An abnormal Barton–Zard reaction leading to the pyrrolo[2,3-b]indole ring system

The reaction of 3-nitro-N-(phenylsulfonyl)indole 1 with ethyl isocyanoacetate 2 under the Barton–Zard pyrrole synthesis conditions gives ethyl 8-(phenylsulfonyl)-1,8-dihydropyrrolo[2,3-b]indole-2-carboxylate 3 rather than the anticipated ethyl 4-(phenylsulfonyl)-2,4-dihydropyrrolo[3,4-b]indole-3-carboxylate 4.