Brønsted-Acid-Catalyzed (3+2)-Cycloannulation of In-Situ-Generated 3-Methide-3H-pyrroles: Asymmetric Synthesis of Cyclopenta[b]pyrroles.

An organocatalytic, highly enantioselective addition of cyclic enamides to in-situ-generated 3-methide-3H-pyrroles with subsequent cyclization and elimination has been developed. This protocol represents a novel and straightforward route toward polycyclic cyclopenta[b]pyrroles with high yields and excellent enantioselectivity. Central to the success is the formation of a chiral, hydrogen-bonded 3-methide-3H-pyrrole upon phosphoric-acid-catalyzed dehydration of the starting 1H-pyrrol-3-yl carbinol that reacts with the enamide in a stepwise cycloannulation process.