Alkaloids of the Australian Rutaceae: Melicope fareana. V. The Structure of the Alkaloids

Abstract

Melicopine, melicopidine, and melicopicine are shown to be members of a new group of alkaloids derived from acridine. The structure of melicopicine, 1,2,3,4-tetramethoxy-10-methylacridone (II), is deduced from data reported in earlier papers. The presence of the same 10-methylacridone skeleton in melicopidine and melicopine is established by conversion of the trimethoxyphenols obtained from them by fission of the methylenedioxy ring with methanolic potash to the respective dimethoxy-o- and p-quinones previously prepared from melicopicine. This conversion also establishes the position of the methylenedioxy group in melicopine relative to the hydroxyl group in normelicopicine. Similar considerations applied to the ethoxydimethoxyphenols show the position of the methylenedioxy group in melicopidine relative to the hydroxyl group in normelicopicine, and in this case, lead to the complete structure for the alkaloid (XIII). The action of nitrous acid on normelicopine and normelicopidine gives two hydroxymethoxyquinones isomeric with that obtained by the action of sodium carbonate on the dimethoxy-o- and p-quinones. The same two hydroxymethoxyquinones also result from the action of caustic soda on the dimethoxy-o- and p-quinones respectively. Their structures, which can be deduced from the second method of preparation, confirm the positions of the methylenedioxy group in melicopine and melicopidine relative to the hydroxyl group of normelicopicine, and prove that the hydroxyl groups in normelicopine and normelicopidine are in the same position as that in normelicopicine, but they do not make possible a choice between alternative structures for melicopine. This choice depends on the position of the hydroxyl group of the noralkaloids relative to the remainder of the acridone molecule. By consideration of the mechanism of fission of the methylenedioxy ring, the hydroxyl group of the noralkaloids is shown to be situated peri to the acridone oxygen atom, i.e. at position 4. This is confirmed by the occurrence of hydrogen bonding. Consequently the complete structural formula for melicopine is XXIII. The properties of the alkaloids and a number of the degradation products are discussed.