On the use of C2-symmetric aziridines as chiral auxiliaries

Article abstract of DOI:10.1016/S0040-4020(01)85546-0

A systematic study has been made of the utility of readily available C₂-symmetric aziridines as auxiliaries for asymmetric alkylation and aldol reactions of amide enolates. Aziridines with suitably placed oxygen atoms in the side chains proved to be useful for alkylation reactions (d.e. values up to >98%) and the results are explained in terms of an intramolecularly chelated Z-enolate species, which could be observed directly by means of NMR spectroscopy. In contrast, aziridine auxiliaries lacking side-chain oxygens performed better in aldol reactions (syn selectivity up to 98% d.e.) for which a Zimmerman-Traxler transition state is proposed. After reaction, the axuiliaries can be cleaved off non-destructively under mild conditions to afford either optically pure aldehydes or carboxylic acids.