CONSTRUCTING NANOSTRUCTURES WITH ATOMIC PRECISION: THE SYNTHESIS OF SPIROLIGOMER-BASED MACROCYCLES

Abstract

This dissertation presents the development of a synthetic strategy to produce various spiroligomer-based macrocycles that bridge the gap between organic molecules and small proteins. “Spiroligomers” (formerly known as “bis-peptides”) are a class of molecules produced by the assembly of “bis-amino acids”, molecules containing two amino acid regions on a single cyclic core. Each bis-amino acid is connected through pairs of amide bonds to form a diketopiperazine consequently eliminating single bond rotation and, therefore, avoids the complicated folding process common to the field of peptidomimetics. Spiroligomers are shape-programmable since the three-dimensional structure is controlled by the stereochemistry of the bis-amino acid monomers used in the synthesis, the connectivity of the monomers, and the number of monomers used. Furthermore, bis-amino acids can contain additional functional groups attached to multiple locations on the monomer which allows each spiroligomer, once synthesized, the ability to display these functional groups in predictable three-dimensional coordinates, with respect to each other. The synthesis of large spiroligomer-based structures requires the production of large amounts of bis-amino acid monomers. To this end, the scale of the synthesis of proline-based bis-amino acids from inexpensive trans-4-hydroxy-L-proline has been increased roughly 5-fold with respect to the previously published method. In addition to the time and solvent savings as a result of increasing the scale, the synthetic steps have been altered with considerations to ensure the production takes place in a convenient and environmentally friendly manner. Additionally, the desire to synthesize large spiroligomer-based structures means that the synthesis of each spiroligomer fragment must be as efficient and high-yielding as possible. To achieve this goal, a new synthetic approach to highly functionalized spiroligomers on solid support has been developed that results in increased yields relative to previously published methods. This new approach makes use of a protecting group, para-nitrobenzyl carbamate, which has not previously been incorporated in bis-amino acids as well as a pentafluorophenol ester activation strategy that also has not been in the synthesis of spiroligomers. Finally, an extendable synthetic route to spiroligomer-based macrocycles has been developed and representative macrocycles have been synthesized. This approach uses solid support to assemble multiple spiroligomers together through amino acids linkers before being cyclized in solution at dilute concentration to yield the desired macrocycles. Minimal functionality was included in the representative macrocycles to simplify structural information, confirmed by NMR and other means, and the macrocyclic structures were further investigated for host-guest activity using fluorescent, solvatochromic dyes.