Natural and semisynthetic oxyprenylated aromatic compounds as stimulators or inhibitors of melanogenesis

It has been very recently shown how naturally occurring oxyprenylated coumarins are effective modulators of melanogenesis. In this short communication we wish to generalize the potentialities as skin tanning or whitening agents of a wider panel of natural and semisynthetic aromatic compounds, including coumarins, cinnamic and benzoic acids, cinnamaldehydes, benzaldehyde, and anthraquinone derivatives. A total number of 43 compounds have been tested assaying their capacity to inhibit or stimulate melanin biosynthesis in cultured murine Melan A cells. The wider number of chemicals herein under investigation allowed to depict a detailed structure-activity relationship, as the following: (a) benzoic acid derivatives are slightly pigmenting agent, for which the effect is more pronounced in compounds with longer O-side chains; (b) independently from the type of substitution, cinnamic acids are able to increase melanin biosynthesis, while benzaldehydes are able to decrease it; (c) coumarins with a 3,3-dimethylallyl or shorter skeletons as substituents in position 7 are tanning agents, while coumarins with farnesyloxy groups are whitening ones; (d) double oxyprenylation in position 6 and 7 and 3,3-dimethylallyl or geranyl skeletons have slight depigmenting capacities, while farnesyl skeletons tend to marginally increase the tanning effect; (e) the presence of electron withdrawing groups (acetyl, COOH, and -Cl) and geranyl or farnesyl oxyprenylated chains respectively in positions 3 and 7 of the coumarin nucleus lead to a whitening effect, and finally (f) oxyprenylated anthraquinones have only a weak depigmenting capacity.