Publicação
Synthesis of Building Blocks for Assembly of Protozoan Galactofuranose-containing Surface Glycans
| Resumo: | Glycosylphosphatidylinositols (GPIs) are eukaryote glycolipids which attach proteins and glycans to the cell membrane. Trypanosomatide parasites of the Trypanosoma cruzi (T. cruzi) species and the Leishmania genus, the etiologic agents of American trypanosomiasis (also known as Chagas' Disease, ChD) and Leishmaniasis, contain many GPIs and the structurally related glycoinositolphospholipids (GIPLs). These protein-free glycolipids are key membrane glycoconjugates found throughout the parasites' life cycle. Some of these glycan structures contain D-Galactofuranose (Galf) residues, which are not present in mammalian glycans. Galf-containing GPIs and GIPLs are potential virulence factors for T. cruzi and Leishmania. However, there is little information about their bioactivity. Further research with these glycoconjugates requires pure and structurally defined samples. These cannot be obtained from natural sources and must be accessed by chemical synthesis. This dissertation covers the syntheses of four orthogonally protected thioglycoside building blocks required to assemble oligosaccharides from T. cruzi and Leishmania major (L. major) GIPLs. These structures include a galactofuranose having a 3-O-(2-naphtylmethyl) group (Nap), two mannopyranose units having either a Nap or Benzyl (Bn) ether as protecting group in the 3-O-position, and a galactopyranose with two orthogonal groups, a 6-O-Levulinoyl ester (Lev) and a O-4 benzoyl ester for remote participation. These structures were designed for use in conventional oligosaccharide synthesis and Automated Glycan Assembly (AGA) on a solid support. The synthesis of the Galf building block using two reported methods, starting from 1,2:5,6-DiO-isopropylidene-α-D-galactofuranose and benzyl α-D-galactofuranoside, was inefficient to deliver the target molecule. Thus, an approach for synthesizing Galf thioglycosides was developed using an openform D-galactose diethyldithioacetal as a precursor. A critical step in this strategy was the cyclization of 5,6-O-isopropylidene-D-galactose diethyldithioacetal into a galactofuranoside, which required extensive optimization to achieve a scalable and reproducible reaction. In addition, access to the Galf building block was hindered by poor regioselectivity and product degradation in subsequent reaction steps. Thus, a change in the protecting group strategy from benzyl to acetyl was required to access a building block suitable for synthesizing the target oligosaccharides. A mannose building block bearing a Nap group was synthesized from ethyl 4,6-O-benzylidene-1-thio-β-D-mannopyranoside in four steps. The process involved a regioselective etherification using a stannyl ether-assisted nucleophilic substitution on NapBr and the benzoylation of the O-2 position. Removal of the benzylidene acetal and benzylation under basic conditions induced side reactions involving the O-2 benzoyl group. Alternatively, a sequence of reductive cleavage of benzylidene and one-pot silylation and reductive benzylation delivered the benzylated product. This strategy was applied to obtain a second mannopyranose building block containing three benzyl groups. The galactopyranose building block was assembled from ethyl 4,6-O-benzylidene-1-thio-β-D-galactopyranoside in four steps, with a regioselective Steglich esterification of the O-6 position as the key reaction step. The results of this work include the syntheses of the four monosaccharide building blocks and the required optimization of critical cyclization and protection steps. In addition, these compounds are valuable materials for the assembly of GIPL glycans, which are required for evaluating the activity of these glycolipids and their potential application for the diagnosis of ChD and leishmaniasis. |
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| Autores principais: | Sousa, Eduardo Cláudio de |
| Assunto: | Trypanosoma cruzi Leishmania major Fosfolípidos de glicoinositol Síntese Galactofuranose Teses de mestrado - 2023 |
| Ano: | 2023 |
| País: | Portugal |
| Tipo de documento: | dissertação de mestrado |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade de Lisboa |
| Idioma: | inglês |
| Origem: | Repositório da Universidade de Lisboa |
| Resumo: | Glycosylphosphatidylinositols (GPIs) are eukaryote glycolipids which attach proteins and glycans to the cell membrane. Trypanosomatide parasites of the Trypanosoma cruzi (T. cruzi) species and the Leishmania genus, the etiologic agents of American trypanosomiasis (also known as Chagas' Disease, ChD) and Leishmaniasis, contain many GPIs and the structurally related glycoinositolphospholipids (GIPLs). These protein-free glycolipids are key membrane glycoconjugates found throughout the parasites' life cycle. Some of these glycan structures contain D-Galactofuranose (Galf) residues, which are not present in mammalian glycans. Galf-containing GPIs and GIPLs are potential virulence factors for T. cruzi and Leishmania. However, there is little information about their bioactivity. Further research with these glycoconjugates requires pure and structurally defined samples. These cannot be obtained from natural sources and must be accessed by chemical synthesis. This dissertation covers the syntheses of four orthogonally protected thioglycoside building blocks required to assemble oligosaccharides from T. cruzi and Leishmania major (L. major) GIPLs. These structures include a galactofuranose having a 3-O-(2-naphtylmethyl) group (Nap), two mannopyranose units having either a Nap or Benzyl (Bn) ether as protecting group in the 3-O-position, and a galactopyranose with two orthogonal groups, a 6-O-Levulinoyl ester (Lev) and a O-4 benzoyl ester for remote participation. These structures were designed for use in conventional oligosaccharide synthesis and Automated Glycan Assembly (AGA) on a solid support. The synthesis of the Galf building block using two reported methods, starting from 1,2:5,6-DiO-isopropylidene-α-D-galactofuranose and benzyl α-D-galactofuranoside, was inefficient to deliver the target molecule. Thus, an approach for synthesizing Galf thioglycosides was developed using an openform D-galactose diethyldithioacetal as a precursor. A critical step in this strategy was the cyclization of 5,6-O-isopropylidene-D-galactose diethyldithioacetal into a galactofuranoside, which required extensive optimization to achieve a scalable and reproducible reaction. In addition, access to the Galf building block was hindered by poor regioselectivity and product degradation in subsequent reaction steps. Thus, a change in the protecting group strategy from benzyl to acetyl was required to access a building block suitable for synthesizing the target oligosaccharides. A mannose building block bearing a Nap group was synthesized from ethyl 4,6-O-benzylidene-1-thio-β-D-mannopyranoside in four steps. The process involved a regioselective etherification using a stannyl ether-assisted nucleophilic substitution on NapBr and the benzoylation of the O-2 position. Removal of the benzylidene acetal and benzylation under basic conditions induced side reactions involving the O-2 benzoyl group. Alternatively, a sequence of reductive cleavage of benzylidene and one-pot silylation and reductive benzylation delivered the benzylated product. This strategy was applied to obtain a second mannopyranose building block containing three benzyl groups. The galactopyranose building block was assembled from ethyl 4,6-O-benzylidene-1-thio-β-D-galactopyranoside in four steps, with a regioselective Steglich esterification of the O-6 position as the key reaction step. The results of this work include the syntheses of the four monosaccharide building blocks and the required optimization of critical cyclization and protection steps. In addition, these compounds are valuable materials for the assembly of GIPL glycans, which are required for evaluating the activity of these glycolipids and their potential application for the diagnosis of ChD and leishmaniasis. |
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