Carbohydrates are some of the most abundant organic molecules on earth. They are ubiquitous compounds, most commonly known for providing energy for working and are biologically stockpiled as glycogen. Modern Era advancement in disciplines of glycomics, glycobiology, and bio-renewable studies has played a pivotal role in generating robust interest in the study of carbohydrate chemistry. The advances in the field have progressed to cover an expansive range of scientific areas. For example, there is strong interest in i) developing synthetic methodology, ii) understanding reactivity, iii) performing structural studies, iv) exploring enzyme catalysis, v) biological and medical applications, as well as exploiting their use in food and polymer industries. From a biological perspective it is recognized that carbohydrates play a fundamental role in cell recognition, cell-cell interaction, cell differentiation, cell growth, survival of living cells and organisms. Synthetically, a variety of natural products and antibiotics contains carbohydrate moieties, providing a vast platform for glyco-based synthesis. In addition, biochemical inquiries over last few decades have established the association of carbohydrates with various disease conditions and identified glycan(s) as untapped tools in drug discovery. Development of the carbohydrate-based drug discovery can be categorized by one of the following approaches. i) producing native glycans with desired biological activity, ii) structural modification via glycomimetic approaches, iii) generating a glyco-scaffold for achieving carbohydrate diversity, and iv) developing glyco-fused therapeutics.
The scope of work covered in this dissertation covers several carbohydrate-related areas. First, we desired to i) develop a series of ß-glycosyl amide linkages demonstrating the use of ß-glycosyl-2,4-dinitrobenzenesulfonamides as key intermediates in the chemoselective formation of these motifs, ii) produce glycosyl fluoride moieties to study their potential inhibitory activity against Mycobacterium tuberculosis (Mtb) GlgE, iii) generate oligosaccharides to study their interaction with the glucan binding site of Mtb GlgE, and iv) demonstrate the utilization of furfural (degradation product of pentose sugars from hemicellulose, bio-resource) to access a bisfuran monomer, in order to generate furan based polymeric materials.
Glycosyl amide linkages are key motifs found in glycosyl amino acids, N-linked glycopeptides besides several natural products. Some of the popular strategies to access these linkages include Lansbury aspartylation involving direct coupling of glycosylamine with activated aspartic acid on protected amino acid. Alternatively, a route to access ß-glycosyl amide linkage is via Staudinger reduction-acylation or traceless Staudinger ligation of ß-glycosyl azide. This method ends up producing a mixture of a/ß glycosyl amides. 2,4-Dinitrobenzenesulfonyl chloride (dNBS-Cl) has been used to protect the primary amines producing the 2,4-dinitrobenzenesulfonamides (dNBS-NH2, dNBS). dNBS can be used to produce secondary amines and diamines as noted by Fukuyama et al. dNBS were later explored for producing amides, thioamides, ureas, and thioureas, and only very recently this strategy has been taken as a possible chemoselective approach to form peptide linkage and neoglycoconjugates. To further this chemistry in our chemoselective studies producing the ß-glycosyl amides, we explored the scope of this methodology by reacting a series of ß-dNBS glycosides with thioacetic acid. The N-glycosyl-2,4-dinitrobenzenesulfonamides were accessed via benzoyl-protected ß-glycosyl azides. The azides were i) reduced with Adams’ catalyst to the corresponding amines, ii) the glycosylamines were sulfonated with 2,4-dinitrobenzenesulfonyl chloride to form N-glycosyl-2,4-dinitrobenzenesulfonamides in moderate yields, and iii) the ß-glycosyl amides were then prepared in 67 - 81 % yields by treatment of the sulfonamides with thioacetic acid and cesium carbonate. The conversion of the glycosylsulfonamide to the glycosyl amide proceeded with high stereoselectivity.
A second project area involved new approaches to preapare oligosaccharide. Mtb is the causative agent of tuberculosis (TB), which is one of the leading cause of death due to an infectious disease. Designing susbtrates, inhibitors and biochemical probes to understand and overcome this disease condition is of significant interest. Mtb GlgE an a-1,4-glucan: phosphate maltosyltransferase is a genetically validated tuberculosis target. Underlining this target, we designed glycosyl fluoride derivatives, as biochemical probes to determine the active-site residues and better understand the Mtb GlgE catalytic mechanism. 5-fluoro-L-idosyl fluoride, an epimer of 5-fluoro-D-glucosyl fluoride has been synthesized over 5-steps. Starting with D-glucose a key step was photobromination at the C-5 using NBS and AIBN followed by substituting the bromide with fluoride using AgF.
Mtb GlgE transfers the maltosyl units on to the elongating the a-1,4-glucan polymer. As a concurrent project we sought to access the oligosaccharide units mimicking the a-glucan polymer in the Mtb GlgE. The oligosaccharides are envisioned to be useful for probing the a-glucan binding site. During the oligosaccharide synthesis, we propose a stereoselective glycosylation strategy was pursued applying a preactivation methodology. Starting with D-maltose, the building blocks for the a-1,4-glucan oligosaccharide were accessed via a divergent synthetic route by creating the donor (4’,6’-O-benzylidene-ß-maltosyl thioglycoside) and the acceptor (4’-OH-ß-maltosyl thioglycoside) substrates. During synthesis the donor is first accessed, which was further regioselectively reduced at the 4’,6’-O-benzylidene to create 4’-OH (non-reducing end), generating the acceptor substrate. Glycosylation between the donor and acceptor substrate is then envisioned to proceed using a [2+2] preactivated method to build the tetrasachharide unit. This tetrasaccharide on regioselective reduction at the 4’,6’-O-benzylidene creates the 4’-OH (non-reducing end), which then acts as a template to produce [4+2] hexasaccharide and/or [4+4] octasaccharide, and so on to access a-1,4-glucan oligosaccharide units of desired length.
In a final area of research, carbohydrates were explored as feedstocks for polymer synthesis Starch is commonly understood as a first generation biorenewable feedstock and lignocellulose as second generation biorenewable. Lignocellulose consists of cellulose (40 – 45 %), hemicellulose (24 – 40 %), and lignin (18 – 25 %). Hemicellulose consists chiefly of the pentose sugar xylose and arabinose. The degradation of these pentose sugars yields furfural. The latter is considered to be a leading candidate for bio-renewable building blocks and as a platform chemical for its application in diverse commercial sectors. In the same direction there is a substantial interest and progress in the utilization of this bio-based feedstock to fulfill the growing demands in the sector of fuel, chemical, pharmaceutical and polymer industries. As a part of our preliminary investigation, starting with commercial furfural, we have designed a four steps synthetic route to access a bisfuran diol (BFD) as a monomer feedstock to generate polymeric materials. Starting with commercial furfural i) reaction with 1,2-ethanedithiol in recyclable glycerol produced the 1,3-dithiane protected furfural in 93 % yield after distillation, ii) treatment with acetone and 50% aqueous sulfuric acid produced a dimerized Friedel Crafts product in 58% yield, iii) deprotection of the 1,3-dithiane was accomplished using selenium dioxide along with acetic acid to generate dialdehyde in 90% yield, and iv) the penultimate dialdehyde was reduced to afford the final diol, the BFD in 96 % yield. BFD was crystallized in the monoclinic space group and X-ray studies were undertaken to understand the molecular arrangement in space, hydrogen bonding and packing of the molecule. These set of data were compared with Bisphenol A (BPA, a known endocrine disruptor), and its known derivatives to predict the potential estrogenic or anti-estrogenic activities in BFD. Further, BFD was employed in polymerization studies to successfully generate bisfuran polyester (BFPE-1), a low-molecular weight novel polyester material of amorphous nature.