Elimination reactions sulfolene dioxides

The most synthetically useful cheletropic elimination involves 2,5-dihydrothiophene-1,1-dioxides (sulfolene dioxides). At moderate temperatures they fragment to give dienes and sulfur dioxide.301 The reaction is stereospecific. For example, the dimethyl derivatives 22 and 23 give the E,E- and Z,E-isomers of 2,4-hexadiene, respectively, at temperatures of 100°-150°C.302 This stereospecificity corresponds to disrotatory elimination. 

The term "cheletropic reaction" defines reactions in which two a bonds to a single atom are made or broken conceitedly. A widely studied example, theoretically as well as experimentally, is the cheletropic elimination of sulfur dioxide from a cyclic sulfolene to generate stereochemically pure dienes or polyenes. ... 

Hydrogen shifts can also interfere in other reactions when the o-quinodimethane is generated thermally. An example is shown in Scheme 8 here the o-quinodimethane is generated by elimination of sulfur dioxide from a sulfolene, and a 1,51 hydrogen shift competes with the desired intramolecular Diels-Alder reaction <93JOC493>. 

Sol 1. (b) Selenites on oxidation with hydrogen peroxide (or ozone or m-CPBA) give selenoxides. The latter in the presence of a p-hydrogen undergoes an intramolecular syn elimination to leave behind an alkene and selenenic acid. The diene component is generated in situ by thermal sulfur dioxide extmsion from sulfolene in a reverse reaction and is then trapped by the dienophile in a Diels—Alder reaction. 

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