DIVERGENT PATHWAYS IN THE INTRAMOLECULAR REACTIONS BETWEEN RHODIUM-STABILIZED VINYLCARBENOIDS AND PYRROLES - CONSTRUCTION OF FUSED TROPANESAND 7-AZABICYCLO[4.2.0] OCTADIENES

The rhodium(II)-catalyzed intramolecular reaction between vinyldiazome thanes and pyrroles leads to a novel synthesis of fused tropanes. The reaction occurs by a stepwise 3 + 4 annulation mechanism between a rho dium-stabilized vinylcarbenoid intermediate and the pyrrole rather tha n by the typical tandem cyclopropanation/Cope rearrangement sequence. The outcome of the reaction is very sensitive to the vinylcarbenoid st ructure. In particular, the presence of a 2-siloxy substituent on the vinylcarbenoid strongly favors the formation of fused tropanes. In the absence of such functionality, the formation of fused 7-azabicyclo[4. 2.0]octadienes becomes the dominant reaction pathway.