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
°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.
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