Syntheses of strychnan- and aspidospermatan-type alkaloids. 11. Total syntheses of (-)-lochneridine and (-)- and racemic 20-epi-lochneridine

Article abstract of DOI:10.1021/jo981401o

Enantioselective syntheses of (-)-lochneridine (1) and (-)-epi- lochneridine (2) were obtained by two alternative C-20 diastereoselective syntheses of the respective pentacyclic (3aS,5S,11bR)-methyl 12-oxo-3,5- ethanol-2,3,3a,4,5,7-hexahydro-3H-pyrrolo[2,3-d]carbazole-6-carboxylate ((- )-8) and tetracyclic methyl (2S,3aS,5R,11bR))-3-benzyl-2,3,3a,4,5,7- hexahydro-5-(2-(1-butenyl))-1H-pyrrolo[2,3-d]carbazole-6-carboxylate ((-)- 12b) precursors from tryptophan derived 2-[(methoxycarbonyl)methyl]-3-[2(S)- (methoxy or benzyloxycarbonyl)-2-(N(b)-benzyl)aminoethyl]indole (15 or 16). While a modified Grignard reaction of the ketone (-)-8 provided (-)- lochneridine (1), dihydroxylation and cyclization of the olefin ()-12b gave its C-20 epimer (2).