Extrusion of sulfur dioxide, from

The methiodide of 2,5-dihydrothiophene (239) is transformed in high yield to Z)-l-(methylthio)buta-l,3-diene (240) on treatment with alkali (81AJC1017). The thermal cheletropic extrusion of sulfur dioxide from both cis and trans isomers of 2,5-dihy-drothiophene 1,1-dioxides is highly stereospecific. For example, c/5-2,5-dimethyl-2,5-dihydrothiophene 1,1-dioxide (241) yields ( , )-hexa-2,4-diene (242) and sulfur dioxide (75JA3666, 75JA3673). 

An a-halosulfone 1 reacts with a base by deprotonation at the a -position to give a carbanionic species 3. An intramolecular nucleophilic substitution reaction, with the halogen substituent taking the part of the leaving group, then leads to formation of an intermediate episulfone 4 and the halide anion. This mechanism is supported by the fact that the episulfone 4 could be isolated. Subsequent extrusion of sulfur dioxide from 4 yields the alkene 2 ... 

A corresponding extrusion of sulfur dioxide from disulfones has been reported486 (equation 98). Extrusions of sulfur have also been observed from thiolsulfinates yielding sulfones488,489. 

Ready extrusion of sulfur dioxide from fused thiirane dioxides is well known and was observed in the formation of pyrazoles from 19b and diazoalkanes6,179. A ring expansion... 

The thiophene ring system can be utilized as a synthetic scaffold for the preparation of nonthiophene materials as the sulfur moiety can be removed by reduction (desulfurization) or extrusion (loss of SO2). The extrusion of sulfur dioxide from 3-sulfolenes (2,5-dihydrothiophene 1,1-dioxides) give dienes (butadienes or o-quinodimethanes) that can be utilized to prepare six-membered rings by cycloaddition chemistry. For example, thermolysis of 3-sulfolene 120 provided tricyclic pyrazole 122 via an intramolecular cycloaddition of the o-quinodimethane 121 that results by extrusion of sulfur dioxide <00JOC5760>. Syntheses of 3-sulfolenes 123 and 124 <00S507> have recently been reported. 

A series of novel pyrroloxazines were prepared by thermal extrusion of sulfur dioxide from pyrrolo[l,2< [l,3]thi-azole 2,2-dioxides. The reaction on the acyl derivatives 369, carried out at 600 °C and 10 3mmHg, generated a transient azafulvenium methide 370 that electrocyclized to oxazines 371 <2001J(P1)1795>. The reaction was compatible with different aromatic and aliphatic groups <2000CC675>. 

A further example of a concerted thermal elimination reaction of a thiepane derivative was the formation of c/s-hexatriene and the extrusion of sulfur dioxide from heating 2,7-dihydrothiepin 1,1-dioxide (116) (67JA1281). That this reaction was under orbital symmetry control was deduced from the results obtained by heating cis- (117) and trans-... 

Extrusion of sulfur dioxide from l,3-thiazolidin-4-ones 1,1-dioxides (17) can be used in the synthesis of [3-lactams (83JOC494). 

In the 1,2-series, there are some conflicting reports concerning the extrusion of sulfur dioxide from 1,2-oxathiin 2,2-dioxides. In some instances the reaction proceeds smoothly to afford the corresponding furan, whereas in others pyrolysis merely produces an intractable... 

A novel synthesis of chroman-2,3-dicarboxylic acid derivatives has been performed by thermal extrusion of sulfur dioxide from benzosultones via o-quinone methides.27... 

Extrusion of sulfur dioxide from a ring-fused dihydiothiophene derivative provides an exceptionally easy method for the preparation of heterocyclic o-dimethylene compounds. These compounds are valuable intermediates in intermolecular Diels-Alder reactions. This extrusion method was used to prepare the o-dimethylene compounds 44-46 <95CC1349> as well as quinolinone derivative 47 <95TL5983>. TTie dihydrothiophene dioxide moiety also played an important role in the formation of the intramolecular Diels-Alder reaction of N-substituted pyrrole 48 <95CC807>. 

Another elegant application of pericyclic ring opening reactions for an in situ release of 1-aza-l,3-butadienes has been worked out by Wojciechowski by extrusion of sulfur dioxide from benzosultams [241,242]. 

Paquette and co-workers have reported on the facile cheletropic extrusion of sulfur dioxide from the heterocyclic annulated semibullvalene 134 which furnishes 135.206) This tetraene is seen to be a double-bond isomer of 1,6-dimethylpentalene. 

Oxidation of the cyclic sulfides to cyclic sulfones and subsequent photochemical extrusion of sulfur dioxide from suspensions of the sulfones in benzene [9] this method is only applicable for the synthesis of [2.2] phanes without photolabile functional groups. 

In a related study Gaioni generated the divinyl dichloride by cheletropic extrusion of sulfur dioxide from the dichlorocarbene/3-sulfolene adduct (49). Under the reaction conditions the divinyl dichloride solvolyzes to the cyclopentenone as outlined previously (equation 32). 

The first thiete fused to an aromatic system 219 (and an anthracene derivative) was reported in 1965 and was prepared by reduction of a naphthothiete sulfone. An attempt to prepare benzothiete by desulfurization of benzo-l,2-dithiolane was unsuccessful. Photolysis of benzo-l,2-dithiolane 1,1-dioxide in an attempt to extrude sulfur dioxide to give benzothiete also was unsuccessful. The naphthothiete 205, however, was prepared by extrusion of sulfur dioxide from 220 and nitrogen from 221. Thiete 205 also is obtained in 6-8% yield by treatment of 1,8-dehydronaphthalene with carbon disulfide. The unstable thiolactone 211 was formed by a photochemical extrusion of benzaldehyde from 222 and from 222a at 350°C. The benzothietes 209, 223, " and 224 have been... 

Extrusion of sulfur dioxide from the products resulting from the alkylation of tetrahydrothiophene 1,1-dioxides " and the 2-phenylthietane 1,1-dioxides, using n-butyllithium and lithium aluminum hydride - or photochemically leads to the synthesis of the corresponding cyclobutenes - and cyclopropanes, respectively (Scheme 107, entries a and b). 

Five membered ring sulfones (thiolene-1, 1-dioxides, sulfolenes or dihydrothiophene-1, 1-dioxides) can be obtained by peracid oxidation of tetrahydrothiophene. The extrusion of sulfure dioxide from dihydrothiophene-1,1 dioxides or sulfolenes like (178) has been much studied as it provides a synthetic route to dienes, e.g. butadiene (152) (Scheme 70). The reverse reaction provides a method of synthesis of sulfolene (178) from butadiene (152) and sulfur dioxide. Thiolene dioxides or sulfolenes can be generally prepared by addition of sulfur dioxide to conjugated dienes for example, 1,4-dimethylbutadiene (179) in the presence of sulfure dioxide, triethylamine and formic acid affords 2,5-dimethylsulfolene (180) (Scheme 71). 

Cycloaddition of thiophene dioxides with alkenic dienophiles often directly leads to the formation of a benzene ring. This can be attained by extrusion of sulfur dioxide from the initial adducts, followed by 1) elimination of a small molecule, 2) an intramolecular hydrogen transfer, and 3) (accidental) oxidative dehydrogenation under applied conditions. 

The extrusion of sulfur dioxide from heterocyclic sulfones is probably the most generally used method for the generation of urt/io-quinodimethanes and many examples have now been reported. Such sulfones are generally stable and easy to synthesise, by various routes. In addition, the acidity of the protons adjacent to the sulfone unit allows for base-promoted introduction of substituents, before thermolytic extrusion and the Diels-Alder step. Three examples of sulfur dioxide extrusion are shown below. " ... 

Extrusion of Sulfur Dioxide from Heterocyclic Sulfones... 

Extrusion of sulfur dioxide from oxidized thiophene derivatives is an exceptional method to prepare cis-dienes as components for Diels-Alder reactions. An example of this approach utilizes the Diels-Alder reactivity of the furan ring in substituted 4//,6ff-thieno[3,4-c]furan-3,S-dioxides to react with a variety of dienophiles such as DMAD, dimethyl maleate and dimethyl fumarate which then lose SO2 to form another reactive diene (Eq. 17) <94H961>. A review of the preparation and use of 4i/,6f/-thieno[3,4-c]furan-S,5-dioxides as well as other heteroaromatic-fused 3-suIfolenes is report <94H1417>. The preparation of dihydrothienooxazole 80 requires the careful control of the reaction time and temperature as well as the reactants molar ratio <94JOC2241>. Specific control of the alkylation conditions for 81 (X = COCH3) allows for the preparation of either 1,4-disubstituted or 1,6-disubstituted 4, 6//-thieno[3,4-c]furan-S,S-dioxides. These molecules could be used as intermediates for the preparation of novel pentacyclic compounds <94JCS(P1)1371>. 

Thermal extrusion of sulfur dioxide from 81 has been reported to give 2 methyi-S,6-dimethylenepyrimidone (82) which could be trapped with dieno-philes and nucleophiles [93M1347]. Sensitised photooxygenation of 83 affords the conesponding 5-hydroperoxy derivatives (94BCJ(67) 12041. 

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