Synthesis and structure–activity relationships and effects of phenylpropanoid amides of octopamine and dopamine on tyrosinase inhibition and antioxidation

Phenylpropanoid amides of octopamine (OA) 1a–1e and dopamine (DA) 2a–2e were synthesised and the structure–activity relationships (SARs) for antioxidant and tyrosinase inhibition activities were analysed. Among synthesised compounds, 2c, which contains two catechol moieties, exhibited the most DPPH radical-scavenging activity (EC50 = 16.2 ± 2.4 μM), and 1d exhibited significant tyrosinase inhibitory activity (IC50 = 5.3 ± 1.8 μM). Interestingly, with the same acid moiety, OA derivatives showed more inhibitory effect on tyrosinase than did compounds derived from DA, whereas DA derivatives were found to have higher antioxidant activity than compounds derived from OA. The relationship between their structures and their potencies, demonstrated in the current study, will be useful for the design of optimal agents.