Desymmetrization of N-sulfonated aziridines by alkyllithium reagents in the presence of chiral ligands

The stereochemical course of the rearrangement of the N-sulfonylaziridines 5, 15, and 25 in the presence of s-BuLi/(-)-sparteine to the bicyclic sulfonamides 4, 16, and 17, respectively, has been investigated chemically and by X-ray structure analysis. The absolute configurations of the products were, in all cases, opposite to those of the alcohols formed upon rearrangement of the corresponding epoxides. Similarily, the allylic sulfonamide 10, resulting from rearrangement of 7-[(4-methylphenyl)sulfonyl]-7-azabicyclo[4.1.0]heptane (7) under the same reaction conditions, had the (R)-configuration, while cyclohexen-3-ol, obtained upon rearrangement of cyclohexene oxide, is known to be (S)-configured. Deuterium labelling showed that the rearrangement of 7 proceeds via enantioselective alpha-elimination to a carbene, which undergoes a 1,2-H shift.