Prodrug of fluorine-containing amino acid

A compound represented by formula (I):

• wherein R1 and R2, which may be the same or different, are each a hydrogen atom, —(CR4R4′)—O—CO—R5, —(CR6R6′)—O—CO—O—R7, (IIa) or (IIb):
• R3 is a hydrogen atom, —CO—O—(CR9R9′)—O—CO—R10, —CO—O—(CR9R9′)—O—CO—O—R11, or (III):
• wherein R4 and R4′, which may be the same or different, are each a hydrogen atom or a C1-6 alkyl group;
• R5 is a C1-10 alkyl group, a C3-8 cycloalkyl group optionally substituted with one to three C1-6 alkyl groups, an adamantyl group optionally substituted with one to three C1-6 alkyl groups, or a phenyl group optionally substituted with one to three groups selected from a halogen atom and a C1-6 alkyl group;
• R6 and R6′, which may be the same or different, are each a hydrogen atom or a C1-6 alkyl group;
• R7 is a C1-10 alkyl group, a C3-8 cycloalkyl group optionally substituted with one to three C1-6 alkyl groups, an adamantyl group optionally substituted with one to three C1-6 alkyl groups, or an aryl group optionally substituted with one to three groups selected from a halogen atom and a C1-6 alkyl group;
• R8 is a C1-6 alkyl group or a phenyl group;
• R9 and R9′, which may be the same or different, are each a hydrogen atom or a C1-6 alkyl group;
• R10 is a C1-10 alkyl group, a C3-8 cycloalkyl group optionally substituted with one to three C1-6 alkyl groups, an adamantyl group optionally substituted with one to three C1-6 alkyl groups, or a phenyl group optionally substituted with one to three groups selected from a halogen atom and a C1-6 alkyl group;
• R11 is a C1-10 alkyl group, a C3-8 cycloalkyl group substituted with one to three C1-6 alkyl groups, an adamantyl group optionally substituted with one to three C1-6 alkyl groups, or an aryl group optionally substituted with one to three groups selected from a halogen atom and a C1-6 alkyl group;
• and
• R12 is a C1-6 alkyl group or a phenyl group,
• provided that compounds in which R1, R2 and R3 are each a hydrogen atom is excluded, or a pharmaceutically acceptable salt thereof.