Episulphonium ion

It is likely that this factor reflects a fundamental difference in the nature of the nucleophilic centres concerned. This difference is responsible also for the wide differences in the relative rates of formation of three and five-membered rings containing differing hetero-atoms noted by Bird and Stirling (1973). These authors showed that the formation of the episulphonium ion from 2-chloroethyl /7-tolyl sulphide (9) is actually faster than the formation of the corresponding five-membered ring from / -tolyl-S(CH2)4Cl. The EM for the... 

The intervention of episulphonium ion intermediates in the synthesis of azidosugars has been postulated by Christensen and Goodman in order to rationalize the product (140), formed on interaction of azide ion with the furanoside (141). Direct replacement with inversion at C(2) did not occur rather, a mixture of the diazides 140 2ind 142 was obtained by nucleophilic attack of azide ion on the episulphonium ion (143). A similar episulphonium intermediate (144) has been invoked to rationalize the conversion of methyl 4,6-0-benzylidene-... 

This reaction is employed in the ring closure, as in Eq. (55) [228], where XXI is anodically oxidized in methanol. It might be surprising that a five-membered ring is formed in preference to a six-membered ring an episulphonium ion is probably formed as an intermediate. 

The chemical and metabolic reactions of sulphur mustard are dominated by reactions with nucleophiles at its two electrophilic carbon atoms, plus oxidation of the electron-rich sulphur atom. Nucleophilic reactions proceed by an internal SNi type mechanism, via the episulphonium ion shown in Figure 2. 

Trichloroethylene is metabolized similarly and gives rise to dichlorovinyl cysteine. It has been found that S- 1, 2-dichlorovinyl)-L-cysteine (DCVC) and V-(2-chloroethyl)-DL-cysteine (CEC) (figure 7,20) are both nephrotoxic when administered to animals causing renal proximal tubular necrosis. S-(2-chloroethyl)-DL-cysteine does not require /3-lyase activation in order to be nephrotoxic, but can rearrange, possibly to a reactive episulphonium ion, by nucleophilic displacement of the chlorine atom. These compounds decrease the activity of the renal tubular anion and cation transport system. 

The Chemistry of Thiiranium Ions. - The chemistry of episulphonium ions and the reactions of alkenes with sulphenyl derivatives have been reviewed Stable thiiranium and thiirenium chlorides have been generated in S02. Kinetic studies for the quantitative generation of (329 R =R = Bu, X = Cl) from the ionization of (328) and the equilibrium between (330) and (331) in SO2 were followed, using n.m.r. spectrometry. Compounds (329 R = Bu, X = SbCls or BF4) have been isolated as stable salts at room temperature... 

New methods for the generation of episulphonium ions have been... 

Livinghouse has extended his studies on episulphonium ions (cf. Scheme 76) to include the corresponding episelenonium ions. The reagent of choice in this latter situation was... 

Two reviews of thiiranium ions have appeared. The main conclusion of reference Ab is that episulphonium ions are generally not involved in reactions unless their counter-ions are very poorly nucleophilic. Additional studies supporting this point have appeared. The existence of two AdE 2 mechanisms has been established, additional examples of the reactions of nucleophiles with pre-formed thiiranium ions are reported, and a new method for the preparation of thiiranium ions by the reaction of alkenes with aryl bis(arylthio)sulphonium ions (55) has been described. ... 

In aqueous solution the p-chloroethyl functional group of SM forms a reactive cyclic intermediate, an episulphonium ion (ESI), that subsequently reacts with a nucleophile to form an alkylated product. This is a nucleophilic substitution (SN) reaction. If the rate determining step in the reaction is unimolecu-lar these reactions are denoted as SNl, if bimolecular, SN2 (Scheme 2.1). 

HD is converted chemically from bis(2-chloroethyl)chloride (a simple substi-mted straight chain compound) by cycUsation into episulphonium ions which rapidly attack a wide range of chemical moieties such as thiols, amino and carboxylic groups. 

Acid-catalysed cyclization of 2-allyl- or 2-vinyl-thiopyridine or 2-j8-hydroxy-alkylthio-pyridines (91) proceeds without rearrangement, to give the corresponding dihydrothiazolo[3,2-a]pyridinium derivative (93). By contrast, 2-j8-hydroxyalkylthio-pyridines (91) react with cold thionyl chloride to give a mixture of isomeric dihydrothiazolo[3,2-a]pyridinium derivatives and isomeric 2-j8-hydroxyalkylthio-pyridines. These results are in part rationalized by reversible formation of the episulphonium ion (92). 

The ring-expansion of the 1,4-benzothiazine (379) to the 1,5-benzothiazepine (381) occurs in 88% yield, probably through the intermediacy of the episulphonium ion (380) (Scheme 85). ... 

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