Alkaloid syntheis via novel azabicycles.

A basic introduction to pyrrolizidine and indolizidine alkaloids has been described along with a selection of recent syntheses of the said compounds.Cycloalkene synthesis by intramolecular Wittig reaction has been reviewed and we describe the utility of this strategy in the formation of nitrogen-bridgehead bicycles which can then be used in alkaloid synthesis. Our initial studies on the viability of this strategy in the synthesis of fused pyrrolidone systems employed a Wittig reaction between 5-acetylpyrrolidin-2-one and vinylphosphonium salts. A comparison was made between three different vinylphosphonium salts but in each case the bicycle formed was present as a mixture with the Wittig by-product triphenylphosphine oxide. Various solutions to this problem were investigated including modification of the bicycle and also the formation of a water soluble vinylphosphonium salt. The difficulties encountered led us to the use of a vinylphosphine oxide which resulted in the formation of 5,6,7,7a-tetrahydro-1-methyl-2-phenylthio-3H-pyrrolizin-5-one in good yields. Desulphurisation gave 5,6,7,7a-tetrahydro-1-methyl-3H-pyrrolizin-5-one which has previously been converted into (+/-)-supinidine. Thus our method constitutes a formal total synthesis of (+/-)-supinidine.Attempts at the preparation of analogues of 5-acetylpyrrolidin-2-one were unsuccessful but were in their preliminary stages at the close of the work.Reaction of 5-acetylpyrrolidin-2-one with cyclopropylphosphonium salts were attempted without success but it is felt that the use of cyclopropylphosphine oxides would solve this problem. Attempts at performing an intramolecular Wittig reaction on the imide carbonyl of succinimide using both cyclopropylphosphonium salts and butadienylphosphonium salts were unsuccessful but our investigations gave an insight into the cause of the failure forcing conditions need to be employed to effect the cyclisation.