undergoes keto–enol tautomerism to form (S)-scoulerine, which is then methylated by SAM to form (S)-tetrahydrocolumbamine 7. Product 7 is then oxidized to form the methylenedioxy ring from the ortho-methoxyphenol, via an O2-, NADPH- and cytochrome P450-dependent enzyme, giving (S)-canadine 8. (S)-canadine is then oxidized to give the quaternary isoquinolinium system of berberine. This happens in two separate oxidation steps, both requiring molecular oxygen, with H2O2 and H2O produced in the successive processes. References Aromatase inhibitors Benzodioxoles CYP2D6 inhibitors CYP3A4 inhibitors DNA-binding substances Benzylisoquinoline alkaloids Nitrogen heterocycles Quaternary ammonium compounds