NEW MATERIAL: A compd. represented by formula I (wherein R1 and R2 are H, alkyl, aryl or a functional group or form an alicyclic ring together with adjacent carbon but are not together IH).

EXAMPLE: (+)-4(R),7(R)-dimethyl-2-oxa-4-4a(s),8a(R)bicyclo[4,4,0]-decan-1-one.

USE: An antimalarial drug and a parasiticide.

PROCESS: A 2-oxabicyclo [4,4,0] codeca-1,8a-ene compd. is subjected to photo- oxidation (pref. performed in methylene chloride by using methylene blue as a sensitizer) by using singlet oxygen and the obtained product is reacted with a carbonyl compd. represented by formula II to obtain a seco-desoxyarteannuin derivative wherein a second position of the compd. represented by formula I is substituted by R1 and R2.