Epoxidation in vivo of hyoscyamine to scopolamine does not involve a dehydration step

Hyoscyamine is epoxidized to scopolamine via 6β-hydroxyhyoscyamine in several solanaceous plants. 6,7-Dehydrohyoscyamine has been proposed to be an intermediate in the conversion of 6β-hydroxyhyoscyamine to scopolamine on the basis of the observation that this unsaturated alkaloid is converted to scopolamine when fed to a Datura scion. To determine whether a dehydration step is involved in scopolamine biosynthesis, 6-¹⁸O]6β-hydroxyhyoscyamine was prepared from l-hyoscyamine and ¹⁸O₂ using hyoscyamine 6β-hydroxylase obtained from root cultures of Hyoscyamus niger L. When 6-¹⁸O]6β-hydroxyhyoscyamine was fed to shoot cultures of Duboisia myoporoides R. B_R., the labeled alkaloid was converted to scopolamine which retained ¹⁸O in the epoxide oxygen. It is concluded that 6β-hydroxyhyoscyamine is converted in vivo to scopolamine without a dehydration step.