Isoindoline compound

A method for producing an isoindoline compound comprising a radiocarbon atom 14C and a group represented by the following formula (A):

• in the formula (A), R1 to R4 each independently represent a hydrogen atom, a linear or branched alkyl group having 1 to 12 carbon atoms, a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, or a phenyl group that is optionally substituted with an alkyl group, R2 and R3 are optionally ring-closed to form a 5- to 8-membered ring, R7 and R8 each independently represent a hydrogen atom, a linear or branched alkyl group having 1 to 12 carbon atoms, or a phenyl group that is optionally substituted with an alkyl group, and * represents a bonding hand,
• the method comprising reacting a compound represented by the following formula (I-2) with barbituric acid represented by the following formula (B) under an acid-free solvent or under a solvent containing at least one kind of acid selected from the group consisting of hydrochloric acid, sulfuric acid, methanesulfonic acid, p-toluenesulfonic acid, acetic acid, triflic acid, phosphoric acid, polyphosphoric acid, pyrophosphoric acid, and nitric acid to obtain an isoindoline compound comprising a group represented by the formula (A):
• in the formula (I), R1 to R4 represent same as R1 to R4 above, and
• in the formula (B), R5 and R6 each independently represent a hydrogen atom, a linear or branched alkyl group having 1 to 12 carbon atoms, or a phenyl group that is optionally substituted with an alkyl group.