A compound represented by formula: wherein R 1 is C 1-30 alkyl, C 2-30 alkenyl, C 2-30 alkynyl, or a group represented by formula: -(C 1-15 alkylene)-Q 1 -Z 1 -(C 1-15 alkylene)-Z 2 -Q 2; in which Q 1 is C 3-10 cycloalkylene, 5-10 membered heterocyclylene, C 6-10 arylene, or 5-10 membered heteroarylene, Q 2 is a hydrogen atom, C 3-10 cycloalkyl, 5-10 membered heterocyclyl, C 6-10 aryl, or 5-10 membered heteroaryl, and Q 2 may be substituted with -Z 3 -Q 3, Q 3 is C 3-10 cycloalkyl, 5-10 membered heterocyclyl, C 6-10 aryl, or 5-10 membered heteroaryl, Z 1, Z 2, and Z 3 are independently selected from the group consisting of an oxygen atom, a sulfur atom, -NR 9 -, difluoromethylene, and a direct bond, wherein the alkyl, alkenyl, alkynyl, cycloalkylene, heterocyclylene, arylene, heteroarylene, cycloalkyl, heterocyclyl, aryl, and heteroaryl may be independently substituted with one or more substituents selected from halo and hydroxy; R 2 is a hydrogen atom, C 1-6 alkyl, C 7-14 aralkyl, or C 1-6 alkoxy C 1-6 alkyl; R 3 is a hydrogen atom or C 1-6 alkyl; R 4 and R 5 are independently selected from the group consisting of a hydrogen atom, C 1-6 alkyl, C 7-14 aralkyl, formyl, C 1-6 alkylcarbonyl, arylcarbonyl, C 1-6 alkoxycarbonyl, C 1-6 alkoxy C 1-6 alkyl, and C 7-14 aralkyloxycarbonyl; R 6 is a hydrogen atom, C 1-6 alkyl, C 1-6 alkoxy C 1-6 alkyl, or C 7-14 aralkyl; R 7 is a hydrogen atom or C 1-6 alkyl, R 8 is C 1-6 alkyl, in which the alkyl may be substituted with one or more substituents selected from the group consisting of hydroxy, C 1-6 alkoxy, C 3-10 cycloalkyloxy, 5-10 membered heterocyclyloxy, C 7-14 aralkyloxy, C 6-10 aryloxy, and 5-10 membered heteroaryloxy, or R 7 and R 8 together with the carbon atom which they are attached may form a ring selected from a C 3-10 saturated carbocyclic ring, and 5-10 membered saturated or partially-saturated heterocyclic ring; R 9 is a hydrogen atom or C 1-6 alkyl; X is CH 2 or a direct bond, with the proviso that when X is CH 2, R 7 and R 8 together with a carbon atom which they are attached form a ring; and Y is CH 2 or a direct bond, or a salt thereof. wherein R 1 is C 1-30 alkyl, C 2-30 alkenyl, C 2-30 alkynyl, or a group represented by formula: -(C 1-15 alkylene)-Q 1 -Z 1 -(C 1-15 alkylene)-Z 2 -Q 2; in which Q 1 is C 3-10 cycloalkylene, 5-10 membered heterocyclylene, C 6-10 arylene, or 5-10 membered heteroarylene, Q 2 is a hydrogen atom, C 3-10 cycloalkyl, 5-10 membered heterocyclyl, C 6-10 aryl, or 5-10 membered heteroaryl, and Q 2 may be substituted with -Z 3 -Q 3, Q 3 is C 3-10 cycloalkyl, 5-10 membered heterocyclyl, C 6-10 aryl, or 5-10 membered heteroaryl, Z 1, Z 2, and Z 3 are independently selected from the group consisting of an oxygen atom, a sulfur atom, -NR 9 -, difluoromethylene, and a direct bond, wherein the alkyl, alkenyl, alkynyl, cycloalkylene, heterocyclylene, arylene, heteroarylene, cycloalkyl, heterocyclyl, aryl, and heteroaryl may be independently substituted with one or more substituents selected from halo and hydroxy; R 2 is a hydrogen atom, C 1-6 alkyl, C 7-14 aralkyl, or C 1-6 alkoxy C 1-6 alkyl; R 3 is a hydrogen atom or C 1-6 alkyl; R 4 and R 5 are independently selected from the group consisting of a hydrogen atom, C 1-6 alkyl, C 7-14 aralkyl, formyl, C 1-6 alkylcarbonyl, arylcarbonyl, C 1-6 alkoxycarbonyl, C 1-6 alkoxy C 1-6 alkyl, and C 7-14 aralkyloxycarbonyl; R 6 is a hydrogen atom, C 1-6 alkyl, C 1-6 alkoxy C 1-6 alkyl, or C 7-14 aralkyl; R 7 is a hydrogen atom or C 1-6 alkyl, R 8 is C 1-6 alkyl, in which the alkyl may be substituted with one or more substituents selected from the group consisting of hydroxy, C 1-6 alkoxy, C 3-10 cycloalkyloxy, 5-10 membered heterocyclyloxy, C 7-14 aralkyloxy, C 6-10 aryloxy, and 5-10 membered heteroaryloxy, or R 7 and R 8 together with the carbon atom which they are attached may form a ring selected from a C 3-10 saturated carbocyclic ring, and 5-10 membered saturated or partially-saturated heterocyclic ring; R 9 is a hydrogen atom or C 1-6 alkyl; X is CH 2 or a direct bond, with the proviso that when X is CH 2, R 7 and R 8 together with a carbon atom which they are attached form a ring; and Y is CH 2 or a direct bond, or a salt thereof.