Glycosyl donor

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A glycosyl donor is a carbohydrate mono- or oligosaccharide that will react with a suitable glycosyl acceptor to form a new glycosidic bond. By convention, the donor is the member of this pair that contains the resulting anomeric carbon of the new glycosidic bond. [1] The resulting reaction is referred to as a glycosylation or chemical glycosylation.

Carbohydrate organic compound that consists only of carbon, hydrogen, and oxygen

A carbohydrate is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula Cm(H2O)n (where m may be different from n). This formula holds true for monosaccharides. Some exceptions exist; for example, deoxyribose, a sugar component of DNA, has the empirical formula C5H10O4. The carbohydrates are technically hydrates of carbon; structurally it is more accurate to view them as aldoses and ketoses.

An oligosaccharide is a saccharide polymer containing a small number of monosaccharides. Oligosaccharides can have many functions including cell recognition and cell binding. For example, glycolipids have an important role in the immune response.

A glycosyl acceptor: is any suitable nucleophile-containing molecule that will react with a glycosyl donor to form a new glycosidic bond. By convention, the acceptor is the member of this pair which did not contain the resulting anomeric carbon of the new glycosidic bond. Since the nucleophilic atom of the acceptor is typically an oxygen atom, this can be remembered using the mnemonic of the acceptor is the alcohol. A glycosyl acceptor can be a mono- or oligosaccharide that contains an available nucleophile, such as an unprotected hydroxyl.

In a glycosyl donor, a leaving group is required at the anomeric position. The simplest leaving group is the OH group that is naturally present in monosaccharides, but it requires activation by acid catalysis in order to function as leaving group (in the Fischer glycosylation). More effective leaving groups are in general used in the glycosyl donors employed in chemical synthesis of glycosides. Typical leaving groups are halides, thioalkyl groups, or imidates, but acetate, phosphate, and O-pentenyl groups are also employed. Natural glycosyl donors contain phosphates as leaving groups. [1]

In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules, but in either case it is crucial that the leaving group be able to stabilize the additional electron density that results from bond heterolysis. Common anionic leaving groups are halides such as Cl−, Br−, and I−, and sulfonate esters such as tosylate (TsO−). Fluoride (F−) functions as a leaving group in the nerve-agent sarin gas. Common neutral molecule leaving groups are water and ammonia. Leaving groups may also be positively charged cations (such as H+ released during the nitration of benzene); these are also known specifically as electrofuges.

A series of common glycosyl donor types. All are very reactive when activated with a promoter or catalyst: The bromide can be activated by silver salts, the trichloroacetimidate with acid catalysis, and thioglycosides with N-iodosuccinimide/triflic acid. The reactivity of glycosyl donors depend very much on their protection groups, which led to classification of these compounds into three categories of reactivity: Disarmed, armed, and superarmed. Glycosyl donors 1.tif
A series of common glycosyl donor types. All are very reactive when activated with a promoter or catalyst: The bromide can be activated by silver salts, the trichloroacetimidate with acid catalysis, and thioglycosides with N-iodosuccinimide/triflic acid. The reactivity of glycosyl donors depend very much on their protection groups, which led to classification of these compounds into three categories of reactivity: Disarmed, armed, and superarmed.

The so-called “armed-disarmed” principle

The concept of armed and disarmed glycosyl donors refers to the increased reactivity of benzylated over benzoylated glycosyl donors, a phenomenon observed very early, [2] and which originates from the greater electron-withdrawing capability of ester blocking groups over ether blocking groups. However, it was Bertram Fraser-Reid who realised that benzylated glycosyl donors can be activated when benzoylated donors are not, and invented the terms armed glycosyl donor for the former, and disarmed glycosyl donor for the latter. He and his group showed that armed glycosyl donors could be coupled to a glycosyl acceptor, that was at the same time a disarmed glycosyl donor, without self-coupling of the disarmed donor/acceptor. [3] This approach allowed him to carry out a one-pot synthesis of a trisaccharide by the n-pentenyl glycoside method. [4]

Armed and disarmed saccharides

The armed/disarmed approach to glycosylation is an effective way to prevent sugar molecules from self-glycosylation when synthesizing disaccharides. This approach was first recognized when acetylated sugars only acted as glycosyl acceptors when reacted with benzylated sugars. The acetylated sugars were termed “disarmed” while the benzylated sugars were termed “armed”.

Bertram Oliver "Bert" Fraser-Reid is a Jamaican synthetic organic chemist who has been widely recognised for his work using carbohydrates as starting materials for chiral materials and on the role of oligosaccharides in immune response.

The concept has been extended to superarmed glycosyl donor by Mikael Bols and his collaborators. He realised that the hydroxy groups of carbohydrates are less electron-withdrawing towards the anomeric center when they are axial than when they are equatorial, which means that glycosyl donor conformers with more axial oxy functions are more reactive. [5] Protection of a glycosyl donor with bulky silyl groups (tert-butyldimethylsilyl or triisopropyl) cause it to change conformation to a more axial-rich conformation that, as a consequence, is more reactive, which Bols and his group called superarmed. They showed that a superarmed donor can be coupled to an armed glycosyl donor/acceptor. [6]

Mikael Bols is a synthetic organic chemist who is mainly known for his work on carbohydrates and artificial enzymes.

Silylation is the introduction of a (usually) substituted silyl group (R3Si) to a molecule. The process is the basis of organosilicon chemistry.

See also

Carbohydrate chemistry is a subdiscipline of chemistry primarily concerned with the synthesis, structure, and function of carbohydrates. Due to the general structure of carbohydrates, their synthesis is often preoccupied with the selective formation of glycosidic linkages and the selective reaction of hydroxyl groups; as a result, it relies heavily on the use of protecting groups.

Related Research Articles

In chemistry, a glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate.

Glycoside molecule in which a sugar is bound to another functional group

In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. These can be activated by enzyme hydrolysis, which causes the sugar part to be broken off, making the chemical available for use. Many such plant glycosides are used as medications. Several species of Heliconius butterfly are capable of incorporating these plant compounds as a form of chemical defense against predators. In animals and humans, poisons are often bound to sugar molecules as part of their elimination from the body.

Glycosylamine class of biochemical compounds consisting of a glycosyl group attached to an amino group, -NR2

Glycosylamines are a class of biochemical compounds consisting of a glycosyl group attached to an amino group, -NR2. They are also known as N-glycosides, as they are a type of glycoside. Glycosyl groups can be derived from carbohydrates. The glycosyl group and amino group are connected with a β-N-glycosidic bond, forming a cyclic hemiaminal ether bond (α-aminoether).

Amino sugar monosaccharide having one alcoholic hydroxy group (commonly but not necessarily in position 2) replaced by an amino group; systematically known as x-amino-x-deoxymonosaccharides. (Glycosylamines are excluded)

In organic chemistry, an amino sugar is a sugar molecule in which a hydroxyl group has been replaced with an amine group. More than 60 amino sugars are known, with one of the most abundant being N-Acetyl-d-glucosamine, which is the main component of chitin.

Glycal is a name for cyclic enol ether derivatives of sugars having a double bond between carbon atoms 1 and 2 of the ring. The term “glycal” should not be used for an unsaturated sugar that has a double bond in any position other than between carbon atoms 1 and 2.

An Endoglycosidase is an enzyme that releases oligosaccharides from glycoproteins or glycolipids. It may also cleave polysaccharide chains between residues that are not the terminal residue, although releasing oligosaccharides from conjugated protein and lipid molecules is more common.

Glycosyltransferase

Glycosyltransferases are enzymes that establish natural glycosidic linkages. They catalyze the transfer of saccharide moieties from an activated nucleotide sugar to a nucleophilic glycosyl acceptor molecule, the nucleophile of which can be oxygen- carbon-, nitrogen-, or sulfur-based.

The Koenigs–Knorr reaction in organic chemistry is the substitution reaction of a glycosyl halide with an alcohol to give a glycoside. It is one of the oldest and simplest glycosylation reactions. It is named after Wilhelm Koenigs (1851–1906), a student of von Bayer and fellow student with Hermann Emil Fischer, and Edward Knorr, a student of Koenigs.

Glycosynthase

The term Glycosynthase refers to a class of proteins that have been engineered to catalyze the formation of a glycosidic bond. Glycosynthase are derived from glycosidase enzymes, which catalyze the hydrolysis of glycosidic bonds. They were traditionally formed from retaining glycosidase by mutating the active site nucleophilic amino acid to a small non-nucleophilic amino acid. More modern approaches use directed evolution to screen for amino acid substitutions that enhance glycosynthase activity.

Glycoside hydrolase InterPro Domain

Glycoside hydrolases catalyze the hydrolysis of glycosidic bonds in complex sugars. They are extremely common enzymes with roles in nature including degradation of biomass such as cellulose (cellulase), hemicellulose, and starch (amylase), in anti-bacterial defense strategies, in pathogenesis mechanisms and in normal cellular function. Together with glycosyltransferases, glycosidases form the major catalytic machinery for the synthesis and breakage of glycosidic bonds.

Glycorandomization, is a drug discovery and drug development technology platform to enable the rapid diversification of bioactive small molecules, drug leads and/or approved drugs through the attachment of sugars. Initially developed as a facile method to manipulate carbohydrate substitutions of naturally occurring glycosides to afford the corresponding differentially glycosylated natural product libraries, glycorandomization applications have expanded to include both small molecules and even macromolecules (proteins). Also referred to as 'glycodiversification', glycorandomization has led to the discovery of new glycoside analogs which display improvements in potency, selectivity and/or ADMET as compared to the parent molecule.

A chemical gycosylation reaction involves the coupling of a glycosyl donor, to a glycosyl acceptor forming a glycoside. If both the donor and acceptor are sugars, then the product is an oligosaccharide. The reaction requires activation with a suitable activating reagent. The reactions often result in a mixture of products due to the creation of a new stereogenic centre at the anomeric position of the glycosyl donor. The formation of a glycosidic linkage allows for the synthesis of complex polysaccharides which may play important roles in biological processes and pathogenesis and therefore having synthetic analogs of these molecules allows for further studies with respect to their biological importance.

Carbohydrate conformation refers to the overall three-dimensional structure adopted by a carbohydrate (saccharide) molecule as a result of the through-bond and through-space physical forces it experiences arising from its molecular structure. The physical forces that dictate the three-dimensional shapes of all molecules—here, of all monosaccharide, oligosaccharide, and polysaccharide molecules—are sometimes summarily captured by such terms as "steric interactions" and "stereoelectronic effects".

Intramolecular aglycon delivery is a synthetic strategy for the construction of glycans. This approach is generally used for the formation of difficult glycosidic linkages.

The Crich β-mannosylation is a synthetic strategy which is used in carbohydrate synthesis to generate a 1,2-cis-glycosidic bond. This type of linkate is generally very difficult to make, and specific methods like the Crich β-mannosylation are used to overcome these issues.

Carbohydrate synthesis is a sub-field of organic chemistry concerned specifically with the generation of natural and unnatural carbohydrate structures. This can include the synthesis of monosaccharide residues or structures containing more than one monosaccharide, known as oligosaccharides.

Glycoside hydrolase family 65

In molecular biology, glycoside hydrolase family 65 is a family of glycoside hydrolases.

References

  1. 1 2 T. K. Lindhorst "Essentials of Carbohydrate Chemistry and Biochemistry" 2007 Wiley-VCH Verlag, Weinheim
  2. ↑ H. Paulsen, Angew. Chem. Int. Ed. Engl. 1982, 155-173.
  3. ↑ D. R. Mootoo, P. Konradsson, U. Udodong, B. Fraser-Reid, J. Am. Chem. Soc. 1988, 110, 5583-5584.
  4. ↑ B. Fraser-Reid, Z. Wu, U. E. Udodong, H. Ottosson, J. Org. Chem. 1990, 55, 6068-6070.
  5. ↑ H. H. Jensen, L. Lyngbye, M. Bols, Angew. Chem. Int. Ed. 2001 40 3447-3449.
  6. ↑ H.H.Jensen, C. M. Pedersen, M. Bols Chem. Eur. J. 2007, 13, 7576-7582.
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