Synthesis of [3,3‑2H2]‑Dihydroartemisinic Acid to Measure the Rate of Nonenzymatic Conversion of Dihydroartemisinic Acid to Artemisinin

Dihydroartemisinic acid is the biosynthetic precursor to artemisinin, the endoperoxide-containing natural product used to treat malaria. The conversion of dihydroartemisinic acid to artemisinin is a cascade reaction that involves C–C bond cleavage, hydroperoxide incorporation, and polycyclization to form the endoperoxide. Whether or not this reaction is enzymatically controlled has been controversial. A method was developed to quantify the nonenzymatic conversion of dihydroartemisinic acid to artemisinin using LC-MS. A seven-step synthesis of 3,3-dideutero­dihydroartemisinic acid (23) was accomplished beginning with dihydroartemisinic acid (1). The nonenzymatic rates of formation of 3,3-dideuteroartemisinin (24) from 3,3-dideuterodihydro­artemisinic acid (23) were 1400 ng/day with light and 32 ng/day without light. Moreover, an unexpected formation of nondeuterated artemisinin (3) from 3,3-dideuterodihydro­artemisinic acid (23) was detected in both the presence and absence of light. This formation of nondeuterated artemisinin (3) from its dideuterated precursor (23) suggests an alternative mechanistic pathway that operates independent of light to form artemisinin, involving the loss of the two C-3 deuterium atoms.