Cinnamic acids are widely distributed in plants, including crops for human use, and exhibit a variety of activities that are beneficial to human health. They also occupy a pivotal position in the biosynthesis of phenylpropanoids such as lignins, anthocyanins, flavonoids, and coumarins. In this context, deuterium‐labeled cinnamic acids have been used as tracers and internal standards in food and medicinal chemistry as well as plant biochemistry. Therefore, a concise synthesis of deuterium‐labeled cinnamic acids would be highly desirable. In this study, we synthesized deuterium‐labeled cinnamic acids using readily available deuterium sources. We also investigated a hydrogen–deuterium exchange reaction in an ethanol‐d1/Et3N system. This method can introduce deuterium atoms at the ortho and para positions of the phenolic hydroxy groups as well as at the C‐2 position of alkyl cinnamates and is applicable to various phenolic compounds. Using the synthesized labeled compounds, we demonstrated that the benzenoid volatiles, such as 4‐methoxybenzaldehyde, in the scent of the flowers of the Japanese loquat Eriobotrya japonica are biosynthesized from phenylalanine via cinnamic and 4‐coumaric acids. This study provides easy access to a variety of deuterium‐labeled (poly)phenols, as well as to useful tools for studies of the metabolism of cinnamic acids in living systems.

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