Tyrosinase Inhibitors from Rhododendron collettianum and Their Structure-Activity Relationship (SAR) Studies

A new coumarinolignoid 8′-epi-cleomiscosin A (1) together with the new glycoside 8-O-β-D-glucopyranosyl-6-hydroxy-2-methyl-4H-1-benzopyrane-4-one (2) have been isolated from the aerial parts of Rhododendron collettianum and their structures determined on the basis of spectroscopic evidences. Tyrosinase inhibition study of these compounds and their structure–activity relationship (SAR) were also investigated. The compounds exhibited potent to mild inhibition activity against the enzyme. Especially, the compound 1 showed strong inhibition (IC50=1.33 μM) against the enzyme tyrosinase, as compared to the standard tyrosinase inhibitors kojic acid (IC50=16.67 μM) and L-mimosine (IC50=3.68 μM), indicating its potential used for the treatment of hyperpigmentation associated with the high production of melanocytes.