Synthesis of medium-bridged twisted lactams via cation-pi control of the regiochemistry of the intramolecular Schmidt reaction.

Medium-bridged twisted amides can be synthesized by the intramolecular Schmidt reaction of 2-azidoalkyl ketones. In these reactions, the regiochemistry of the Schmidt reaction is diverted into a typically disfavored pathway by the presence of an aromatic group at the alpha-position adjacent to the ketone, which stabilizes the predominantly reactive conformation of the azidohydrin intermediate by engaging in a nonbonded cation-pi interaction with the positively charged diazonium cation. This results in the rarely observed rearrangement of the C-C bond distal to the azidoalkyl chain. This reaction pathway also requires the azide-containing tether to be situated in the axial orientation in the key azidohydrin intermediate. Examination of the effect of substitution of aromatic rings on the regiochemistry of the Schmidt reaction shows an increase in the migratory selectivity with more electron-rich aromatic groups. The selectivity is lower when an electron-withdrawing substituent is placed on the aromatic ring. The ability of cation-pi interactions to act as a controlling element decreases when Lewis acids coordinate to substituents on the aromatic ring. The developed version of the Schmidt reaction provides a direct access to a family of medium-bridged twisted amides with a [4.3.1] bicyclic system, compounds which are very difficult to access with use of other currently available methods.