Direct Access to <i>N</i>-H-Aziridines from Asymmetric Catalytic Aziridination with Borate Catalysts Derived from Vaulted Binaphthol and Vaulted Biphenanthrol Ligands

Abstract

The asymmetric catalytic aziridination reaction (AZ reaction) of N-dianisylmethylimines (N-DAM-imines) with ethyl diazoacetate is developed with chiral catalysts prepared from triphenylborate and both the vaulted binaphthol (VANOL) and vaulted biphenanthrol (VAPOL) ligands. Catalysts derived from both ligands were equally effective in terms of asymmetric induction, but the VANOL catalyst was slightly faster. Up to 400 turnovers could be achieved with the VANOL catalyst while still maintaining>or=90% ee in the aziridine product. The ligand could be recovered in 95% yield with no loss in optical purity. Excellent asymmetric inductions were observed with arylimines, and although slightly lower inductions were observed for alkyl-substituted imines, the optical purity of the aziridines from all of the imine substrates could be enhanced to>or=99% ee with a single crystallization. Methods were developed for deprotection of the N-DAM-aziridines under acidic conditions without causing an acid-promoted opening of the ring. Excellent yields of the N-H-aziridines could be obtained with both alkyl- and aryl-substituted aziridines. Finally, activation of the N-H-aziridines was achieved with Boc, tosyl, and Fmoc groups. The activated aziridines can be converted to beta3-amino esters, and unexpectedly, the N-Boc-protected aziridine-2-carboxylate 16b with a phenyl substituent in the 3-position cis to the ester group was found to undergo ring expansion to a mixture of cis- and trans-oxazolidinones.