Arteannuin (1), a new sesquiterpene lactone containing a peroxide group, is the antimalarial active principle isolated from Artemisia annua L. Deoxyarteannuin (2) occurs in the same plant and was isolated as a metabolite from the urine of patients treated with arteannuin. We now report a stereocontrolled synthesis of these two naturally occurring sesquiterpene lactones starting from arteannuic acid (5) by the reaction sequence as shown in Scheme 2 (see Chinese Text). Irradiation of the methanolic solution of the key intermediate 3 in the resence of oxygen and a sensitizer (Rose Bangal) with 200 W high pressure mercury lamp followed by acid treatment gave 1 in 28% yield (2 steps), m.p. 151-153`c,

Key words: LACTONES, ANTIMALARIALS, SESQUITERPENE, ARTEANNUIN