Rapid and Systematic Exploration of Chemical Space Relevant to Artemisinins: Anti-malarial Activities of Skeletally Diversified Tetracyclic Peroxides and 6-Aza-artemisinins.

To achieve both structural changes and rapid synthesis of the tetracyclic scaffold relevant to artemisinins, we explored two kinds of de novo synthetic approaches that generate both skeletally diversified tetracyclic peroxides and 6-aza-artemisinins. The anti-malarial activities of the tetracyclic peroxides with distinct skeletal arrays, however, were moderate and far inferior to artemisinins. Given the privileged scaffold of artemisinins, we next envisioned element implantation at the C6 position with a nitrogen without the trimmings of substituents and functional groups. This molecular design allowed the deep-seated structural modification of the hitherto unexplored cyclohexane moiety (C-ring) while keeping the three-dimensional structure of artemisinins. Notably, this approach induced dramatic changes of retrosynthetic transforms that allow an expeditious catalytic asymmetric synthesis with generation of substitutional variations at three sites (N6, C9, and C3) of the 6-aza-artemisinins. These de novo synthetic approaches led to the lead discovery with substantial intensification of the in vivo activities, which undermine the prevailing notion that the C-ring of artemisinins appears to be merely a structural unit but to be a functional area as the anti-malarial pharmacophore. Furthermore, we unexpectedly found that racemic 6-aza-artemisinin (33) exerted exceedingly potent in vivo efficacies superior to the chiral one and the first-line drug, artesunate.

Design and De Novo Synthesis of 6-Aza-artemisinins.

Development of designer natural product variants, 6-aza-artemisinins, enabled us to achieve structural modification of the hitherto unexplored cyclohexane moiety of artemisinin and concise de novo synthesis of the tetracyclic scaffold in just four steps from the modular assembly of three simple building blocks. This expeditious catalytic asymmetric synthetic approach generated lead candidates exhibiting superior in vivo antimalarial activities to artemisinin.

Synthesis of (2ß,3α,6-²H3cholesteryl linoleate and cholesteryl oleate as internal standards for mass spectrometry.

The accurate analysis of trace component in complex biological matrices requires the use of reliable standards. For liquid chromatography/mass spectrometry analysis, the stable isotope-labeled derivatives of the analyte molecules are the most appropriate internal standards. We report here the synthesis of (2ß,3α,6-(2)H3)cholesteryl linoleate and oleate containing three non-exchangeable deuterium in the steroid ring. The principal reactions used were: (1) trans diaxial opening of 2α,3α-epoxy-6-oxo-5α-cholestane with LiAlD4 and subsequent oxidation of the resulting (2ß,6α-(2)H2)-3α,6ß-diol with Jones' reagent, followed by reduction of the resulting (2ß-(2)H)-3,6-dione with NaBD4 leading to the (2ß,3α,6α-(2)H3)-3ß,6ß-dihydroxy-5α-cholestane, (2) selective protection of the 3ß-hydroxy group as the tert-butyldimethylsilyl ether, (3) dehydration of the 6ß-hydroxy group with POCl3 and removal of tert-butyldimethylsilyloxy groups with 5M HCl in acetone, and (4) esterification of the resultant (2ß,3α,6-(2)H3)cholesterol with linoleic and oleic acids using 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide. The isotopic purity was found to be satisfactory by mass spectrometry, and nuclear magnetic resonance properties of the new compounds were tabulated. The labeled compounds can be used as internal standards in liquid chromatography/mass spectrometry assays for clinical and biochemical studies.

Generation of anti-trypanosomal agents through concise synthesis and structural diversification of sesquiterpene analogues.

To access high-quality small-molecule libraries to screen lead candidates for neglected diseases exemplified by human African trypanosomiasis, we sought to develop a synthetic process that would produce collections of cyclic scaffolds relevant to an assortment of natural products exhibiting desirable biological activities. By extracting the common structural features among several sesquiterpenes, including artemisinin, anthecularin, and transtaganolides, we designed six types of scaffolds with systematic structural variations consisting of three types of stereochemical relationships on the sp(3) ring-junctions and two distinct arrays of tricyclic frameworks. A modular and stereodivergent assembly of dienynes exploiting a versatile manifold produced a series of cyclization precursors. Divergent cyclizations of the dienynes employing tandem ring-closing metathesis reactions overrode variant reactivities of the cyclization precursors, leading to the six canonical sets of the tricyclic scaffolds incorporating a diene group. Screenings of trypanosomal activities of the canonical sets, as well as regio- and stereoisomers of the tricyclic dienes, allowed generation of several anti-trypanosomal agents defining the three-dimensional shape of the pharmacophore. The candidate tricyclic dienes were selected by primary screenings and further subjected to installation of a peroxide bridge, which generated artemisinin analogues that exhibited potent in vitro anti-trypanosomal activities comparable or even superior to those of artemisinin and the approved drugs, suramin and eflornithine.

Skeletal and stereochemical diversification of tricyclic frameworks inspired by Ca(2+)-ATPase inhibitors, artemisinin and transtaganolide D.

Inspired by the common skeletal motifs of Ca(2+)-ATPases inhibitors involving artemisinin and transtaganolide D, small molecule collections with the three-dimensional structural diversity of tricyclic systems were designed and expeditiously synthesized (4-5 steps). A synthetic strategy featuring stereochemical diversification of ring-junctions and control of cyclization modes was devised to access varied molecular architectures in a systematic fashion.