Glycal

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Glucal, the glycal formed from glucose Glucal.svg
Glucal, the glycal formed from glucose

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. [1]

Contents

History

The first glycal was synthesized by Hermann Emil Fischer and Karl Zach in 1913. [2] They synthesized this 1,2-unsaturated sugar from D-glucose and named their product D-glucal. Fischer believed he had synthesized an aldehyde, and therefore he gave the product a name that suggested this. [3] 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. [4]

Conformation

Glucal in its preferred half-chair conformation Half-chair conformation of glucal.png
Glucal in its preferred half-chair conformation

Glycals can be formed as pyranose (six-membered) or furanose (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. [5] 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, [6] a result which has been confirmed by quantum mechanical calculations. [7]

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. [8] Some other synthetic routes include:

A general example of each synthetic route is given below (drawn with first discussed synthesis bottom right, moving clockwise):

Some available methods to synthesize glycals Literature Glycal Syntheses.png
Some available methods to synthesize glycals

Reactions and uses

The double bond of a glycal allows many other functional groups to be introduced into a monosaccharide. Like an alkene, a glycal can undergo electrophilic addition across the double bond to add in these new atoms such as halogens, epoxides, 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. [8]

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. [11]

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. [12]

Related Research Articles

<span class="mw-page-title-main">Carbohydrate</span> 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 and thus with the empirical formula Cm(H2O)n, which does not mean the H has covalent bonds with O. However, not all carbohydrates conform to this precise stoichiometric definition, nor are all chemicals that do conform to this definition automatically classified as carbohydrates.

<span class="mw-page-title-main">Disaccharide</span> Complex sugar

A disaccharide is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose, lactose, and maltose.

Monosaccharides, also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built.

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

<span class="mw-page-title-main">Maltose</span> Chemical compound

Maltose, also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the two-unit member of the amylose homologous series, the key structural motif of starch. When beta-amylase breaks down starch, it removes two glucose units at a time, producing maltose. An example of this reaction is found in germinating seeds, which is why it was named after malt. Unlike sucrose, it is a reducing sugar.

<span class="mw-page-title-main">Amino sugar</span>

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.

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.

<span class="mw-page-title-main">Anomeric effect</span>

In organic chemistry, the anomeric effect or Edward-Lemieux effect is a stereoelectronic effect that describes the tendency of heteroatomic substituents adjacent to a heteroatom within a cyclohexane ring to prefer the axial orientation instead of the less hindered equatorial orientation that would be expected from steric considerations. This effect was originally observed in pyranose rings by J. T. Edward in 1955 when studying carbohydrate chemistry.

<span class="mw-page-title-main">Pyranose</span> Class of chemical compounds

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. There may be other carbons external to the ring. The name derives from its similarity to the oxygen heterocycle pyran, but the pyranose ring does not have double bonds. A pyranose in which the anomeric OH at C(l) has been converted into an OR group is called a pyranoside.

<span class="mw-page-title-main">Glycosyltransferase</span> Class of enzymes that catalyze the transfer of glycosyl groups to an acceptor

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.

Nucleotide sugars are the activated forms of monosaccharides. Nucleotide sugars act as glycosyl donors in glycosylation reactions. Those reactions are catalyzed by a group of enzymes called glycosyltransferases.

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Oligosaccharides and polysaccharides are an important class of polymeric carbohydrates found in virtually all living entities. Their structural features make their nomenclature challenging and their roles in living systems make their nomenclature important.

Carbohydrate NMR spectroscopy is the application of nuclear magnetic resonance (NMR) spectroscopy to structural and conformational analysis of carbohydrates. This method allows the scientists to elucidate structure of monosaccharides, oligosaccharides, polysaccharides, glycoconjugates and other carbohydrate derivatives from synthetic and natural sources. Among structural properties that could be determined by NMR are primary structure, saccharide conformation, stoichiometry of substituents, and ratio of individual saccharides in a mixture. Modern high field NMR instruments used for carbohydrate samples, typically 500 MHz or higher, are able to run a suite of 1D, 2D, and 3D experiments to determine a structure of carbohydrate compounds.

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. The resulting reaction is referred to as a glycosylation or chemical glycosylation.

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<span class="mw-page-title-main">Ribose</span> Group of simple sugar and carbohydrate compounds

Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, d-ribose, is a component of the ribonucleotides from which RNA is built, and so this compound is necessary for coding, decoding, regulation and expression of genes. It has a structural analog, deoxyribose, which is a similarly essential component of DNA. l-ribose is an unnatural sugar that was first prepared by Emil Fischer and Oscar Piloty in 1891. It was not until 1909 that Phoebus Levene and Walter Jacobs recognised that d-ribose was a natural product, the enantiomer of Fischer and Piloty's product, and an essential component of nucleic acids. Fischer chose the name "ribose" as it is a partial rearrangement of the name of another sugar, arabinose, of which ribose is an epimer at the 2' carbon; both names also relate to gum arabic, from which arabinose was first isolated and from which they prepared l-ribose.

References

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  9. Calimente, D.; Postema, M.H.D. (1999). "Preparation of C-1 Glycals via Olefin Metathesis. A Convergent and Flexible Approach to C-Glycoside Synthesis". The Journal of Organic Chemistry. 64 (6): 1770–1771. doi:10.1021/jo982331o. PMID   11674258.
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See also