Bozhena S. Komarova

Is an acyl group at O-3 in glucosyl donors able to control α-stereoselectivity of glycosylation? The role of conformational mobility and the protecting group at O-6

The stereodirecting effect of a 3-O-acetyl protecting group, which is potentially capable of the remote anchimeric participation, and other protecting groups in 2-O-benzyl glucosyl donors with flexible and rigid conformations has been investigated. To this aim, an array of N-phenyltrifluoroacetimidoyl and sulfoxide donors bearing either 3-O-acetyl or 3-O-benzyl groups in combination with 4,6-di-O-benzyl, 6-O-acyl-4-O-benzyl, or 4,6-O-benzylidene protecting groups was prepared. The conformationally flexible 3-O-acetylated glucosyl donor protected at other positions with O-benzyl groups demonstrated very low or no α-stereoselectivity upon glycosylation of primary or secondary acceptors. On the contrary, 3,6-di-O-acylated glucosyl donors proved to be highly α-stereoselective as well as the donor having a single potentially participating acetyl group at O-6. The 3,6-di-O-acylated donor was shown to be the best α-glucosylating block for the primary acceptor, whereas the best α-selectivity of glycosylation of the secondary acceptor was achieved with the 6-O-acylated donor. Glycosylation of the secondary acceptor with the conformationally constrained 3-O-acetyl-4,6-O-benzylidene-protected donor displayed under standard conditions (-35°C) even lower α-selectivity as compared to the 3-O-benzyl analogue. However, increasing the reaction temperature essentially raised the α-stereoselectivities of glycosylation with both 3-O-acetyl and 3-O-benzyl donors and made them almost equal. The stereodirecting effects of protecting groups observed for N-phenyltrifluoroacetimidoyl donors were also generally proven for sulfoxide donors.

Synthesis of a Pentasaccharide and Neoglycoconjugates Related to Fungal α‐(1→3)‐Glucan and Their Use in the Generation of Antibodies to Trace Aspergillus fumigatus Cell Wall

3-Aminopropyl α-(1→3)-pentaglucoside, a fragment of α-(1→3)-glucan of the cell wall of Aspergillus fumigatus, has been synthesized in a blockwise approach. The application of mono- and disaccharide N-phenyltrifluoroacetimidates bearing a stereodirecting 6-O-benzoyl group was essential for stereoselective α-glucosylations. In the products, p-methoxyphenyl and levulinoyl groups served as orthogonal protecting groups for the anomeric position and 3-OH group, respectively. Their removal from shared blocks led to donors and acceptors that were used for the synthesis of pentasaccharides. Coupling of free α-(1→3)-pentaglucoside with biotin and bovine serum albumin (BSA) gave glycoconjugate tools for mycological studies. Immunization of mice with the BSA conjugate induced the generation of antibodies that recognize α-(1→3)-glucan on A. fumigatus cell wall and distinguish its morphotypes. This discovery represents a first step to the development of a diagnostic test system and a vaccine to detect and fight this life-threatening pathogen.

First Synthesis of Pentasaccharide Glycoform I of the Outer Core Region of the Pseudomonas aeruginosa Lipopolysaccharide

The synthesis of a pentasaccharide representing the glycoform I, which is one of two naturally occurring glycoforms of the outer core of Pseudomonas aeruginosa lipopolysaccharide, and its analogues, differing in the N-substituent in the galactosamine unit, is reported. The main features of the synthetic scheme included the assembly of the pentasaccharide backbone by successive introduction of monosaccharide units, the use of glucosyl donors with specific location of acyl protecting groups capable of the remote anchimeric participation for highly stereoselective alpha-glucosylation, and efficient reduction of the azido group allowing high-yielding transformation of the intermediary azido pentasaccharide into final products.

Design of α‐Selective Glycopyranosyl Donors Relying on Remote Anchimeric Assistance

Oligosaccharides have a variety of versatile biological effects, but unlike peptides and oligonucleotides, investigation of the roles of oligosaccharides is not easy. Since biosynthesis of oligosaccharides does not comply with direct genetic control, their isolation from natural sources and biotechnological preparation are complicated, due to the heterogeneous composition of raw carbohydrates. Oligosaccharide synthesis is needed for the establishment or confirmation of the structure of natural glycocompounds. Also, synthetically prepared, defined oligosaccharides and their derivatives are becoming increasingly important tools for many biological and immunological research projects. The key step of oligosaccharide synthesis involves glycosylation, a reaction that builds glycosidic bonds. Usually, construction of 1,2-trans-bonds is easy, and therefore, this reaction can even be included into automated syntheses. At this time, installation of the 1,2-cis-bond remains simultaneously frustrating and compelling. In our and other laboratories, a strategy of α-selective (1,2-cis) glycosylation, relying on remote anchimeric assistance with acyl groups, is studied. The state of the art in this work is summarized in this review.

Synthesis of pentasaccharides corresponding to the glycoform II of the outer core region of the Pseudomonas aeruginosa lipopolysaccharide

Cystic fibrosis (CF) is a congenital disease caused by a mutation in a gene responsible for the synthesis of a membrane protein called the cystic fibrosis transmembrane conductance regulator (CFTR). Resistance to Pseudomonas aeruginosa infection is closely related to the biological properties of CFTR; however, these properties have not been clearly linked to the known role of CFTR as a chloride and bicarbonate ion channel. Indeed, data indicate that CFTR is an epithelial cell receptor for P. aeruginosa, with CFTR binding to the oligosaccharide of the outer core region of the bacterial lipopolysaccharide (LPS), of which two distinct glycoforms have been identified. Binding leads to effective innate immunity to clear this pathogen in individuals with wild-type CFTR. To reveal the molecular basis of elimination of the bacterium through this interaction, the synthesis of pentasaccharides corresponding to both glycoforms of the outer core region of P. aeruginosa LPS was undertaken. Here we report the synthesis of the glycoform II. Like glycoform I, it was prepared as three pentasaccharides bearing naturally occurring N-alanyl and N-acetyl substituents in the galactosamine moiety as well as unnatural N-acetylalanine to reveal the role of the amino group in the alanyl substituent. Key features of the synthesis were two α-glucosylations with glucosyl donors bearing α-stereodirecting acyl groups at O-6 and/or O-3 and high-yielding reduction of the azido group followed by N-acylation and final O-debenzylation.

1,3-syn-Diaxial Repulsion of Typical Protecting Groups Used in Carbohydrate Chemistry in 3-O-Substituted Derivatives of Isopropyl d-Idopyranosides

The strength of 1,3-syn-diaxial repulsion was evaluated for main types of protecting groups (alkyl, silyl, and acyl) usually used in carbohydrate chemistry. As molecular probes for this study, derivatives of isopropyl 2-O-benzyl-4,6-O-benzylidene-α-d-idopyranoside bearing allyl, acetyl, and tert-butyldiphenylsilyl (TBDPS) protecting groups at O-3 were prepared from p-methoxyphenyl d-galactopyranoside. The equilibrium between OS2 and 4C1 conformations in these compounds was investigated using 3JH,H and 3JC,H coupling constants that were determined from 1D 1H NMR and 2D J-resolved HMBC spectra in various solvents. The analysis of the corresponding coupling constants calculated using DFT/B3LYP/pcJ-1 approximation applied to conformations optimized at DFT/B3LYP/6-311++G** level supported the investigation. Proportions of conformers in the equilibrium revealed the highest repulsion between the 3-allyloxy group and the isopropoxy aglycon and its dependence on the solvent polarity. Differences in the conformational behavior of 3-O-allyl and 3-O-acetyl-α-d-idopyranoside derivatives complied with the notion that higher electron density on O-3 increased 1,3-syn-diaxial repulsion. 3-O-TBDPS derivative existed mainly in 4C1 conformation. The attenuation of the 1,3-syn-diaxial repulsive interaction indicates that TBDPS has stereoelectronic properties that may have significance in context of fixing unnatural pyranoside conformation with the help of silyl groups but have been disregarded until now.

Synthesis of a biotinylated penta-α-(1→6)-D-glucoside based on the rational design of an α-stereoselective glucosyl donor

Two glucosyl donors, which allowed for the α-selective glucosylation of primary hydroxyl groups, were designed using a combination of acyl groups capable of remote participation, an electron-withdrawing pentafluorobenzoyl substituent and a bulky TBDPS protecting group. The first had a participating levulinoyl group at O-3 and the TBDPS protecting group at O-6, and the second bore an acetyl group at O-3 and the pentafluorobenzoyl one at O-6. With the help of these donors, two synthetic schemes, convergent and linear, towards a pentaglucoside representing the α-(1→6)-linked glucans of Helicobacter pylori were evaluated. The linear scheme proved to be preferential over the convergent one because of its insufficiently high α-stereoselectivity demonstrated by disaccharide donors. For the linear scheme, the pentafluorobenzoyl group turned out to be more convenient than TBDPS at the removal step. The α-(1→6)-linked pentaglucoside was prepared by iteration of glucosylation with the donor bearing 3-O-acetyl and 6-O-pentafluorobenzoyl and the subsequent liberation of the 6-OH group. Biotin was attached to the pentaglucoside via the spacer amino group.

Contribution of carbohydrate chemistry to assessment of the biological role of natural α-glucosides

Significant progress was achieved in the field of the carbohydrate and oligosaccharide synthesis in the past decade. Complex fragments of natural glycopolymers with unusual stereochemistry and labile substituents have been successfully prepared by a large number of research groups and used for bioassays targeted for the investigation of molecular mechanisms of their biological activities. A part of these structures contains an α-glucose linkage that represents one of the main synthetic challenges. Surprisingly, despite tremendous success of synthetic oligosaccharide chemistry, enzymatic and chemical approaches proved to be of equal importance for the preparation of polysaccharides and glycoconjugates containing α-glucose units. Interest in enzymatic methods in this field is due to the practical demand of large amounts of synthetic probes that are not otherwise available through synthetic chemistry. This review surveys syntheses of glycopolymers related to reserve polysaccharides, molecules of recognition and immunogenic glycol-conjugates that possess one or more α-glucose residues. The special attention is paid to the contribution of these synthetic probes for exploring diverse biological mechanisms at the molecular level. The consideration of the most relevant and modern methods for the α-selectivity control is also included.