Diene with thiophenes, 2,5-dihydro

In a neat manipulation of oxidation levels, the reaction of a 1,4-dihydro-1,2-diaroylbenzenes, such as are available from Diels-Alder additions of buta-1,3-dienes with 1,2-diaroylalkynes, with a sulfur source, produces benzo[c]thiophenes note that no reductant is required as would be necessary if a 1,2-aroylbenzene were utilised. These same Diels-Alder adducts react with primary amines to give 2-substituted isoindoles. 

In an ingenious application of the extrusion reaction, 1-alkenyl-1,3-dihydro-benzo[c]thiophene 2,2-dioxides have been thermolyzed the diene system so generated undergoes an intramolecular Diels-Alder reaction with the isolated double bond in the side chain (79HCA2017). An (E) configuration of the diene is necessary for this purpose (Scheme 233). The by-products are styrenes arising from the (Z)-isomer. The same approach has also been used to prepare a steroid derivative (Scheme 233) (80JOC1463). 

Dihydro thiophenes 1,3-dienes thiophenes. The salt reacts with a-mercap-to ketones to give dehydrothiophenes (1) in moderate yields (about 50-75%). 

In the reactions with cyclic alkenes, such as benzo-fused 2,5-dihydrothiophenes, 2,5-dihydrofurans, 2,5-dihydro-pyrroles, and bicyclo[2.2.2]octa-2,5-diene, the initially formed fused dihydropyridazines may be unstable and undergo cycloreversion reaction via elimination of the pyridazine derivative to give the corresponding thiophenes, furans, pyrroles, fulvalenes, and benzenes (Scheme 41) <1984CHEC(3)531, 1996CHEC-II(6)901>. 

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