Sulfenic acid elimination

Sulfoxides containing P-hydiogen atoms, eg, di-Abutylsulfoxide [2211 -92-9] react with strongly basic systems, eg, potassium /-butoxide, in DMSO by sulfenic acid elimination to produce olefins (eq.l2) (44) ... 

In other cases, sulfenic acid elimination can involve y-hydrogen atoms with the formation of cyclopropane derivatives. y-Klimination is favored when DMSO is the reaction solvent. An example involving l-methylsulfinyl-2-ethyl-3-phenyl propane [14198-15-3] is shown in equation 13 (45) ... 

Reactions of 112 (R = H) with divinyl benzenes and naphthalenes have allowed the enantioselective synthesis of helicenebisquinones in high optical purity (80-88% ee) [118b]. In Scheme 66 is also depicted the results obtained from 1,4-divinyl naphthalene. The formation of (M)-(-)-133 involves a sequential process consisting of the cycloaddition of 112, pyrolytic sulfenic acid elimination, and final aromatization, in each of the two dienic moieties of the starting naphthalene. 

As already mentioned, vinyl sulfides are electron-rich dienophiles and react, therefore, preferentially with electron-poor dienes, i.e. in inverse-electron-demand processes such as those depicted in Scheme The auxiliary sulfur substituent may be removed at some stage after the cycloaddition, either by hydrogenolysis (e.g. -+ 81) or oxidation/sulfenic acid elimination (e.g. - 84), illustrating the potential of vinyl sulfides as ethylene or acetylene equivalents. 

The most common procedure previously employed to effect the elimination of thiols from thioacetals has been heating in the presence of a protic acid. For example, propionaldehyde diethyl thioacetal is converted to 1-ethylthio-l-propene on heating at 175° in the presence of phosphoric acid. The relatively high temperature and acidic conditions of such procedures are, however, distinct disadvantages of this method. Another approach consists of oxidation of a thioacetal to the mono S-oxide and thermal elimination of a sulfenic acid at 140-150°. ... 

The thermolysis of acyclic- and/or six- and larger ring sulfoxides to yield olefins and sulfenic acids is well documented . The formation of allylic sulfenic acids and thiosulfinates in the thermolysis of thiirane oxides containing hydrogen on the a-carbon of the ring substituent (which is syn to the S—O bond) has been discussed previously in terms of /i-elimination of hydrogen, which is facilitated by relief of strain in the three-membered ring (Section llI.C.l). 

Direct elimination of toluene-/>-sulfenic acid (/>-TolSOH) from 210 occurred during its purification using column chromatography on silica gel, to give the unsaturated lactam 211 (Scheme 26) <1990CL1547>. 

A range of other sulfur(IV) and (VI) derivatives formally obtained by the oxidation of thioglycosides have been prepared, but they are apparently not employed to date in O-glycosylation reactions. These include glycosyl sulfenamides and sulfonamides, as well as sulfinates and sulfonates [289]. S-Glycosyl sulfenic acids have been prepared as transients by the syn-elimination of S-(2-cyanoethyl) glycosyl sulfoxides [311]. 

In recent years, besides the thermolysis of sulfoxides, several other E,-type eliminations have been discovered that also result in the generation of sulfenic acids. Block (1972a) showed that alkyl thiolsulfinates rather easily undergo thermolysis in the manner shown in (7) to generate sulfenic acids. These could... 

JOC4620). The reaction is considered to be initiated by protonation at the nitrogen atom of the benzimidazole ring. The sulfenic acid is eliminated... 

Taking advantage of a tandem sulfoxide elimination-sulfenic acid addition approach to cyclic sulfoxides <1977J(P1)1574>, the synthesis of a number of novel 1,4-oxathiane oxides 229 and 230 based on the intramolecular addition of sulfenic acids to alkenes or alkynes tethered through an ether linkage has been reported (Equation 38) <20050BC404>. 

Saito et al. <1995S87> described a new method for the synthesis of heterocycle-fused[c]thiophenes via reaction of aryl heteroaryl thioketones with the carbene precursors. Heteroaromatic thioketones A react with carbenoids generated from bis(arylsulfonyl)diazomethanes or phenyliodonium bis(phenylsulfonyl)methylides to give heterocycle-fused[f]thiophenes B. The reaction involves the ring closure of the intermediary thiocarbonyl ylides, followed by restorative aromatization via the elimination of a sulfenic acid (Equation 11). 

As a consequence of the conformational mobility of the thiepane 1-oxide ring (115) it was possible to form the necessary planar five-membered cyclic transition state for a thermal E elimination reaction (equation 19). The acyclic sulfenic acid intermediate was not isolated but rearranged to c/s-2-methylthiane 1-oxide by an intramolecular cyclization mechanism (75TL2235). 

An interesting procedure has been developed by Vat le to obtain C-5 branched-chain sugars [189]. It also takes advantage of the stereospecificity of the Claisen rearrangement of allyl (vinylsulfinyl) ether 158 derived from a 4,5-unsaturated sugar. An acrylic derivative 159 is formed in the process by elimination of sulfenic acid (Scheme 53). 

On treatment with weak base, 1,3-oxathiolane sulfoxide (80) was shown to undergo a reversible intramolecular ene reaction to sulfenic acid (81). Using stronger base, it was found that this pericyclic reaction cannot compete with elimination to enone (82), which then oxidizes or dimerizes to the observed products (83) and (84). The diastere-omeric sulfoxide (79) was found to undergo retro-ene elimination then dimerization to (85) or to (83) and (84) with even a weak base.85... 

The authors suggested a stepwise mechanism in which precomplexation of the sulfoxide to the lithium azaenolate would take place, thus allowing conjugate addition to follow. This notion was based on Posner s and Paquette s earlier work on conjugate additions of nucleophiles to 57. As before, thermal elimination of toluene sulfenic acid led to the conjugated products (not shown) in 92-93% yields for R2 = methyl.30 However, for R2 = H thermal elimination of arylsulfenic acid did not afford any dihydropyrrole product but rather led to the formation of pyrroles. Treating 58 or 59 with Raney nickel at low temperature led to the rapid formation of 60 and 61, respectively, in 82-86% yield. 

Reactions of optically pure sulfinyl enones with acyclic dienes have never been reported, perhaps as a result of the discouraging results obtained from racemic substrates [56]. The reaction of a-phenylsulfinyl a,/i-unsaturated ketones with butadiene and 2,3-dimethyl butadiene, under Lewis acid catalysis, yielded the cyclohexadienes resulting from spontaneous elimination of sulfenic acid (precluding any conclusion about the stereoselectivity of the reaction). Analogous results were found by Nishio et al. [57] from reactions of /i-phenylsulfinyl a,/l-unsaturated ketones with acyclic dienes. The bicyclic adducts derived from cyclopentadiene are more stable [57]. In this case, the reactions gave only the exo-sulfinyl adduct,but these stereochemical results were neither confirmed nor discussed in the paper. 

It is remarkable that elimination of sulfenic acid was not observed during the formation of unsaturated sulfinyl dithioesters. These results are now being extended to enantiopure starting materials, with diacetone glucose as a chiral source of the starting sulfoxides [203]. 

An interesting disulfide cleavage is performed with 7,7/-dithio-bis[l,3,6-trimethyllumazine] (276) in basic medium yielding the relatively stable silver 7-sulfenate (277) and thiol (278) (Scheme 44). The free 1,3-dimethyllumazine-7-sulfenic acid (279) has been prepared from 7-isopropylthio and 7-[2-(4-nitrophenyl)ethylthio]-l,3-dimethyllumazine first by oxidation to the corresponding sulfoxides (282, 283) then by a modified Cope elimination and dbu treatment, respectively (Scheme 44). 

In garlic, a simple sulfoxide elimination creates an unstable sulfenic acid. When we looked at sulfoxide eliminations before, we ignored the fate of the unstable sulfenic acid, but here it is important. It dimerizes with the formation of an S-S bond and the breaking of a weaker S-O bond. 

Another simple elimination reaction on the thiosulfinate ester makes another molecule of the sulfenic acid and a highly unstable unsaturated thioaldehyde, which promptly dimerizes to give a thioacetal found in garlic as a potent platelet aggregation inhibitor. 

In onions, things start much the same way but the initial amino acid is not quite the same. The skeleton is the same as that of the garlic compound but the double bond is conjugated with the sulfoxide. Elimination and dimerization of the sulfenic acid produce an isomeric thiosulfinate. 

Early investigations by Davis and co-workers demonstrated that arylsulfinimines 39 undergo thermo-elimination to produce sulfenic acids 99, key intermediates in biological transformations, when heated for 24 h at 77-110 °C. These thermolyti-cally generated sulfenic acids can be trapped as silyl sulfenates 101 or vinyl sulfoxides 103.67-69... 

Reaction of heteroaromatic thioketones 638 with the carbenoids, generated from phenyliodonium bis(phenylsul-fonyl)methane or bis(arylsulfonyl)diazomethanes in the presence of a copper acetylacetonate catalyst, affords heterocycle-fused [c]thiophenes 639. The reaction involves ring closure of the intermediary thiocarbonyl ylides and elimination of a sulfenic acid (Equation 33) <1995S87>. 

The sulfenic acids usually generated in this way are very unstable and react instantaneously with alkenes present in the reaction medium to form sulfoxides88,89. A number of reversible intramolecular cyclic eliminations of sulfenic acids from cyclic sulfoxides have been reported, for example, thiepane 1-oxide decomposes at 140 C with a half-life of ca. 28 hours to give cis-2-methyTtetrahydro-2//-thiopyran l-oxide (1), which is stable under these conditions for 6 days90. 

The closely related 27/-azirine-2-carboxylate esters 871 have been prepared in enantiomerically pure form via the base-induced elimination of sulfenic acid from nonracemic A-sulfmylaziridine 2-carboxylate esters 869 <1997JOC3796,... 

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