Quinodimethanes from benzo thiophene dioxides

The synthetic utility of o-quinodimethane generated by cheletropic elimination of S02 has been amply demonstrated by Oppolzer and Nicolaou, who have conducted an intramolecular cycloaddition coupled with the alkylation of 1,3-dihydrobenzo[c]thiophene 2,2-dioxide122. When 1,3-dihydro-l-(4-pentenyl)benzo[c]thiophene 2,2-dioxide (201) prepared from 1,3-dihydrobenzo[c]thiophene 2,2-dioxide and 4-pentenyl bromide is heated in di-n-butyl... 

The pyranothiophenes (577), prepared in reasonable yields from commerically available thio-phene-3-acetic acid in a single step, are stable thiophene-2,3-quinodimethane equivalents <88TL5817, 90JCS(PI)68I>. Thus, they undergo Diels-Alder reaction with DMAD to give benzo[6]thiophenes (578) after loss of carhon dioxide, and with ethyl propiolate to give isomeric mixtures of (57 ) and... 

Rhodium-catalyzed intramolecular insertion in aromatic C-H bonds offers an interesting entry to the preparation of benzo[c] thiophene-2-dioxides [86] which are precursors of o-quinodimethane. The intramolecular trapping of the latter molecule, generated thermally from sulfones was the key step in a synthesis of (+)-estradiol [86]. 

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