The Attempted Synthesis of Indolizidine and Pyrrolizidine Natural Products

Aza-sugars are naturally occurring polyhydroxylated alkaloids in which the ring oxygen is replaced by nitrogen. They are reported to have a wide range of biological properties, most importantly as glycosidase inhibitors; these glycosidases play a key role in various diseases like HIV, cancer and lysosomal storage disorders.

This thesis will describe an approach to the synthesis of analogues and precursors of azasugar natural products in the indolizidine (for example castanospermine) and pyrrolizidine (for example hyacinthacine) using cyclopropenones and cyclic imines as key intermediates.

This thesis contains work that is an extension of the work pioneered by Eicher and Heimgartner and followed by our group for the reaction of cyclic imines with diphenylcyclopropenone. The methodology was extended towards the synthesis of more complex bicyclic heterocycles like indolizidine and pyrrolizidine aza-sugars and is summarised by the following Scheme. In this thesis, cyclopropenones other than diphenylcyclopropenone were used.

This work also extended the range of cyclic imines that can be reacted by using for the first time, the parent aldimines, polyhydroxylated cyclic aldimines synthesised from sugars and other substituted cyclic imines. The reactions gave bicyclic products but always with an extra oxygen at the bridge head postion (X= OH) via aerial oxidation of the initial product (X= H).