New cycloadditions of dihydroimidazolinium ylides

The thesis introduces the azepinoindole as a subunit of the Stemona alkaloid family of natural products, and outlines a cycloaddition strategy towards this heterocycle. The strategy is based on azomethine imine cycloaddition, and the generation and reactivity of azomethine imines are introduced. Previous work on 1 ,3-dipolar cycloaddition in one-pot, on l-benzyl-4,5-dihydroimidazole (113a) and l-benzyl-4-phenyl-4,5-dihydroimidazole (113b) as starting materials with ester-activating dipolarophile and giving achiral and chiral pyrrolidines, respectively is discussed. The work carried out during the course of this PhD. has been on the 1,3-dipolar cycloadditions of new dihydroimidazolium ylides prepared by N-alkylation of different Nsubstituted dihydroimidazoles. Several new N-substituted dihydroimidazoles have been prepared both achiral and chiral. New routes to the chiral dihydroimidazoles have been developed. This thesis shows the various cycloaddition reactions that have been performed using esteractivated and sui fur-activated dipolarophiles with different N-substituted achiral dihydroimidazoles giving achiral cycloadducts. The ester-activated dipolarophiles gave endo-cycloadducts whereas the sui fur-activated dipolarophiles have shown exocycloadducts. When the cycloaddition was repeated using sui fur-activated dipolarophiles but this time with different chiral N-substituted dihydroimidazoles, an unexpected enda stereochemistry was observed with all except one adduct. Structural studies on the chiral cycloadducts indicated that racemisation had occurred at an early stage in the precursor dihydroimidazole synthesis. Work has been carried out for the extension of the side chain of the pyrrolidine allowing for the formation of six and seven-membered rings required in the synthesis of Stemana alkaloids.