Syntheses and Reactions of 5-Unsubstituted 11b-Methyl-1,2,3,11b-tetrahydro-pyrido[3,2,1-jk]carbazoles Having a Strychnos Alkaloids Partial Structure

4a-Methyl-2,3,3,4a-tetrahydro-1H-carbazoles 3, obtained from arylhydrazines 1 and 2-methylcyclohexanone 2, react with diethyl malonate 5a by cyclocondensation to 7-hydroxy-13b-methyl-tetrahydro-pyrano[2',3':4,5]pyrido[3,2,1-jk]carbazolediones 6. With methanetricarboxylates 5b,c, cyclocondensation forms a mixture of pyrano-pyrido-carbazole-carboxylates 7 and pyrido-carbazole-carboxylates 8. Degradation of pyrano-pyrido-carbazolediones 6 by alkaline ring-opening and decarboxylation leads to 5-acetyl derivatives 10. Deacetylation with sulfuric acid reveals the 5-unsubstituted 4-hydroxy-11b-methyl-1,2,3,11b-tetrahydro-6H-pyrido[3,2,1-jk]carbazol-6-one (9). Pyridocarbazolone 9 was also obtained in a single step reaction from tetrahydrocarbazole 3a and malonic acid 5d with phosphoryl chloride as catalyst in low yields.
Electrophilic halogenation of 9 takes place at position 5 and gives 5-bromo compound 15 or 5,5-dihalogenated products 14, 16 or 17, depending on the reaction conditions. Nitration takes also place in an electrophilic reaction to give 4-hydroxy-5-nitro-pyrido-carbazolone 20. Nucleophilic exchange of the 4-hydroxy group in pyridocarbazolones 9 and 20 by reaction with phosphoryl chloride gives reactive 4-chloro intermediates 18 and 21, which react with sodium azide to 4-azido derivatives 19 and 22. 4-Azido-5-nitro-pyrido-carbazolone 22 with the reactive ortho-nitro group in the neighborship of the azido group cyclizes thermolytically to furoxane 23. This reaction was studied by differential scanning calorimetry (DSC).