NEW MATERIAL:A compound shown by formula I [R1 is COOR11 (R11 are H, alkyl, cycloalkyl, admantyl, etc.) hydroxymethylcarbonyl, hydroxymethyl, CO-AA (AA is amino acid residue); Z is O, S or NR21 (R21 is H or alkyl); R2 is single bond or 1-4C alkylene; R3 is 1-7C alkyl, 4-7C cycloalkyl which may be replaced, phenyl or phenoxy which may be replaced], an atoxic salt and a cyclodextrin salt thereof.

EXAMPLE: 5-Oxa-6-oxo-17S,20-dimethyl-PGE1.

USE: Having PG-like activity, especially strong cell protecting action and useful for preventing and treating various diseases caused by cytotoxicity.

PREPARATION: For example, a compound shown by formula II (R4 is tert.- butylmethylsilyl, 2-tetrahydropyranyl, etc.) is hydrolyzed under an acidic condi tion and optionally further esterified or amidated to give a corresponding com pound shown by formula I.