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Chemical structure of major cinnamic acids. Cinnamic acid: R1 = R2 = R3 = R4 = H. o-Coumaric acid: R1 = OH; R2 = R3 = R4 = H. m-Coumaric acid: R1 = R3 = R4 = H; R2 = OH. p-Coumaric acid: R1 = R2 = R4 = H; R3 = OH. Caffeic acid: R1 = R4 = H; R2 = R3 = OH. Ferulic acid: R1 = R4 = H; R2 = OCH3; R3 = OH. Synapic acid: R1 = H; R2 = R4 = OCH3; R3 = OH.
