Synthesis and Biological Evaluation of Lactimidomycin and Its Analogues

The studies culminating in the total synthesis of the glutarimide-containing eukaryote translation elongation inhibitor lactimidomycin are described. The optimized synthetic route features a Zn(II)-mediated intramolecular Horner-Wadsworth-Emmons (HWE) reaction resulting in a highly stereoselective formation of the strained 12-membered macrolactone of lactimidomycin on a 423 mg scale. The presence of the E,Z-diene functionality was found to be key for effective macrocyclizations as a complete removal of these unsaturation units resulted in exclusive formation of the dimer rather than monocyclic enoate. The synthetic route features a late-stage installation of the glutarimide functionality via an asymmetric catalytic Mukaiyama aldol reaction, which allows for a quick generation of lactimidomycin homolog 55 containing two additional carbons in the glutarimide side chain. Similar to lactimidomycin, this analog was found to possess cytotoxicity against MDA-MB-231 breast cancer cells (GI50 =1-3 μM) using in vitro 2D and 3D assays. Although lactimidomycin was found to be the most potent compound in terms of anticancer activity, 55 as well as truncated analogues 50-52 lacking the glutarimide side-chain were found to be significantly less toxic against human mammary epithelial cells.