| Summary: | A representative naturally occurring coumarin, 4-methylumbelliferone (<b>5</b>), was exposed to 50 kGy of gamma ray, resulting in four newly generated dihydrocoumarin products <b>1</b>–<b>4</b> induced by the gamma irradiation. The structures of these new products were elucidated by interpretation of spectroscopic data (NMR, MS, [α]<sub>D</sub>, and UV). The unusual bisdihydrocoumarin <b>4</b> exhibited improved tyrosinase inhibitory capacity toward mushroom tyrosinase with IC<sub>50</sub> values of 19.8 ± 0.5 μM as compared to the original 4-methylumbelliferone (<b>5</b>). A kinetic analysis also exhibited that the potent metabolite <b>4</b> had non-competitive modes of action. Linkage of the hydroxymethyl group in the C-3 and C-4 positions on the lactone ring probably enhances the tyrosinase inhibitory effect of 4-methylumbelliferone (<b>5</b>). Thus, the novel coumarin analog <b>4</b> is an interesting new class of tyrosinase inhibitory candidates that requires further examination.
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