Enantioselective synthesis of functionalized tropanes by rhodium(II) carboxylate-catalyzed decomposition of vinyldiazomethanes in the presence of pyrroles

A series of enantiomerically enriched tropanes was synthesized by the rhodium(II) octanoate-catalyzed reaction of various N-BOC-protected pyrroles with vinyldiazomethanes. The overall 3 + 4 annulation occurs by a tandem cyclopropanation/Cope rearrangement. Asymmetric induction was best achieved in these transformations by using either (S)-lactate or (R)-pantolactone as a chiral auxiliary on the vinyldiazomethanes. Reactions carried out with the chiral catalyst tetrakis[N-( 4-tert-butylbenzenesulfonyl)-(L)-prolinato]dirhodium (2) provided moderate asymmetric induction, but also resulted in the formation of isomeric azabicyclooctane side products. The utility of the synthetic process was demonstrated through the asymmetric synthesis of(-)-anhydroecgonine methyl ester and (-)-ferruginine.