Thioacetals from, mechanism

However, the switchover from an A2 to an A1 hydrolysis is a very common mechanistic pathway in strong acid media, probably more common than the pure A2 mechanism. Excess acidity analyses have shown that thioacetic acid, several thiobenzoic acids, and many thiolbenzoate and thionbenzoate esters show this sort of mechanism switch.179 Acylals and thioacylals also show this behavior,116 with thioacylals using two water molecules and acylals one. Many hydroxamic acids react this way,127,216 as do esters of various types,41,217,218 episulfoxides219 and aryloxatriazoles.220 Acylhydrazines can also show a mechanism switch of this sort, although with these substrates the situation is somewhat more complex.221... 

A study of the species present in these solutions and the mechanism of the deposition has been presented [71]. Under the conditions of the depositions, the main solution indium species (in the absence of thioacetamide) are In-Cl (mainly [InCU] ) complex species. Only ca. 1% of the total In content is present as free In. No ln(OH)3 or hydroxy-complexes were calculated to be present if acetic acid was present (in the absence of acetic acid, the hydroxide could form). From a kinetic analysis of the deposition reaction, it was concluded that the deposition occurred by direct reaction between the thioacetamide and the chloro-indium complexes. It was noted that thioacetic acid was the main by-product and that no acetamide was detected (see 8ec. 3.2.1.3 for a description of the possible mechanisms and by-products of thioacetamide hydrolysis). Acetonitrile (CH3CN), a less common by-product, was also detected at the higher pH values (these depositions took place between a pH of 2 and 3) but not at the lower ones. 

Merckl6 et al. <2001JOC5058> proposed a mechanism for the formation of taxol-thietanes 57 and 33 from spiro taxol-oxiranes 118 and 122. According to these authors, the reaction starts with the attack of the sulfur atom from a thioacetate anion on an oxirane carbon atom to form the intermediate A. Then, an acetyl group migrates from sulfur to oxygen to form the intermediate B, and the X group leaves with simultaneous formation of a fused thietane product G (Scheme 19). 

Thioacetals readily eliminate to vinylsnlfides at room temperature in the presence copper(I) triflate [46]. Acyclic (75) and cyclic (77) substrates react equally well (Sch. 20). The a-ketothioacetal 79 furnished furan 80, presumably via a carbocation mechanism, and the diene 82 resulted from sequential elimination of two sulfides from 81. 

Homolysis of the S-acyl bond is responsible for formation of phenylthio-cyclohexane and 3-phenylthiocyclohexene from. SiS -diphenyl dithiocarbonate by irradiation in cyclohexene.117 A cyclic mechanism, however, has been proposed to account for the photodecomposition of certain carbohydrate dithiobisthio-formates.118 On irradiation, 0-ethyl thioacetate is converted into cis- and / 5-2,3-diethoxybut-2-ene, together with small amounts of 2,3-diethoxybut-l-ene, 2,3-diethoxybuta-1,3-diene, and 1-ethoxyethane-l-thiol.119 121 Free sulphur was trapped by reaction with 1,2-dimethyIenecyclohexane. Details of the photoelimination of sulphur dioxide from D-glucofuranosyl phenyl sulphone acetates have been published.180 An efficient synthesis of another 1,8-bridged naphthalene... 

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