Abstract: The synthesis of 5-unsubstituted pyrido[3,2,1-jk]carbazol-6-one can be achieved by the reaction of carbazole and malonate derivatives, either in a 3-step synthesis via 5-acetyl-pyrido[3,2,1-jk]carbazolone or in a 1-step reaction from carbazole and malonic acid. The 5-acetyl-4-hydroxy derivative can be transformed to 4-azido-pyrido[3,2,1-jk]carbazolone, which cyclizes by thermal decomposition to isoxazolo-pyrido[3,2,1-jk]carbazolone. The thermolysis conditions were investigated by differential scanning calorimetry (DSC). Nitration of pyrido[3,2,1-jk]carbazol-6-one and subsequent introduction of azide leads to 4-azido-5-nitro-pyrido[3,2,1-jk]carbazol-6-one , which cyclizes on thermolysis to a furazan-oxide derivative. Again the thermolysis conditions were investigated by DSC. 5-Chloro-5-nitro-pyrido[3,2,1-jk]carbazole-4,6-dione, obtained from pyrido[3,2,1-jk]carbazol-6-one by subsequent nitration and chlorination, forms on oxidative thermolysis reaction pyrido[3,2,1-jk]carbazole-4,5,6-trione, which reacts e.g. with phenole by acylation to give 5-hydroxy-5-(p-hydroxyphenyl)-pyrido[3,2,1-jk]carbazole-4,6-dione. A further C-C-coupling at position 4 starts from 4-chloro-5-nitro-pyrido[3,2,1-jk]carbazol-6-one, which gives with diethyl diethyl malonate 5-nitro-6-oxopyrido[3,2,1-jk]carbazol-4-yl)-malonate; with diemethyl malonate, 5-nitro-6-oxopyrido[3,2,1-jk]carbazol-4-yl)-acetate was obtained by loss of carbon dioxide.