Artemisinin is a plant natural product that contains an endoperoxide bridge which gives the compound its ability to kill the parasite that causes malaria, Plasmodium falciparum. Dihydroartemisinic acid (DHAA) is a known precursor to artemisinin. The conversion of DHAA to artemisinin is speculated to be either non-enzymatic or enzymatic. Recently it was shown that DHAA spontaneously converts to artemisinin over time in the light and in the dark, with a 40-fold faster conversion in the light. The purpose of this project is to investigate the mechanism of endoperoxide formation to artemisinin from DHAA further using deuterated isotopologues and 18O-molecular oxygen-labeling techniques. It was hypothesized that the use of a C15-trideuterated version of DHAA would allow for the measurement of the rate of non-enzymatic conversion to artemisinin using non-deuterated artemisinin as an internal standard. The synthesis of 15,15,15-trideutero-DHAA from DHAA is reported in 25 steps. Key chemical transformations include: (i) oxidative cleavage of the C4-C5 double bond of DHAA, (ii) Rubottom oxidation at the C15-position of a silyl enol ether intermediate, (iii) Grubbs ring closing metathesis to access the cis-decalin ring system, and (iv) deuteration of a C15-carboxylic acid intermediate with LiAlD4 and a mesylate to ultimately incorporate the three deuteriums at C15. The desired 15,15,15-trideutero-DHAA isotopologue was placed in vials in the light and in the dark to form artemisinin over time. Several new observations were made: (i) ultraviolet-C (UV-C) radiation initially accelerates artemisinin formation and promotes rearrangement to a different product, (ii) di-deuterated and tri-deuterated DHAA isotopologues at C3 and C15 converted to artemisinin at a slower rate compared to nondeuterated DHAA, revealing a kinetic isotope effect towards endoperoxide formation, (iii) the rate of conversion from DHAA to artemisinin increased with concentration, suggesting an intermolecular interaction to promote endoperoxide formation, and (iv) 18O2-labeling showed incorporation of three and four oxygen atoms from molecular oxygen into the endoperoxide bridge of artemisinin. These results aid in the elucidation of the mechanism of endoperoxide formation of artemisinin in nature.

Synthesis of [15,15,15-2H3]-dihydroartemisinic acid and isotope studies support a mixed mechanism in the endoperoxide formation to Artemisinin

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.