Journal of the American Chemical Society, Vol.118, No.41, 9876-9883, 1996
An Asymmetric Route to the Conanine Bcde Ring-System - A Formal Total Synthesis of (+)-Conessine
The first enantioselective synthesis of the known (+)-conessine precursor (+)-benzohydrindan 23 from the chiral nonracemic bicyclic lactam 1 is described, The key transformation was the highly diastereoselective (3 + 2) cycloaddition of azomethine ylide 11a to lactam 7 in order to construct the pyrrolidine E ring system at any early stage in the synthesis. The requisite pyrrolidine methyl group at C-21 was stereoselectively installed late in the synthesis by lithiation of N-Boc-pyrrolidine intermediate 20 with sec-BuLi/TMEDA followed by quenching at -90 degrees C with iodomethane, furnishing the tetracyclic pyrrolidine 21. Reduction of t-Boc 21 with lithium aluminum hydride; affords (+)-N-methyl tetracycle 23 in a concise 13-step synthesis from 1.
Keywords:CHIRAL BICYCLIC LACTAMS;AZOMETHINE YLIDE EQUIVALENTS;HIGH-PRESSURE;ENANTIOSELECTIVE SYNTHESES;CONFIGURATIONAL STABILITY;ALPHA-AMINOORGANOLITHIUMS;FORMAMIDINES;ALKYLATIONS;METALATION;REDUCTION