The biosynthesis of the tropane alkaloid hyoscyamine in datura stramonium

Abstract

This thesis investigates the biosynthesis of the tropane alkaloid hyoscyamine in Datura stramonium. The biosynthesis of the medicinally important tropane alkaloid hyoscyamine has been investigated for many years. However, the complete biosynthetic pathway is still to be unravelled. This thesis concentrates primarily on the last step of hyoscyamine biosynthesis - the rearrangement of littorine to hyoscyamine. In Chapter 1, an introduction to the biosynthesis of hyoscyamine and other related alkaloids is discussed. In Chapter 2, (^18)O-labelling studies are utilised in order to probe the nature of the rearrangement. The results reveal that intriguingly that the rearrangement is uidirect and perhaps involves an aldehyde intermediate, which requires to be reduced by a dehydrogenase to furnish hyoscyamine. In Chapter 3, aryl substituted fluorophenyllactates were used in order to probe the mechanism of the rearrangement of littorine to hyoscyamine. The working hypothesis suggests the involvement of a carbocation uitermediate following literature reports on chemical and enzymatic models. The results reveal that the mechanism of the rearrangement is not as clear cut as predicted, and that perhaps binding affinities to the littorine synthase and littorine mutase affect the efficiency of the rearrangement. In Chapter 4, attention turned back to the tropane moiety of hyoscyamine. Its biosynthesis has been thoroughly investigated although there still remains the mystery as to how acetate units are condensed with the N-methylpyrrolinium salt to generate the bicyclic tropane ring.