THE TOTAL SYNTHESIS OF (+/-)-INDOLIZIDINES 235B AND 235B'

The total synthesis of racemic indolizidines 235B 1 and 235B' 2 is described, in which the key step is the intramolecular thermal cycloaddition of the (Z)-N-alkenylnitrones 10b and 10a, respectively. Cyclisation of the isoxazolidines 12, followed by reductive N-O bond cleavage, epimerisation of the resulting axial hydroxymethyl side chain to the equatorial configuration, and further reduction to a methyl group gave the target molecules. Intramolecular cyclisation of the related nitrone 18, a potential precursor to the solenopsins, was less regioselective, affording a 6:1 mixture of the adducts 19 and 20, which indicates a dependence on the nature of the substituent alpha to nitrogen.