The present invention provides a compound represented by general formula (1) or a salt thereof. [In general formula (1), R1 and R2 represent an aldehyde group, a carboxyl group, or a C1-6 alkyl group, and at least one of R1 and R2 is an aldehyde group or a carboxyl group. R3 represents a hydrogen atom, a C1-6 alkyl group, a C2-6 alkenyl group, a C2-6 alkynyl group, a C6-10 aryl group, a C7-11 aralkyl group, a C1-6 alkyl carbonyl group, or a C7-11 aralkyl carbonyl group. R4 represents a hydrogen atom or a C1-6 alkyl group.].本發明係提供一種化合物或其鹽,該化合物以下述通式(1)表示。[化1][通式(1)中,R1及R2表示醛基、羧基或C1-6烷基,其中,至少一者為醛基或羧基;R3表示氫原子、C1-6烷基、C2-6烯基、C2-6炔基、C6-10芳基、C7-11芳烷基、C1-6烷羰基、或C7-11芳烷羰基;R4表示氫原子或C1-6烷基]。