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Figure 2: (a) Basic structure of catecholamine, arrow shows commonly present hydroxyl group at position 3 and 4 of the aromatic ring (b) Structure of tapentadol, arrow shows one hydroxyl group at position 3 of the aromatic ring

Figure 2: (a) Basic structure of catecholamine, arrow shows commonly present hydroxyl group at position 3 and 4 of the aromatic ring (b) Structure of tapentadol, arrow shows one hydroxyl group at position 3 of the aromatic ring