Calpain inhibitor

A compound represented by the following general formula (I) or a salt thereof:

• in the formula (I),
• R1 represents a hydrogen atom, a linear, branched, or cyclic alkyl group that may be substituted, a linear or branched alkenyl group that may be substituted, a linear or branched alkynyl group that may be substituted, an aryl group that may be substituted, or a heterocyclic group that may be substituted, or may be bonded to R2 to form a ring,
• R2 and R3 each independently represent a hydrogen atom, a hydroxyl group, an alkoxy group that may be substituted, or a linear, branched, or cyclic alkyl group that may be substituted, or R2 and R3 may be bonded to form a ring, provided that when R2 is bonded to R1 to form a double bond-containing ring, R3 may be absent,
• R4 represents a hydrogen atom, a linear or branched alkyl group having 4 or more carbon atoms that may be substituted, or an alkyl group having 1 or 2 carbon atom(s) substituted with a cycloalkyl group having from 3 to 5 carbon atoms,
• R5 represents a hydrogen atom, a linear or branched alkyl group that may be substituted, a linear or branched alkenyl group that may be substituted, a linear or branched alkynyl group that may be substituted, an aryl group that may be substituted, or a heterocyclic group that may be substituted,
• R6 represents a linear, branched, or cyclic alkyl group that may be substituted, a linear or branched alkenyl group that may be substituted, a linear or branched alkynyl group that may be substituted, an aryl group that may be substituted, a heterocyclic group that may be substituted, a fused polycyclic hydrocarbon group that may be substituted, an alkoxy group that may be substituted, or a hydrogen atom,
• X represents O or S,
• when R1 and R2 are bonded to form a ring, R1 may represent O, S, or N, and R2 may represent O or S, and
• L represents an amide bond, an ester bond, —NH—, —N-(linear, branched, or cyclic alkylene)-, or -(linear or branched alkylene-O—)n— where “n” represents an integer of from 1 to 10, or L represents a single bond.