Unusual formation of β-carboline dimers under Bischler–Napieralski reaction conditions: an old reaction with a new direction

l-N-Formyl tryptophan methyl ester (3) underwent a Bischler–Napieralski reaction with POCl3 at room temperature or under microwave irradiation, resulting in the unusual formation of β-carboline dimers 5 and 6. Most importantly, acetylation using Ac2O of each of the dimers 5 and 6 separately afforded 1-[3′-carbomethoxy-β-carbolinyl]-3-carbomethoxy-9-acetyl-β-carboline (7) as the only product, the structure of which was confirmed by X-ray crystallography studies.