N-ALKENYL NITRONE DIPOLAR CYCLOADDITION ROUTES TO PIPERIDINES AND INDOLIZIDINES .5. PREPARATION OF A GEPHYROTOXIN PRECURSOR

The synthesis of the B/C indolizidine ring skeleton of the alkaloid gephyrotoxin 3 is described. The route involved oxidative cleavage of the racemic bicyclic isoxazolidine 2 to form the nitrone 4 and its dipolar addition with methyl acrylate to give the adduct 7. Reductive cleavage of 7 and cyclisation gave the lactam 21 which was deoxygenated and converted into the toluene-p-sulfonate 37. Attempts to effect the intramolecular sulfone alkylation of 37 to form ring A of gephyrotoxin are summarised.