Efficient Biomimetic\nTotal Synthesis, Characterization,\nand Antioxidant Activity of Ommatins

Ommatins, natural colorants found in cephalopods and\narthropods,\nare biosynthesized from tryptophan with uncyclized xanthommatin (Uc-Xa)\nas the key biosynthetic precursor. These pigments change color under\noxidative or reductive conditions. Xanthommatin (Xa) and dihydro-xanthommatin\n(H<sub>2</sub>-Xa), as well as decarboxylated xanthommatin (Dc-Xa)\nand decarboxylated-dihydro-xanthommatin (Dc-H<sub>2</sub>-Xa), are\nsome of the most common and well-studied ommatins. Herein, we describe\nthe biomimetic total synthesis of Xa/H<sub>2</sub>-Xa on a gram scale\nby using the Mannich reaction and oxidative dimerization as the key\nsteps. The 7-step linear synthetic sequence achieved an overall yield\nof 27%. Dc-Xa/Dc-H<sub>2</sub>-Xa and protected Uc-Xa/Uc-H<sub>2</sub>-Xa were also synthesized from the common intermediate-protected\n3-hydroxykynurenine (3-OHK). The synthesized ommatins underwent thorough\ncharacterization using various spectroscopic techniques, including\nNMR, UV–vis, FTIR, HRMS, and LC–MS. Their optoelectronic\nproperties were studied using spectrophotometry and electrochemical\nanalysis. Furthermore, the antioxidant activity of the synthesized\nommatins was evaluated using an oxygen radical antioxidant capacity\nactivity assay. The results indicated that Dc-Xa exhibited the highest\nantioxidant activity, followed by Xa, while Uc-Xa showed the lowest\nactivity.