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Convenient access to highly enantioenriched substituted quinolizidines has been achieved by chiral dirhodium(II) carboxylate-catalyzed dearomatizing formal 3 + 3-cycloaddition of isoquinolinium/pyridinium methylides and enol diazoacetates. Coordination of Lewis basic methylides to dirhodium(II) prompts the rearrangement of the enol-carbene that is bound to dirhodium to produce a donor-acceptor cyclopropene. The donor-acceptor cyclopropene is in equilibrium with the dirhodium-bound enol-carbene and undergoes both enantioselective 3 + 3-cycloaddition from the dirhodium-bound enol-carbene and diastereoselective 3 + 2-cycloaddition by uncatalyzed reaction of the cyclopropene with isoquinolinium or pyridinium methylides. Increasing the mol % of catalyst loading suppresses the 3 + 2-cycloaddition pathway.
Xu et al. (Thu,) studied this question.