Color developer, thermal recording material, and thermal recording layer coating material

A color developer for a thermal recording layer, comprising:

• a compound of formula (I);
• and
• an N-substituted amino acid derivative of formula (II),
• wherein in the formula (I),
• L represents an imino group (—NH—) or an oxy group (—O—);
• R1, R2 and R3 each represent a hydrogen atom, a halogen atom, a nitro group, an alkyl group, a cycloalkyl group, an alkoxy group, a cycloalkyloxy group, an alkenyl group, a fluoroalkyl group, an N(R4)2 group (wherein R4 represents a hydrogen atom, a phenyl group, a benzyl group, or an alkyl group having 1 to 6 carbon atoms), an NHCOR5 group (wherein R5 represents an alkyl group having 1 to 6 carbon atoms), an optionally substituted phenyl group, an optionally substituted benzyl group, an aryloxy group, an alkylcarbonyloxy group, an arylcarbonyloxy group, an alkylcarbonylamino group, an arylcarbonylamino group, an alkylsulfonylamino group, or an arylsulfonylamino group;
• n1, n2 and n3 each independently represent an integer of 0 to 5;
• R1, R2 and R3 are identical to or different from one another;
• when two or more R1s are present, R1s are identical to or different from each other;
• when two or more R2s are present, R2s are identical to or different from each other;
• and
• when two or more R3s are present, R3s are identical to or different from each other,
• (R0—X)—Y—(Z) (II)
• wherein in the formula (II),
• R0 represents an alkyl group having an aryl group having 6 to 10 carbon atoms or an aryl group optionally substituted with a substituent of an alkyl group having 1 to 8 carbon atoms, an aralkyl group having 7 to 11 carbon atoms, an aryl group having 6 to 10 carbon atoms, or an alkoxy group having 1 to 8 carbon atoms;
• X represents a group bonded to an N-terminal of Y and represents —OCO—, —SO2NHCO—, —NHCO—, —NHCS—, or —SO2—;
• Y represents an amino acid residue or a peptide residue, and an OH group of a serine residue, a threonine residue, an aspartic acid residue, a glutamic acid residue, or a tyrosine residue in the Y group is optionally substituted with an OR9 group or an OR″ group, and an SH group of a cysteine residue is optionally substituted with an SR9 group or an SR″ group, an NH group of a histidine residue is optionally substituted with an NR9 group or an NR′ group, an NH2 group of a lysine residue or an ornithine residue is optionally substituted with an NHR9 group or an NHR′ group, R′ represents an amino protecting group, and R″ represents a carboxy protecting group, provided that Y is an amino acid residue other than a cystine residue or a peptide residue having no cystine residue;
• Z is a group bonded to a C-terminal of Y and represents an OH group or an OR″ group;
• R0, R′ and R″ are identical to or different from one another;
• when two or more R0s are present, R0s are identical to or different from each other;
• when two or more R′s are present, R′s are identical to or different from each other;
• when two or more R″s are present, R″s are identical to or different from each other;
• and
• two or more groups of R0, R′, and R″ are optionally bonded to each other to form a ring.