NEW MATERIAL:The compound of formula I (R1 is H or 1-8C alkyl; R2 is single bond or 1-4C alkylene; R3 is 1-8C alkyl, 4-7C cycloalkyl, phenyl or phenoxy; B is ethylene, trans-vinylene, etc.), its cyclodextrin-clathrate compound or, when R1 is H, its salt.

EXAMPLE: (11α)-(15S)-(13E)-15-[(1S,3S)-3-butylcyclopentyl)-5,5-difluoro-11,15- dihydroxy-6,9-dioxo-16,17,18,19,20-pentanorprost-13-enoic acid methyl ester].

USE: An antiulcer agent having exclusively and selectively strong antiulcer activity.

PREPARATION: The compound of formula I can be produced by hydrolyzing a compound of formula II (R4 is tetrahydropyran-2-yl, tetrahydrofuran-2-yl, etc.) with an acid (e.g., acetic acid) in a solvent (e.g., THF) at 0-90°C.