Sulfenate ester trapping

Deprotonation of allylic aryl sulfoxides leads to allylic carbanions which react with aldehyde electrophiles at the carbon atom a and also y to sulfur . With benzaldehyde at — 10 °C y-alkylation predominates , whereas with aliphatic aldehydes at — 78 °C in the presence of HMPA a-alkylation predominates . When the a-alkylated products, which themselves are allylic sulfoxides, undergo 2,3-sigmatropic rearrangement, the rearranged compounds (i.e., allylic sulfenate esters) can be trapped with thiophiles to produce overall ( )-l,4-dihydroxyalkenes (equation 24). When a-substituted aldehydes are used as electrophiles, formation of syn-diols 27 occurs in 40-67% yields with diastereoselectivities ranging from 2-28 1 (equation 24) . ... 

Both Chou (1974) and Davis and Friedman (1976) have shown that sulfenic acids can also be trapped effectively by (CH3)3SiCl to afford trimethylsilyl sulfenate esters, RSOSi(CH3)3. 

This is an unfavourable reaction, because the equilibrium lies over on the sulfoxide side. But the nucleophile traps the sulfenate ester and the methanol ensures that the alkoxide ion formed is immediately protonated so that we get another allylic alcohol. 

The product obtained is an allylic alcohol with the hydroxyl group at the other end of the allyl system from where the sulfur started—-a rearrangement has taken place. We have observed the rearrangement in this case because the P(OMe)3 has trapped the rearrangement product but, even without this reagent, allylic sulfoxides are continually and reversibly rearranging into sulfenate esters by the mechanism shown below. 

Thiophilic agents, such as trimethyl phosphite, phosphorous triamides, secondary amides (piperidine, diethylamine) or anionic thioderivatives (thiophenoxide, sodium sulfide) are commonly used2. They differ in their efficiency of trapping the sulfenate ester in the following order1 ... 

Sultones undergo photolytic ring cleavage and the intermediate sulfenes are trapped by methanol to form esters , viz. 

Even though the sulfenate ester is usually in low equilibrium concentration with the isomeric sulfoxide, it can be trapped by thiophiles like trimethyl phosphite, leading eventually to complete formation of the alcohol. 

These allylic sulfoxides 41 are in equilibrium with a sulfenate ester 43 by a [2,3]-sigmatropic rearrangement 41a. It is not usually possible to detect the sulfenate ester by NMR so there must be less than about 3% of it, but it can be trapped by various nucleophiles that like to attack sulfur. These thiophiles include secondary amines, thiolate anions and, most important, phosphite esters. The reaction is carried out in a protic solvent (usually the alcohol already present in the phosphite ester) and a rearranged allylic alcohol 45 is formed. 

Lithiated allyl sulfide 256-Li reacted with m-xylylene dibromide 257 to afford the bis-sulfide 258. A double Mislow-Evans rearrangement of 258 and subsequent reductive trapping of the sulfenate ester provided the symmetric trans-diol 255. Monoprotection, Sharpless asymmetric epoxidation, and reductive ring-opening of the epoxide gave 1,3-diol 259 in 97% ee. After several functionalizations, the arene was subjected to Birch reduction to provide 1,4-cyclohexadiene 254. Ozonolysis of the diene, followed by reductive workup and treatment of the resulting 1,3-diketone with acid furnished pyra-... 

The formation of cyclic sulfinic esters (sultines) from vinyl sulfenes is known , and the trapping of the expected intermediate vinyl sulfene in the thermolysis of thiete dioxide (6fc and 194) has been convincingly achieved . Specifically, thermolysis of thiete dioxide 6b in the presence of norbornenes gave cycloadducts of the Diels-Alder type (i.e. 252b), resulting from the trapping of the vinyl sulfene formed. The accumulated evidence thus supports the proposed mechanism for these thermolytic reactions. 

Photolysis of 3-thietanone involves both singlet and triplet excited states the initial products are sulfene and ketene, as determined by IR spectroscopy at liquid nitrogen temperatures in a pentane matrix.When the photolysis is done in methanol, isopropyl alcohol, or r-butyl alcohol, these intermediates are trapped as the acetate and methanesulfonate esters. In diphenylmethanol solvent, the bis-diphenylmethyl ether is formed by displacement of the alcohol on the sulfonate ester. Photolysis of tetramethyl-3-thietanone 1,1-dioxide gives dimethylketene. ... 

Sulfenic acids, esters and halides are reduced to thiols by many reagents, e.g. hydrazine, lithium aluminium hydride and metallic sodium. These addition reactions are extensively employed for trapping sulfenic acids, e.g. with ethyl acrylate (57) to give the adduct (58) (Scheme 32). 

An alternate method of preparing the azetidinone sulfenic acid was developed by initially trapping the sulfenic acid as its trimethylsilyl ester (32) (Chou et al., 1976). Careful cleavage of the trimethylsilyl protecting group with methanol afforded the sulfenic acid (31) (Chou et al., 1974). 

The trapping of the penicillin sulfenic acid with dihydropyran has been previously discussed (Cooper and Spry, 1972). Further examples of trapping with vinyl ethers have also been disclosed (Ager et al., 1972 1973). The sulfoxide (106) reacted with isobutyl vinyl ether resulting predominantly in unstable vinyl ether (139). On treatment with a catalytic amount of p-toluenesulfonic acid in benzene, 139 afforded the dihydrothiazine (140). The reaction of 139 with methanolic hydrogen chloride resulted in a smooth transformation to the acetal (141). When 1,1-diethoxyethy-lene was used, the corresponding vinyl ether was not observed. Instead, the preferred course of the reaction was an in situ hydrolysis to the ester (142). 

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