Sulfonium intermediate

Thioglycosides can be activated for gfycosylation reactions with sulfur electrophiles, e.g., with dimethyl(methylthio)sulfonium triflate or with methanesulfenyi bromide and silver(l +) to form reactive sulfonium intermediates (F. Dasgupta, 1988). 

In the alcohol oxidations, the sulfonium intermediate (2, nucleophile = R2C(OH)) loses a proton and dimethyl sulfide to give the carbonyl compound (42). The most common mechanism for the decomposition of (2) is attack by a mild base to remove a proton from one of the methyl groups. Subsequent cycHc coUapse leads to the carbonyl compound and dimethyl sulfide (eq. 9) ... 

A type I one-electron photo-oxidation of methionine-methionine-containing peptides by triplet carboxybenzophenone in air-saturated aqueous solution has been reported the S+ radical cation that is formed then reacts with the other Met-S to form an S-S three-electron complex which reacts with superoxide radical anion before hydrolysis to Met(=0)-Met(=0) bis-sulfoxide. Alternatively, cyclization of the A-terminal NH2 on to the S can occur to give a three-electron S-N complex which can react with superoxide radical anion to give a cyclic sulfonium intermediate. 

Strong transannular sulfur participation is seen in the solvolysis of the p-nitrobenzoate of tetrahydrothiophene-2-methanol (574) the rate is 5.4x 107 times that of the corresponding cyclohexyl derivative. The product is a mixture of (577) and (578), arising through the episulfonium ion (576) (74T2087). The same mixture is also obtained under identical conditions from (575). Even more dramatic is the acetolysis of 2-endo-ch oxo-l -thiabicyclo[2.2.1]heptane, which is accelerated by a factor of 4.7 x 109 compared with the exo-chloro compound. The endo-chloro derivative leads stereospecifically to the endo-product via the sulfonium intermediate (Scheme 239) (75JA2886). 

The presence of an equatorially substituted cyclic sulfonium intermediate 7 that controls the stereoselectivity of glycosylation was confirmed by nuclear magnetic resonance (NMR) spectroscopy. Combined use of this methodology to introduce a-glycosides and traditional /3-glycoside protocols provides a potential route to one-pot multistep glycosylations and automated syntheses for a wide variety of oligosaccharides. 

Optically pure (+)-(i )-3-phenylsulfanyl-l,3-diarylpropan-l-ones are readily available by the enantioselective Michael addition of thiophenols to chalcones. After reduction, acidic dehydration of the racemic alcohol affords a mixture of the racemic cis- and trans- 2,4-disubstituted thiochromans (Scheme 164). A detailed consideration of the stereochemical outcome of the reaction with unsymmetrically substituted diaryl derivatives suggests the involvement of a [1,3] PhS shift via a four-membered sulfonium intermediate and this is backed up by theoretical calculations <2003T3621>. 

The generally accepted mechanism6 of Pummerer rearrangement is the one in which there is an initial attack on the sulfoxide oxygen atom by an electrophilic species, e.g., protonation or acylation. Acylation is followed by proton abstraction by a base from the a-carbon atom of the sulfoxide to form an ylide, which rapidly eliminates an acetate anion to form the a-sulfonium salt. Addition of acetate anion to the sulfonium intermediate completes the formation of the a-functionalized sulfide. Ylide formation from sulfoxonium salts is well recognized, and this aspect... 

Polymer-bound thiol was reacted with the complex of NCS and dimethylsulfide to afford 1,2-dithionane through spontaneous cyclization of the dimethyl(thio)sulfonium intermediate 224 (Scheme 46) <2000TL9989>. 

We also observed similar phenomena in the reaction of silyl enol ethers with cation radicals derived from allylic sulfides. For example, oxidation of allyl phenyl sulfide (3) with ammonium hexanitratocerate (CAN) in the presence of silyl enol ether 4 gave a-phenylthio-Y,5-un-saturated ketone 5. In this reaction, silyl enol ether 4 reacts with cation radical of allyl phenyl sulfide CR3 to give sulfonium intermediate C3, and successive deprotonation and [2,3]-Wittig rearrangement affords a-phenylthio-Y,6-unsaturated ketone 5 (Scheme 2). Direct carbon-carbon bond formation is so difficult that nucleophiles attack the heteroatom of the cation radicals. 

Reaction involving the formation of a sulfonium intermediate (80) followed by a sigmatropic rearrangement was formulated by Trost and Biddlecom (Scheme 8) and cyclization initiated by an allylic copper carbenoid (81) was proposed by Cohen and coworkers (Scheme 9). The common feature of all of these proposed mechanisms relies on the electrophilic addition of the donor to the acceptor prenyl pyrophosphate and is... 

The roles of the various reagents are as follows [1023] The activator converts dimethyl sulfoxide into a reactive sulfonium intermediate, which reacts with the alcohol and forms a complex A. A base, usually triethyl-amine, abstracts a proton from one of the methyl groups linked to sulfur. An intramolecular shift of electrons causes the disintegration of the com-... 

Scheme 2.154 Synthesis of trifluoromethyldibenzothiophenium and -selenophenium triflates (X = S, Se) and their subsequent nitration [28b]. On the extreme left and the extreme right, the reactive sulfonium intermediates for both synthesis routes are depicted undergoing ring-closure by intramolecular electrophilic substitution.

Sulfoxides containing a methylsulfinyl group react with organomagnesium halides to give sulfides. 24,125 The reaction was suggested to involve deprotonation, formation of a methylydene-sulfonium intermediate and addition of the alkyl Grignard on the a-carbon.l25... 

Tris(4-bromophenyl)ammonium hexachloroantimonate (TBPA) is unique among the promoters in that its cation is a radical, and it produces radical-cation sulfonium intermediates in glycosylation reactions. 

The reaction of the sulfonium intermediate with alcohols leads to their protection as hemithioacetals. Treatment of thioglyco-sides with MeOTf gives an efficient glycosylating agent, and p3TUvic acetal formation from a pyruvyl thioacetal has heen achieved in a reaction catalyzed by MeOTf (amongst other electrophiles). ... 

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