O,O-Silyl Group Migrations in Quinic Acid Derivatives: An Opportunity for Divergent Synthesis

The O,O-silyl group migrations on a quinic acid-derived cyclitol have been studied, and the ease of migration was observed to be dependent on the silicon substituents and reaction conditions. Conditions were found to improve the formation of a main isomer during the O,O-silyl group migrations that could be integrated into the formal synthesis of vitamin D receptor modulator VS-105 and in the first total synthesis of a metabolite from the African ant Crematogaster nigriceps.

Poly-Amido-Saccharides (PASs): Functional Synthetic Carbohydrate Polymers Inspired by Nature

Carbohydrates are ubiquitous in nature, playing vital roles in all organisms ranging from metabolism to intercellular signaling. Polysaccharides, repeating units of small molecule carbohydrates, are hydrophilic, densely functionalized, stereoregular, and rigid macromolecules, and these characteristics are simultaneously advantageous in biomedical applications while presenting major hurdles for synthetic methodology and development of structure property relationships. While naturally obtained polysaccharides are widely utilized in the biochemical and medical literature, their poor physicochemical definition and the potential for contaminated samples hinders the clinical translation of this work. To address the need for new methods to synthesize carbohydrate polymers, we reported a novel class of biomaterials (Poly-Amido-Saccharides; PAS) in 2012. PASs share many properties with natural polysaccharides, such as hydrophilicity, dense hydroxyl functionality, stereoregularity, and a rigid backbone. PASs are connected by an α-1,2-amide linkage, instead of an ether linkage, that confers resistance to enzymatic and hydrolytic degradation and leads to a unique helical conformation. Importantly, our synthetic methodology affords control over molecular weight distribution resulting in pure, well-defined polymers. This Account provides an overview of the development of PAS, from the factors that initially motivated our research to current efforts to translate functional PAS to biomedical applications. We detail the synthesis of glucose- and galactose-based PAS and their biophysical properties including conformation analysis, lectin interactions, cell internalization, and water solubility. Additionally, we describe postpolymerization modification strategies to afford PASs that act as protein stabilizers. We also highlight our recent efforts toward a mechanistic understanding of monomer synthesis via [2 + 2] cycloaddition reactions in order to develop novel monomers with different stereochemistry and amine or alkyl functionality, thereby accessing functional carbohydrate polymers. Throughout our work, we apply computational and theoretical analysis to explain how properties at the monomer level (e.g., stereochemistry, functionality) significantly impact polymer properties, helical conformation, and bioactivities. Collectively, the results from the theoretical, synthetic, and applied aspects of this research advance us toward our goal of utilizing PASs in key biomedical applications as alternatives to natural polysaccharides. The importance of carbohydrates in nature and the versatility of their functions continue to inspire our investigation of new monomers, polymers, and copolymers, leveraging the advantageous properties of PAS to develop potential therapies.

Protecting group migrations in carbohydrate chemistry

Protecting groups are valuable in chemo- and regioselective synthetic manipulations. In particular, they are indispensable in carbohydrate chemistry. Although a wide array of protecting groups are available at the disposal of carbohydrate chemists, their stability and orthogonality make the choice of protecting groups challenging. Another important factor is the migratory aptitude of different protecting groups used in carbohydrate chemistry. Migration of commonly used groups like silyl, acetal and acyl groups under various reaction conditions are discussed. Synthetic application of predicted migrations, alternate protecting groups to avoid migration and conditions favoring and disfavoring migrations are discussed in this review.

Total Synthesis of Caprazamycin A: Practical and Scalable Synthesis of syn-β-Hydroxyamino Acids and Introduction of a Fatty Acid Side Chain to 1,4-Diazepanone

The first total synthesis of caprazamycin A (1), a representative liponucleoside antibiotic, is described. Diastereoselective aldol reactions of aldehydes 12 and 25-27, derived from uridine, with diethyl isocyanomalonate 13 and phenylcarbamate 21 were investigated using thiourea catalysts 14 or bases to synthesize syn-β-hydroxyamino acid derivatives. The 1,4-diazepanone core of 1 was constructed using a Mitsunobu reaction, and the fatty acid side chain was introduced using a stepwise sequence based on model studies. Notably, global deprotection was realized using palladium black and formic acid without hydrogenating the olefin in the uridine unit.

Intermolecular silyl migration reactions

Sodium hydride promoted O-alkylation of 2-[(4- t-butyldimethylsilyloxy)phenyl]ethan-1-ol with 1-bromo-2-(bromomethyl)-4,5-dimethoxybenzene depend on the solvent used in the coupling reaction. Mixtures of 2-[4-(2-bromo-4,5-dimethoxybenzyloxy)phenyl]-1- t-butyldimethylsilyloxyethane and 2-[4-(2-bromo-4,5-dimethoxybenzyloxy)phenyl] ethan-1-ol (in DMF), or 2-[4-(2-bromo-4,5-dimethoxybenzyloxy)phenyl]-1- t-butyldimethylsilyloxyethane and 4-[2-(2-bromo-4,5-dimethoxybenzyloxy)ethyl]phenol (in THF), were detected. These results can be explained by an unusual intermolecular silyl migration reaction.