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A diastereodivergent approach for the stereoselective syntheses of all four stereoisomers of galanthamine, (−)-galanthamine 1 , (+)-galanthamine 2 , (−)-epigalanthamine 3 , and (+)-epigalanthamine 4 , from (±)-narwedine 5 is reported. Thus (±)-narwedine 5 was resolved by dynamic kinetic resolution to obtain enantiomerically pure (−)-narwedine 6 and (+)-narwedine 7 . Each enantiomerically pure isomer of narwedine was subjected to Luche and L-selectride reactions to obtain all four isomers of galanthamine. In these reactions, the (−)-galanthamine 1 and (+)-galanthamine 2 isomers were obtained with an enantiomeric purity of >99.5%, whereas (−)-epigalanthamine 3 and (+)-epigalanthamine 4 are obtained with a chiral purity of >97%. The axial hydride attack by the Luche reduction and the equatorial hydride attack by the L-selectride reduction on the cyclic enone system are explored in the stereoselective synthesis of the galanthamine isomers and thus it was demonstrated that the stereoselective synthesis involving the Luche and L-selectride reductions are complementary in yielding enantiomeric stereogenic centers from a prochiral carbonyl group on the cyclic enone system. |
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