Pyrrole-Imidazole Alkaloids from Marine Sponges: Structural Variation and Cytotoxicity of (-)-Dibromophakellstatin, Synthesis of Ugibohlin and Oroidin, and Studies Towards Oxocyclostylidol

Oroidin is considered to be the biogenetic key metabolite of the pyrroleimidazole alkaloids. Several syntheses are known to date, all of which contain steps with only moderate yields. For a study on its reactivity large quantities are needed. Therefore, an improved synthesis would be beneficial. In the present work two different known routes to 8 were investigated. By following the method developed in our group (via Sonogashira coupling) several analogues arose as new products. By treating the two deaminated analogs 1 and 4 with NBS in TFA, no cyclization to the phakellin skeleton 3 took place. Instead, pyrrole oxidation occurred (Scheme 1).