Synthesis of New Anti-melanogenic Compounds Containing Two Molecules of Kojic Acid

Tyrosinase is a copper-containing enzyme, which catalyzes two distinct reactions of melanin synthesis, the hydroxylation of L-tyrosine to L-dopa and the oxidation of L-dopa to dopaquinone. These processes are involved in local hyperpigmentation such as melasma and lentigo. Therefore, the inhibition of tyrosinase to treat the pigmentation disorders has been a recent subject of many studies. Kojic acid (1) is well known as one of the most popular tyrosinase inhibitors and it has been widely used as a skin-whitening agent. However, the inhibitory activity of kojic acid is not potent enough. To overcome this drawback, many semi-synthetic kojic acid derivatives were synthesized usually by the modification of C-2 hydroxyl group into ester, hydroxyphenyl ether, and glycoside. Recently, two molecules of kojic acid were connected by ester and ethylene. Their inhibitory activities were dramatically enhanced. Thus we became interested in new kojic acid derivatives containing two molecules of kojic acid. In this study, we connected two molecules of kojic acid by various chemical bonds such as ester, amide, and thioether. Anti-melanogenic activities of compounds (4a-4f) were compared with that of kojic acid (Figure 1).