Tyrosinase inhibitory cycloartane type triterpenoids from the methanol extract of the whole plant of Amberboa ramosa Jafri and their structure–activity relationship

New tyrosinase inhibitory cycloartane triterpenoids have been discovered from the methanol extract of the whole plant of Amberboa ramosa (Roxb.) Jafri, which is a member from the Compositae family. Utilizing the conventional spectroscopic techniques, including 1D and 2D NMR analysis, and also by comparing the experimental with literature data, the isolated compounds proved to be cycloartane type triterpenoids. These cycloartanes are: (22R)-cycloart-20, 25-dien-2α3β22α triol (1), (22R)-cycloart-23-ene-3β, 22α, 25-triol (2), cycloartenol (3), cycloart-23-ene-3β, 25-diol (4), cycloart-20-ene-3β, 25-diol (5), cycloart-25-ene-3β, (22R) 22-diol (6), 3β, 21, 22, 23-tetrahydroxy-cycloart-24 (31), 25 (26)-diene (7), and (23R)-5α-cycloart-24-ene-3β, 21, 23-triol (8). Out of these eight compounds, compound 3 did not show any activity against the enzyme tyrosinase. Among them compound 7 was found to be the most potent (1.32 μM) when compared with the standard tyrosinase inhibitors kojic acid (16.67 μM) and l-mimosine (3.68 μM). Finally in this paper, we have discussed the structure–activity relationships of these molecules.

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