloride or p-toluenesulphonic acid . An exchange method between 2,2-dimethyl-1,3-oxathiolane or 2,2-dimethyl-1,3-oxathiane and a non-volatile ketone leads to formation of the new mono-thioacetal and acetone . The equilibrium is displaced by continuous distillation of the acetone formed (equation 71). With saturated ketones, mostly steroids, the yields of the above methods are comparable and are usually in the 60-90% range. With a,j3-unsaturated ketones, the yields were significantly lower . 

DCLs to date (a) transesterification, (i) thioacetal exchange, (j) pyrazolotriazone... 

Two exchange processes that are related to acetal exchange have been studied under reversible conditions (Scheme 3.2). The reversible formation of thioacetals has been explored by Hirsch et al. to generate equilibritun mixtures of bis- and tris-(thioacetals) [46]. The condensation of benzene hexathiol with p-tolualdehyde went to completion in 3 days in refluxing CHCI3 in the presence of the Lewis acid Zn(OTf)2. The products could be isolated and are stable in the absence of catalyst. 

Miscellaneous a-Thiocarbanions. l-Lithio-l-(phenylthio)cyclopropane, which can be prepared by lithium-sulphur exchange or by reductive lithiation of cyclo-propanone thioacetals, or by lithiation followed by cyclization of 1,3-bis-(phenylthio)propane, has been used in the generation of cyclobutanone, a tetrahydrochromanone, alkylidenecyclopropanes, ° and various ketones and alcohols. ... 

Coatings, cation-exchange resins Vinyl thioacetate Britain 585,755 1947 Imperial Chemical... 

The C-4 carbon in phthiocerol A has been assigned the 5-configuration. Acetals and Thioacetals.—A catalytic dehydrator for rapid acetal synthesis has been described a combination of an acid ion-exchange resin and a drying agent is used. Ingenious use is made of molecular sieves in a related... 

While orthoesters do not react with trifluoroacetic anhydride, aryltrithio-orthoacetates (ArS)3CMe give keten thioacetals (ArS)2C=CHC(0)CF3, by way of the parent keten dithioacetal (ArS)aC=CH2 acetic anhydride does not react. Aryl trithio-orthoacetates readily exchange with CFsCOa H as a consequence of the reversible loss of ArSH, and this can be exploited in the synthesis of deuteriated hydrocarbons, e.g. (ArS)2CArC H3 -> ArCHaC Hs (after treatment with Raney nickel). ... 

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