The synthesis of pyrrolo[1,2-b][1,2,5]benzothiadiazepines from 1,2-thiazine 1-oxides—sulfonamide analogues of the pyrrolobenzodiazepine antitumour natural products

Pyrrolo[2,1-c][1,4]benzodiazepines (PBDs) and the corresponding pyrrolobenzothiadiazepines (PBTDs) are attractive targets as natural and synthetic antitumour antibiotics and as non-nucleosidic reverse transcriptase inhibitors. A concise synthesis of the PBTD class is presented, which starts from o-azidobenzenesulfonamide and its conversion into 2-(o-azidobenzenesulfonyl)-1,2-thiazine 1-oxides via Diels–Alder reaction. After a one-pot ring contraction, desulfurisation and aromatisation process, accompanied by concomitant same pot conversion of the azide group into a primary amine via the Staudinger reaction, these 1,2-thiazine-1-oxides yield a 1-(o-aminobenzenesulfonyl)pyrrole. N-Formylation of the amine and Bischler–Napieralski ring closure onto the pyrrole completes the PBTD synthesis.