Synthesis of Pyrrolo[4,3,2-de]quinolines from 6,7-Dimethoxy-4-methylquinoline. Formal Total Syntheses of Damirones A and B, Batzelline C, Isobatzelline C, Discorhabdin C, and Makaluvamines A−D

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Abstract
2-Amino-4-nitrophenol and 2-methoxy-5-nitroaniline were converted into the 5-nitroquinolines 6b and 6d, respectively, and then the latter into nitro-acetal 6f. 6,7-Dimethoxy-4-methylquinoline (6g) was nitrated at C-5 and then the methyl substituent converted into aldehyde 6j and then protected giving acetal 6l. Various means, notably a large excess of NiCl2/NaBH4, were used to reduce both nitro group and pyridine ring, forming 1,2,3,4-tetrahydroquinolines such as 7b, 7c, 7d, which under acidic conditions closed to give 1,3,4,5-tetrahydropyrrolo[4,3,2-de]quinolines 9a, 9d, 9c, respectively. In some cases it was unnecessary to protect the aldehyde function, for example quinolinium salt 12c gave 9j and nitro-aldehyde 6j gave 9e (after BOC protection) directly by reaction with NiCl2/NaBH4. Substitution of the indole and aniline nitrogens in the 1,3,4,5-tetrahydropyrrolo[4,3,2-de]quinolines was based on a combination of protection, selective deprotection, and the exploitation of the greater acidity of the indole N-hydrogen. 8-Chlorination of 6h and then conversions, as above, gave chloro-diamine-acetal 7e which on acid treatment produced iminoquinone 11b; formylation of the nitrogens in 7e and then acidic treatment allowed formation of the chlorine-substituted 1,3,4,5-tetrahydropyrrolo[4,3,2-de]quinoline 9m which was then converted into 9p. De-O-methylation and then oxidation of 9b and 9c gave o-quinones 10b and 10a, respectively.