Synthesis and absolute stereochemistry assignment of enantiopure dihydrofuro- and dihydropyrano-quinoline alkaloids

Abstract

The chiral quinoline alkaloids platydesmine 3, platydesmine methosalt 4 and edulinine 9, have been synthesised in enantiopure form via asymmetric dihydroxylation of the achiral alkaloid atanine 1. Chromatographic separation of MTPA diastereoisomers 20 formed from racemic bromohydrin derivatives of atanine 1 was a key step in the synthesis of geibalansine 7, edulinine 9, ribalinine 10, Ψ-ribalinine 11 and araliopsine 12 as single enantiomers. The absolute configurations of (+)-platydesmine methosalt 4 and (−)-Ψ-ribalinine 11 were unequivocally determined by X-ray crystallography while stereochemical correlation and circular dichroism spectroscopy methods were used to assign absolute configurations to platydesmine 3, geibalansine 7, ribalinine 10, araliopsine 12 and edulinine 9. Possible errors which earlier led to the incorrect assignment of absolute configurations of the quinoline alkaloids platydesmine 3, platydesmine methosalt 4, edulinine 9, araliopsine 12 and other related chiral quinoline alkaloids are discussed.