Synthesis of Some Carbazolequinone Alkaloids and Their Analogues. Facile Palladium-Assisted Intramolecular Ring Closure of Arylamino-1,4- benzoquinones to Carbazole-1,4-quinones.

Carbazolequinone alkaloids, murrayaquinone-A (1a), and pyrayaquinone-A (2) and -B (3), as well as pyrano[3, 2-b]carbazolequinone 5, an isomer of 2 and 3, were prepared conveniently by the palladium-assisted intramolecular ring closure of the corresponding 2-arylamino-methyl-1, 4-benzoquinones 9a and 9i-k. A series of their analogues, 6-, 7-, and 8-substituted 3-methylcarbazole-1, 4-quinone derivatives 1b-g and 2-methylcarbazolequinones 6a-g corresponding to 1a-g, was also prepared in the same manner. Comparative analysis of the carbon-13 nuclear magnetic resonance spectra was found to provide useful information for the structural assignment of either 2- or 3-methylcarbazolequinones.