Patent ID: 7067553

Claim:
A compound of formula I: or a pharmaceutically acceptable salt thereof, wherein, X represents a sulfur atom or an oxygen atom; R 1 represents a lower alkyl sulfonyl group having 1 to 5 carbon atoms, an aryl sulfonyl group or a lower alkyl carbonyl group having 1 to 5 carbon atoms, which may be unsubstituted or substituted with a halogen atom; R 2 represents a hydrogen atom, a methoxy group or a halogen atom; when A is —NHCH 2 —, B represents wherein, n is 0 or 1, or when A is —CH 2 —, B represents 4-t-butylbenzyl, 3,4-dimethylphenylethyl, or an oleyl group; Y represents —CH 2 — or —CH 2 CH 2 —; R 3 represents a hydrogen atom or a lower alkyl group having 1 to 5 carbon atoms; and R 4 represents a lower alkyl group having 1 to 5 carbon atoms or a phenyl group.