Synthesis and thermal decomposition of azidovinylbenzo[b]thiophenes

Abstract

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 α-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 α-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[2,3-b]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.