New pentacyclic ring systems: intramolecular cyclization of o,o′-disubstituted bibenzothiazoles

Efficient methods for the preparation of isomeric o,o′-diaminobibenzothiazoles (8a and 11a) and o,o′-diamino-2,2′-dimethylbibenzothiazoles (8b and 11b), potentially valuable building blocks for construction of hitherto unknown dithiazolo annulated pentacyclic heterocycles, have been developed. The dithiazolo annulated benzo[c]cinnolines 9a, 9b, and 12a were prepared from the corresponding diamines by oxidation with PhI(OAc)2 in good yield. The dithiazolo annulated carbazoles 13 and 14 were efficiently prepared from the corresponding diamines by thermal cyclization in H3PO4. The unusual course of reduction and product formation of o,o′-dinitrosubstituted bibenzothiazoles 6a and 6b with SnCl2 under acidic conditions was rationalized by DFT quantum-mechanical calculations. It was suggested that cyclic products are formed from dinitroso derivatives and open-shell species immediately following on a reduction path.