Electrophilic natural products lymphostin and ammosamide C: Total synthesis for mode-of-action studies

Using the method of "reactivity-guided isolation", we identified the pyrroloquinoline alkaloids lymphostin and ammosamide C as putative electrophilic natural products. Electrophilic natural products are often known for their remarkable variety of biological activities, such as antibiotic, antifungal, insecticidal and anticancer. Our thiol probes are not only used as tools to identify new natural products but moreover give an insight in the structural moieties involved in the natural products' mode of action via covalent modification of one of the key residues in the catalytic pocket. The knowledge about important structural moieties enables the design of affinity probes for the determination of their cellular targets without reducing the biological activity or changing the binding mode of action.

Although, several targets of lymphostin such as lymphocyte kinase Lck, phosphatidylinositol 3-kinase (PI3K) and the mammalian target of rapamycin (mTOR) are already identified, the explicit mode of action as a kinase inhibitor is still unknown. Ammosamide C, which harbors an electrophilic iminium moiety and is therefore significantly different from ammosamide A and B structures, suggests consequently a different target protein.

Herein, we describe the work towards the total synthesis of lymphostin and ammosamide C biotin-affinity probes, starting with 3-chloroaniline and yielding 4-hydroxy-6-nitropyrrolo[4,3,2-de]quinolin-2(1H)-one as core structure, to identify potential protein targets. Initial results presented will support the understanding of the mode of action of this compound class as electrophilic natural products.