NEW MATERIAL:A compound of formula I (R1 and R2 are each protective group for H and OH).

EXAMPLE: [3a-S(3a-α,6a-α)[-3,3a,6,6a-hexahydro-4-(t-butyldimethylsilyl)oxy-6a-(4- methoxyphenyl)oxymethyl-2H-cyclopenta[b]furan-2-one of formula II.

USE: Expected as an active ingredient of medicines or pesticides, similar to a group of Corey lactones of formula III (R',R"=R1, R2) known as the synthetic intermediates for prostaglandins having strong physiological activity and didemnenones of formulas IV, V,etc., known for their cytotoxicity.

PREPARATION: A compound of formula VI (X is halogen, acyloxy or sulfonyloxy) or its optically active modification is treated with a base such as diazabicyclononene in a solvent such as THF to obtain the objective compound of the formula I or its optically active modification.