| abstract |
A compound represented by the formula: ** Formula ** wherein R 1a is (a) C 1-6 alkyl which optionally has 1 to 3 substituents selected from C 1-6 alkylcarbonyloxy, hydroxy and a halogen atom, (b) C3-6 cycloalkyl, (c) phenyl or (d) mono- or di- C 1-6 alkyl; R2a is a hydrogen atom or C1-6 alkyl; R2b is a hydrogen or hydroxy atom; R3a is (a) a hydrogen atom, (b) C1-6 alkyl which optionally has 1 to 3 substituents selected from phenyl, hydroxy, a halogen atom, C1-6 alkyl-carbonyl, C7-13 aralkyloxy and pyridyl, ( c) C3-6 cycloalkyl, (d) phenyl, (e) C1-6 alkoxy, (f) mercapto, (g) C1-6 alkylthio or (h) mono- or di- C1-6 amino alkyl; X is an oxygen atom or a sulfur atom; and indicates a single bond or a double bond, or its salt. |