Fabian Feyen

A ring-closing metathesis (RCM)-based approach to mycolactones A/B

The total synthesis of the mycobacterial toxins mycolactonesu2005A/B (1u2009a/b) has been accomplished based on a strategy built around the construction of the mycolactone core through ring-closing metathesis. By employing the Grubbs second-generation catalyst, the 12-membered core macrocycle of mycolactones, with a functionalized C2 handle attached to C11, was obtained in 60-80u2009% yield. The C-linked upper side chain (comprising C12-C20) was completed by a highly efficient modified Suzuki coupling between C13 and C14, while the attachment of the C5-O-linked polyunsaturated acyl side chain was achieved by Yamaguchi esterification. Surprisingly, a diene containing a simple isopropyl group attached to C11 could not be induced to undergo ring-closing metathesis. By employing fluorescence microscopy and flow cytometry techniques, the synthetic mycolactonesu2005A/B (1u2009a/b) were demonstrated to display similar apoptosis-inducing and cytopathic effects as mycolactonesu2005A/B extracted from Mycobacterium ulcerans. In contrast, a simplified analogue with truncated upper and lower side chains was found to be inactive.

Making epothilones fluoresce: design, synthesis, and biological characterization of a fluorescent N12-aza-epothilone (azathilone)

A green fluorescent 12‐aza‐epothilone (azathilone) derivative has been prepared through the attachment of the 4‐nitro‐2,1,3‐benzoxadiazole (NBD) fluorophore to the 12‐nitrogen atom of the azamacrolide core structure. While less potent than natural epothilones or different N12‐acylated azathilone derivatives, NBD‐azathilone (3) promotes tubulin assembly, inhibits cancer cell proliferation in vitro and arrests the cell cycle at the G2/M transition. Most significantly, the binding of 3 to cellular microtubules (MTs) could be directly visualized by confocal fluorescence microscopy. Based on competition binding experiments with laulimalide‐stabilized MTs in vitro, the N12‐Boc substituted azathilone 1, Epo A, and NBD‐azathilone (3) all interact with the same tubulin‐binding site. Computational studies provided a structural model of the complexes between β‐tubulin and 1 or 3, respectively, in which the NBD moiety of 3 or the BOC moiety of 1 directly and specifically contribute to MT binding. Collectively, these data demonstrate that the cellular effects of 3 and, by inference, also of other azathilones are the result of their interactions with the cellular MT network.