Английская Википедия:Glycal
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]
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
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:
- Ring-closing metathesis [9]
- Reaction of thioglycosides with lithium napthalenide.[10]
- Mesylation of the anomeric hydroxyl and formation of the anomeric palladium complex, which undergoes beta-elimination[10]
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 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]
References
See also
