Synthesis and Characterization of an Isoleucine-Containing Macrocycle

Abstract

Large drugs-including macrocycles-are receiving more attention due to the belief that there may be therapeutic targets available to them that are unavailable to small molecules. Here, the synthesis of an isoleucine-containing macrocycle comprising 24 atoms utilizes a three-step synthesis with intermediates that can all be isolated, purified, and characterized. The choice of isoleucine reflects a desire to understand how a large, beta-branched amino acid affects synthesis and conformation. The monomer is available in two reactions. First, a sequential substitution of cyanuric chloride with BOC-protected hydrazine, isoleucine, and dimethylamine gives the carboxylic acid intermediate. The three reactions are performed in a single reaction vessel and are monitored by thin-layer chromatography to ensure that each substitution was successful. The carboxylic acid intermediate is purified using silica gel chromatography. Then, an EDC-mediated coupling reaction between the acetal and an aminoacetal gives the monomer. The monomer can be envisioned to be crescent-shaped. Spontaneous dimerization is initiated by treating the monomer with trifluoracetic acid. Oligomeric and polymeric materials are not observed in synthesis. NMR spectroscopy confirms the successful synthesis of each intermediate and the macrocycle. Both COSY and rOesy spectra are used to probe the change in conformation as a function of solvent. Finally, NMR spectra are measured at various temperatures to help understand the dynamic behavior of the macrocycle.