A glycosyl donor is a
carbohydrate
In organic chemistry, 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 (where ''m'' may o ...
mono- or
oligosaccharide
An oligosaccharide (/ˌɑlɪgoʊˈsækəˌɹaɪd/; from the Greek ὀλίγος ''olígos'', "a few", and σάκχαρ ''sácchar'', "sugar") is a saccharide polymer containing a small number (typically two to ten) of monosaccharides (simple sug ...
that will react with a suitable
glycosyl acceptor 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 t ...
to form a new
glycosidic bond
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.
A glycosidic bond is formed between the hemiacetal or hemiketal group ...
. By convention, the donor is the member of this pair that contains the resulting
anomer
In carbohydrate chemistry, a pair of anomers () is a pair of near-identical stereoisomers that differ at only the anomeric carbon, the carbon that bears the aldehyde or ketone functional group in the sugar's open-chain form. However, in order f ...
ic carbon of the new glycosidic bond.
[T. K. Lindhorst "Essentials of Carbohydrate Chemistry and Biochemistry" 2007 Wiley-VCH Verlag, Weinheim] The resulting reaction is referred to as a
glycosylation
Glycosylation is the reaction in which a carbohydrate (or ' glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not al ...
or
chemical glycosylation
A chemical glycosylation 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 ...
.
In a glycosyl donor, a
leaving group In chemistry, a leaving group is defined by the IUPAC as an atom or group of atoms that detaches from the main or residual part of a substrate during a reaction or elementary step of a reaction. However, in common usage, the term is often limited ...
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.
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, and which originates from the greater electron-withdrawing capability of ester blocking groups over ether blocking groups. However, it was
Bertram Fraser-Reid
Bertram Oliver "Bert" Fraser-Reid (23 February 1934 – 25 May 2020) was 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 oligosac ...
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. This approach allowed him to carry out a one-pot synthesis of a trisaccharide by the n-pentenyl glycoside method.
The concept has been extended to superarmed glycosyl donor by
Mikael Bols
Mikael Bols (born July 28, 1961) is a synthetic organic chemist who is mainly known for his work on carbohydrates and artificial enzymes.
Early life
Mikael Bols was born and grew up in Copenhagen, Denmark. He attended high school (Gentofte Stat ...
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. 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.
[H.H.Jensen, C. M. Pedersen, M. Bols Chem. Eur. J. 2007, 13, 7576-7582.]
See also
*
Glycosyl halide
A glycosyl group is a univalent free radical or substituent structure obtained by removing the hemiacetal hydroxyl group from the cyclic form of a monosaccharide and, by extension, of a lower oligosaccharide.
Glycosyl also reacts with inorganic a ...
*
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 reac ...
*
Carbohydrate chemistry
Carbohydrate chemistry is a subdiscipline of chemistry primarily concerned with the detection, synthesis, structure, and function of carbohydrates. Due to the general structure of carbohydrates, their synthesis is often preoccupied with the sele ...
References
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Carbohydrate chemistry
Organic reactions