NOVEL CASCADE REACTIONS OF ALKENYLZIRCONOCENES AND THEIR APPLICATION TO THE SYNTHESIS OF CYCLOPROPYL PEPTIDE MIMETICS

We have successfully applied to Zr→Zn methodology developed in the Wipf group to the preparation of functionalized allylic amides and alcohols via the 1,2-addition to imines and α-keto esters. During the preparation of allylic amides, concomitant formation of C-cyclopropylalkylamides was observed in CH₂Cl₂. The combination of the Zr→Zn methodology with the Simmons-Smith cyclopropanation reaction has led to the discovery of a novel cascade reaction for the preparation of C,C-dicyclopropylmethylamides from simple, readily available starting materials. These functionalized amides have served as precursors in a diversity-oriented approach for the preparation of 7-, 8-, and 9-membered azaspirocyclic ring structures based on reductive amination, epoxide opening or ring-closing metathesis strategies. Finally, a small library of α,β-cyclopropyl-γ-amino acids were prepared in 6-7 steps from readily available starting materials and evaluated for their potential as peptide mimetics. Simple amide derivatives were found to adopt stable sheet-like structures in the solid state, whereas the structural properties of oligopeptides were not readily assessed using crystallographic techniques. A combination of molecular modeling, circular dichroism and NMR studies was used to ascertain the solution folding preferences of our novel peptides