Highly Enantioselective Catalytic [6+3] Cycloadditions of Azomethine Ylides

Marco Potowski, Jonathan Bauer, Carsten Strohmann, Andrey Antonchick, Herbert Waldmann
Angew. Chem. Int. Ed.
DOI: 10.1002/anie.201204394

Cycloaddition reactions are very useful for generating multiple stereocentres in one step. The fact that they also provide enantiocontrol with the use of chiral Lewis acids or auxiliaries makes these reactions very attractive in most aspects of organic synthesis.

Waldmann and co-workers have reported an enantioselective [6+3] cycloaddition of azomethine ylides with fulvenes using a metal salt with a chiral ferrocene-derived ligand to provide stereocontrol.

Azomethine ylides are readily formed by β-carbonyl imines, which can be prepared simply by condensation between an amino acid ester and an aldehyde.

Waldmann’s group took things a step further; they used the product of the enantioselective [6+3] cycloaddition in a subsequent [4+2] cycloaddition. The second cycloaddition proceeds diastereoselectively allowing the construction of eight stereocentres – up to 98:2 er – in one pot.

A remarkable degree of complexity can be achieved in a single process from three simple starting materials.

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