Electronically stabilized versions of the antimalarial acetal trioxanes artemether and artesunate

From 9-substituted DHA, several new artemisinin-derived C-10 acetal ethers and esters were prepared with either a 9-fluoro or a 9-sulfonyl substituent. The very strong inductive electron-withdrawing C-9 substituent is shown to retard considerably C-10 ionization (acid-promoted etherification) of 9-fluoro-DHA and 9-sulfonyl-DHA.