Diels-Alder reactivity of pyrano[3,4-b]indol-3-ones. Part 4. Synthesis of the carbazole alkaloids carbazomycin A and B and hyellazole

The first synthesis of the carbazole alkaloids carbazomycins A and B (1) is described. The key step is the regioselective Diets-Alder reaction between 1-methylpyrano[3,4-b]indol-3-one (4) and ethyl trimethylsilyl propynoate to give ethyl 1-methyl-3-trimethylsilylcarbazole-2-carboxylate (6). The ester and trimethylsilyl groups in carbazole (6) are converted into methyl and methoxy groups respectively, and the final oxygen substituent is introduced, after protection of the carbazole nitrogen, by bromination, lithiation, formation of the corresponding borate, and oxidation. The carbazole alkaloid hyellazole (3a) is synthesised by a similar route from 1-phenylpyrano[3,4-b]indol3-one.