Patent ID: 8084492

Claim:
A compound represented by the formula (I) wherein R 1 is a C 1-6 alkyl group; R 2 and R 3 are the same or different and each is a C 1-6 alkyl group, wherein the alkyl group is optionally substituted by one or more —COOAlk; R 4 is a C 1-6 alkyl group, wherein the alkyl group is optionally substituted by one or more substituents selected from the group consisting of a halogen atom, —COOAlk, an amino group, wherein the amino group is optionally mono- or di-substituted by a C 1-6 alkyl group, a sulfanyl group, a C 6-14 arylsulfanyl group, wherein aryl of the arylsulfanyl group is optionally substituted by one or more halogen atoms and a C 6-14 aryl group, wherein the aryl group is optionally substituted by one or more substituents selected from the group consisting of a hydroxyl group and a C 1-6 alkoxy group, or a pharmaceutically acceptable salt thereof.