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 refl uxing toluene the azides 4a, b,d smoothly gave solely fused pyrrole pr oducts, 5a, b, d, whereas the azide 4c furnished an isomeric mixture o f the cyclized pyrroles 5c and 6. The observed 1H-pyrroles 5a-d and 6 are assumed to occur through aromatization of initially formed 2H-pyrr oles 7a-d. Attempted condensation of 2-azidobenzo[b]thiophene-3-carbal dehyde 2 with acetone or pentane-2,4-dione gave no vinylic product, bu t instead resulted in reduction of the azide 2 to the amine 8 or in it s conversion into the 1,2,3-triazole adduct 9, respectively. Azido gro up transfer of 3-styrylbenzo[b]thiophene with tosyl azide yielded the ortho-vinyl-substituted azide 10, but in poor yield. Thermolysis of th e azide 10 gave quantitatively the benzothiopyran 12 clearly ascribabl e to an initial ring-opening reaction. It is therefore inferred that, in the presence of ortho-vinyl substituent, an alpha-azidobenzo[b]thio phene can exhibit 'normal' ring-cleavage fragmentation, in contrast wi th previous deoxygenations of 2-nitro-3-vinylbenzo[b]thiophenes report ed to yield cyclized benzothieno[2,3b]pyrroles. Evidence is also prese nted that, upon thermolysis, the o-azidoaldehyde 1 cleanly affords a b enzothienoisoxazole product, whereas the isomeric azide 2 essentially leads to intractable material.,