Antitumour polycyclic acridines. Part 1. Synthesis of 7H-pyrido- and 8H-quino-[4,3,2-kl]acridines by Graebe–Ullmann thermolysis of 9-(1,2,3-triazol-1-yl)acridines: application of differential scanning calorimetry to predict optimum cyclisation conditions

Abstract

The thermal decomposition of a series of acridines substituted in the 9-position with 1,2,3-triazol-1-yl, benzotriazol-1-yl and naphthotriazol-1-yl groups has been studied by differential scanning calorimetry. Whereas the monocyclic triazole 7a shows a discrete melting endotherm followed by a decomposition exotherm corresponding to formation of the 7H-pyrido[4,3,2-kl]acridine 8, in the benzotriazoles 10a–e and naphthotriazole 10f these processes coincide with a single sharp exothermic transition attributed to cyclisation to polycyclic acridines 11a–f, respectively. The optimum conditions for the preparative scale synthesis of polycyclic acridines from triazole precursors utilised boiling diphenyl ether as the decomposition medium. A benzotriazol-1-ylacridine 10e substituted in the peri position with a methyl group behaved anomalously: as well as affording the expected 8H-quino[4,3,2-kl]acridine 11e, cyclisation also led to radical mediated loss of the methyl group to form the unsubstituted 8H-quino[4,3,2-kl]acridine 11a and H-abstraction from the methyl group leading to the benzoazepinoacridine 12. Radical cyclisation of 9-(2-iodoanilino)acridine 16 also gave 8H-quino[4,3,2-kl]acridine 11a. The crystal structure of 11a confirms the 8H tautomer arrangement with intermolecular N8–H · · · N13 hydrogen bonding and exhibits a polycyclic system that is planar with rms deviation 0.044 Å.