2,3-Dimethylene-2,3-dihydrothiophene

Dimethylene-2,3-dihydrothiophene (37, Figure 2.3) is the thiophene analog [38] of o-quinodimethanes and has been used to develop a Diels-Alder-based synthetic approach to benzothiophene derivatives. Generated in situ by treating the trimethylsylyl ammonium derivatives 38 or 39 with Bu4N F , it... 

The fluoride ion-induced fragmentation of 372 is a mild and efficient way of generating 2,3-dimethylene-2,3-dihydrothiophene 373. In the presence of iV-phenylmaleimide it gives the adduct 374. In the absence of dienophile, 373 dimerizes to unstable spiro-compound 375 acting at the same time as diene and dienophile. 

Cycloaddition of Thiophene-2,3-Quinodimethanes (2,3-Dimethylene-2,3-dihydrothiophenes) and Related Compounds... 

Eliminations. When functionalized silanes in which a potential leaving group is attached to a /3-atom or to a vinylogously related atom are treated with TBAF, fragmentation ensues. New uses of this process are preparations of 2,3-dimethylene-2,3-dihydrothiophene," substituted 1,2,3-butatrienes, chiral allylic alcohols, and a-fluoroketones. The precursors for the allylic alcohols are the alkylation products (with aldehydes) of 2-(trimethylsilyl)ethyl sulfoxides, and those for the fluoroketones are 1-silyl-l-hydroxymethyloxiranes. 

Pyrolysis of the quaternary ammonium hydroxide (330) by azeotropic removal of water in toluene in the presence of phenothiazine gave a 19% yield of the dimer (331), probably via 2,5-dimethylene-2,5-dihydrothiophene (60JA1428). Extension of this process to a mixture... 

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>. 

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