STEREOSELECTIVE SYNTHESIS OF (+/-)-INDOLIZIDINES 167B, 205A, AND 207A - ENANTIOSELECTIVE SYNTHESIS OF (-)-INDOLIZIDINE 209B

The first syntheses of the dendrobatid indolizidine alkaloids 167B (3), 205A (4), and 207A (5) are described using as a key step the highly stereoselective intramolecular nitrone cycloaddition of the (Z)-N-alkenylnitrone 10 to prepare the isoxazolidine 11. Mesylate-promoted cyclization of the alcohol 12, followed by reductive cleavage of the resulting mesylate salt, afforded the key axial hydroxymethyl compound 13, which was epimerized via the aldehyde to the equatorial alcohol, and was subsequently reduced to the required 8-methyl-substituted indolizidine. The feasibility of extending this strategy to the enantioselective synthesis of such alkaloids was demonstrated in the first synthesis of (-)-indolizidine 209B (6), whose nitrone precursor 10d was obtained from the (S)-glutamate-derived amine 40.