Glycal is a name for cyclic
enol ether
In organic chemistry an enol ether is an alkene with an alkoxy substituent. The general structure is R2C=CR-OR where R = H, alkyl or aryl. A common subfamily of enol ethers are vinyl ethers, with the formula ROCH=CH2. Important enol ethers incl ...
derivatives of
sugar
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose
Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...
s having a
double bond
In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...
between
carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
s 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.
History
The first glycal was
synthesized by
Hermann Emil Fischer
Hermann Emil Louis Fischer (; 9 October 1852 – 15 July 1919) was a German chemist and List of Nobel laureates in Chemistry, 1902 recipient of the Nobel Prize in Chemistry. He discovered the Fischer esterification. He also developed the Fisch ...
and Karl Zach in 1913. They synthesized this 1,2-unsaturated sugar from D-
glucose
Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
and named their product D-glucal. Fischer believed he had synthesized an
aldehyde
In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...
, and therefore he gave the product a name that suggested this.
By the time he discovered his mistake, the name "glycal" was adopted as a general name for all sugars with a double bond between carbon atoms 1 and 2.
Conformation

Glycals can be formed as
pyranose
In organic chemistry, pyranose is a collective term for saccharides that have a chemical structure that includes a six-membered ring consisting of five carbon atoms and one oxygen atom (a heterocycle). There may be other carbons external to the ...
(six-membered) or
furanose
A furanose is a collective term for carbohydrates that have a chemical structure that includes a five-membered ring system consisting of four carbon atoms and one oxygen atom. The name derives from its similarity to the oxygen heterocycle furan, ...
(five-membered) rings, depending on the monosaccharide used as a starting material to synthesize the glycal. Glycals can also be classified as ''endo''-glycals or ''exo''-glycals. A glycal is an endo-glycal when the double bond is within the ring. If the hydroxyl group on carbon 1 has been replaced with another carbon atom, a double bond can also form outside the ring between carbon 1 and this new carbon. In this case, the product is called an ''exo''-glycal. The glycal conformation that has been studied in most depth is that of the pyranose ''endo''-glycal. The favoured conformation of this glycal is the half-chair,
a result which has been confirmed by quantum mechanical calculations.
Synthesis
The original Fischer glycal synthesis was the reductive elimination with zinc of a glycosyl halide. This glycosyl halide was formed from a monosaccharide starting material.
Some other synthetic routes include:
*Ring-closing metathesis
*Reaction of thioglycosides with lithium napthalenide.
*Mesylation of the anomeric hydroxyl and formation of the anomeric palladium complex, which undergoes beta-elimination
A general example of each synthetic route is given below (drawn with first discussed synthesis bottom right, moving clockwise):
Reactions and uses
The double bond of a glycal allows many other
functional group
In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...
s to be introduced into a monosaccharide. Like an
alkene
In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins.
The Internationa ...
, a glycal can undergo electrophilic addition across the double bond to add in these new atoms such as
halogen
The halogens () are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and tennessine (Ts), though some authors would ...
s,
epoxide
In organic chemistry, an epoxide is a cyclic ether, where the ether forms a three-atom ring: two atoms of carbon and one atom of oxygen. This triangular structure has substantial ring strain, making epoxides highly reactive, more so than other ...
s, and nitrogen. The glycal double bond also allows a deoxy position (carbon in the ring that doesn’t have an oxygen bonded to it) to be easily introduced.
Glycals have many uses in synthetic carbohydrate chemistry. They are commonly used as glycosylation donors, meaning that they can react with other monosaccharides to form a longer chain of monosaccharides called an oligosaccharide.
Glycals can also have interesting applications in studying biological systems, particularly enzymes. D-glucal and radiolabelled D-galactal have been used to selectively bind with amino acids in the active sites of several enzymes. These enzyme-glycal complexes allow these amino acids that are essential for catalysis to be identified and allow for a better understanding of how these enzymes function.
References
{{Reflist
See also
*
Ferrier rearrangement
*
Chemical glycosylation
Carbohydrates
Carbohydrate chemistry