Cortistatin A

I haven’t been doing any retrosynthetic analyses for a long time that my retrosynthesis skills are slowly deteriorating. Also, the number of visitors to this blog have dropped since I stopped Friday Night Retrosynthesis posts. So, to keep myself in good form and to keep this blog more or less interesting a quick retrosynthesis post is exactly what is needed.

After this brief introduction from a renowned group’s web-page:

Cortistatin A. In 2006, the unique natural product cortistatin A was reported as an extremely potent inhibitor of the migration and proliferation of HUVECs (Human Umbilical Vascular Endothelial Cells); IC50 = 180 pM (VEGF-stimulated HUVECs proliferation). It was also shown that cortistatin A is 3000 times more potent at inhibiting the proliferation of HUVECs versus other cancer and normal cell lines, suggesting that cortistatin A may be a highly selective angiogenesis inhibitor. Cortistatin A has an unprecedented structure, with a particularly challenging [3.2.1]oxabicyclooctene ring system.

…on with the retrosynthesis:

UPDATE: I agree with Greg that it is almost not possible to predict which c=c bond will cyclopropanate first. But what I wanted to say is that the core structure could (in principle) be built in this way. Here is the proposed mechanism of the key step:

UPDATE II:

… isomerization step:

BTW, (maybe) the problematic isopropyl group stereochemistry of Guanacastapen A could be installed in this manner, too. But that should be a topic of another retrosynthesis post…