SYNTHESIS AND THERMAL-DECOMPOSITION OF AZIDOVINYLBENZO[B]THIOPHENES

Condensation of 3-azidobenzo[b]thiophene-2-carbaldehyde 1 with acetone, diethyl malonate, ethyl acetoacetate and pentane-2,4-dione furnished the 3-azido-2-vinylic derivatives 4a-d in fairly good yields. In refluxing toluene the azides 4a, b,d smoothly gave solely fused pyrrole products, 5a, b, d, whereas the azide 4c furnished an isomeric mixture of the cyclized pyrroles 5c and 6. The observed 1H-pyrroles 5a-d and 6 are assumed to occur through aromatization of initially formed 2H-pyrroles 7a-d. Attempted condensation of 2-azidobenzo[b]thiophene-3-carbaldehyde 2 with acetone or pentane-2,4-dione gave no vinylic product, but instead resulted in reduction of the azide 2 to the amine 8 or in its conversion into the 1,2,3-triazole adduct 9, respectively. Azido group transfer of 3-styrylbenzo[b]thiophene with tosyl azide yielded the ortho-vinyl-substituted azide 10, but in poor yield. Thermolysis of the azide 10 gave quantitatively the benzothiopyran 12 clearly ascribable to an initial ring-opening reaction. It is therefore inferred that, in the presence of ortho-vinyl substituent, an alpha-azidobenzo[b]thiophene can exhibit 'normal' ring-cleavage fragmentation, in contrast with previous deoxygenations of 2-nitro-3-vinylbenzo[b]thiophenes reported to yield cyclized benzothieno[2,3b]pyrroles. Evidence is also presented that, upon thermolysis, the o-azidoaldehyde 1 cleanly affords a benzothienoisoxazole product, whereas the isomeric azide 2 essentially leads to intractable material.,