N-Tetrachlorophthaloyl (TCP) for ready protection/deprotection of amino sugar glycosides

Protecting group principles suited to late stage functionalization and global deprotection in oligosaccharide synthesis

Chemical synthesis is a powerful tool to access homogeneous complex glycans, which relies on protecting group (PG) chemistry. However, the overall efficiency of chemical glycan assembly is still low when compared to oligonucleotide or oligopeptide synthesis. There have been many contributions giving rise to collective improvement in carbohydrate synthesis that includes PG manipulation and stereoselective glycoside formation and some of this chemistry has been transferred to the solid phase or adapted for programmable one pot synthesis approaches. However, after all glycoside bond formation reactions are completed, the global deprotection (GD) required to give the desired target OS can be challenging. Difficulties observed in the removal of permanent PGs to release the desired glycans can be due to the number and diversity of PGs present in the protected OSs, nature and structural complexity of glycans, etc. Here, we have reviewed the difficulties associated with the removal of PGs from densely protected OSs to obtain their free glycans. In particularly, this review focuses on the challenges associated with hydrogenolysis of benzyl groups, saponification of esters and functional group interconversion such as oxidation/reduction that are commonly performed in GD stage. More generally, problems observed in the removal of permanent PGs is reviewed herein, including benzyl, acyl (levulinoyl, acetyl), N-trichloroacetyl, N-2,2,2-trichloroethoxycarbonyl, N-phthaloyl etc. from a number of fully protected OSs to release the free sugar, that have been previously reported in the literature.

Protecting Groups in Peptide Synthesis

The protection of amino acid reactive functionalities including the α-amino group, the side chain (amines, carboxylic acids, alcohols, and thiols), or the carboxylic acid terminus is an essential strategy in peptide chemistry. This is mandatory to prevent polymerization of the amino acids and to minimize undesirable side reactions during the synthetic process. Proper protecting group manipulation strategies can maximize the yield of the desired product or allow the construction of complex peptide-based structures. Thus, the compatibility and orthogonality of each protecting group are key to achieve the proper control of molecular structure. Herein, we describe some common protecting groups and their general unmasking methods, in order to mask and expose amine, carboxylic acid, alcohol, and thiol functionalities to achieve the synthesis of peptides and related molecules.

Protected sphingosine from phytosphingosine as an efficient acceptor in glycosylation reaction

A convenient, simple, and high-yielding five-step synthesis of a sphingosine acceptor from phytosphingosine is reported, and its behavior in glycosylation reactions is described. Different synthetic paths to sphingosine acceptors using tetrachlorophthalimide as a protecting group for the sphingosine amino function and different glycosylation methods have been explored. Among the acceptors tested, the easiest accessible acceptor, unprotected on the two hydroxyl groups in positions 1 and 3, was regioselectively glycosylated on the primary position, the regioselectivity depending on the donor used.

Recent trends in the synthesis of O-glycosides of 2-amino-2-deoxysugars

The discovery of new methods for stereoselective glycoside synthesis and convergent oligosaccharide assembly has been critical for the area of glycosciences. At the heart of this account is the discussion of the approaches for stereoselective synthesis of glycosides of 2-amino-2-deoxysugars that have emerged during the past two decades. The introductory part provides general background information and describes the key features and challenges for the synthesis of this class of compounds. Subsequently, major approaches to the synthesis of 2-amino-2-deoxyglycosides are categorized and discussed. Each subsection elaborates on the introduction (or protection) of the amino functionality, synthesis of glycosyl donors by introduction of a suitable leaving group, and glycosidation. Wherever applicable, the deprotection of a temporary amino group substituent and the conversion onto the natural acetamido functionality is described. The conclusions part evaluates the current standing in the field and provides a perspective for future developments.

Alpha-N-acetylmannosamine (ManNAc) synthesis via rhodium(II)-catalyzed oxidative cyclization of glucal 3-carbamates

[reaction: see text] Glucal 3-carbamates 1 and 7 underwent oxidative cyclization with iodobenzene diacetate or iodosobenzene in the presence of Rh2(OAc)4, providing mannosamine 2-N,3-O-oxazolidinones. With iodosobenzene, incorporation of 4-penten-1-ol provided a readily separable anomeric mixture of n-pentenyl glycosides, with the anomers exhibiting pronounced differences in reactivity as glycosyl donors. N-acylation of the sugar oxazolidinones led to alpha-selective glycosyl donors for the elaboration of various 2-mannosamine frameworks. Alternatively, the anomeric n-pentenyl glycosides of N-Cbz 2-mannosamine oxazolidinones were converted separately to oxazolidinone-opened derivatives 28alpha and 28beta. These served as stereoconvergent glycosyl donors, and the alpha-linked products were readily advanced to a variety of N-acetylmannosamine (ManNAc) frameworks, using an intramolecular O-->N acetyl transfer as the final step.

Highly diastereoselective oxidation of 2-amino-2-deoxy-1-thio-beta-D-glucopyranosides: synthesis of imino sulfinylglycosides

A synthetic route to imino thioglycosides and to imino sulfinylglycosides has been developed. A detailed study on the diastereoselective oxidation of 2-amino-2-deoxy-1-thio-beta-D-glucopyranosides is reported. It has been shown that the stereochemical outcome of the oxidation is highly dependent on the protective group of the amine function. While the oxidation of iminothioglycosides is slightly diastereoselective (up to 30% de in favor of the R(S) sulfoxide), a single isomer is obtained in the case of tetrachlorophthalimido derivatives. The absolute configuration of the sulfinyl glycoside was ascertained by NMR analysis using our recent model on the basis of the exo-anomeric effect corroborated by X-ray crystallography.

Synthesis of 5-fluoro N-acetylglucosamine glycosides and pyrophosphates via epoxide fluoridolysis: versatile reagents for the study of glycoconjugate biochemistry

Numerous carbohydrate-processing enzymes facilitate catalysis via stabilization of positive charges on or near the C-1, C-4, C-5, or C-6 positions. Substrate analogues differing only in the substitution of a fluorine for the axial C-5 hydrogen would possess reduced electron density at these positions and could be useful mechanistic probes of these enzymes. Introduction of this 5-fluoro substituent after radical halogenation was problematic because of the incompatibility of many protecting groups to the radical halogenation and the instability of the subsequent 5-fluoro hexosamines. Thus, to allow easy access to a wide variety of 5-fluoro glycosides and glycosyl phosphates, a versatile method for the introduction of the 5-fluoro group has been developed, the key step being the fluoridolysis of C-5, 6 epoxides. By use of this method, two fluorinated carbohydrates, uridine 5'-diphospho-5-fluoro-N-acetylglucosamine and octyl 5-fluoro-N-acetylglucosamine, have been synthesized. Initial biochemical investigations of these compounds show that 5-fluoro analogues are useful probes of transition-state charge development in several enzyme-catalyzed reactions.

A versatile, three-component-reaction route to N-glycosylamines

[figure: see text] Under the agency of N-bromosuccinimide, n-pentenyl glycosides, acetonitrile, and carboxylic acids participate in three-component-reactions that afford N-acylated glycosylamines. The procedure tolerate diverse donors, and C2-tetrachlorophthalimido and C2-azido groups effectively control anomeric stereoselectivity. Success of the procedure does not appear to depend on the acid's strength, but for an aromatic acid, substitution pattern affects the rate, while the presence of a lone pair on the para substituent inhibits the process.

Access to tetrachlorophthalimide-protected ethyl 2-amino-2-deoxy-1-thio-beta-D-glucopyranosides

Ethyl 2-deoxy-2-tetrachlorophthalimido-1-thio-beta-D-glucopyranoside (7) was prepared from glucosamine hydrochloride in four steps with a 20-25% overall yield. Formation of 1,3,4,6-tetra-O-acetyl-2-deoxy-2-tetrachlorophthalimido-beta-D- glucopyranoside (5) was found to be crucial for this reaction sequence since the corresponding alpha-1-acetate did not react in Lewis-acid-catalyzed ethylthio glycosidations. Formation of the beta-1-acetate (5) was achieved by treatment of 3,4,6-tri-O-acetyl-2-deoxy-2-tetrachlorophthalimido-alpha-D-glucop yranosyl bromide (4) with acetic acid under silver zeolite promotion. This was necessary because conditions normally used for beta-1-acetate formation were not tolerated by the tetrachlorophthalimido (TCP) group.

Synthetic methods of glycopeptide assembly, and biological analysis of glycopeptide products

The technology of glycopeptide synthesis has recently developed into a fully mature science capable of creating diverse glycopeptides of biological interest, even in combinatorial displays. This has allowed biochemists to investigate substrate specificity in the biosynthetic processing and immunology of various protein glycoforms. The construction of all the mucin core structures and a variety of cancer-related glycopeptides has facilitated detailed analysis of the interaction between MHC-bound glycopeptides and T cell receptors. Novel dendritic neoglycopeptide ligands have been shown to demonstrate high affinity for carbohydrate receptors and these interactions are highly dendrimer specific. Large complex N-linked oligosaccharides have been introduced into glycopeptides using synthetic or chemoenzymatic procedures, both methods affording pure glycopeptides corresponding to a single glycoform in preparative quantities. The improved availability of glycosyl transferases has led to increased use of chemoenzymatic synthesis. Chemical ligation has been introduced as a method of attaching glycans to peptide templates. Combinatorial synthesis and the analysis of resin-bound glycopeptide libraries have been successfully carried out by applying the ladder synthesis principle. Direct quantitative glycosylation of peptide templates on solid phase has paved the way for the synthesis of templated glycopeptide mixtures as libraries of libraries.