Provided are a compound represented by Formula (I): wherein R1, R2, R3, and R4 are each independently selected from a hydrogen atom, a C1-6 alkyl group which may be substituted, a C7-14 aralkyl group which may be substituted and —C(═O)Rx; n denotes an integer selected from 1 and 2; and ring Ar is selected from the groups represented by the following Formula (a) to (f). or a prodrug thereof or a pharmaceutically acceptable salt thereof as well as a pharmaceutical agent and a pharmaceutical composition containing such a compound or a prodrug thereof, or a pharmaceutically acceptable salt thereof.