Editing Glucose (section)

===Cyclic forms===
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| image1 = Alpha-D-Glucopyranose.svg | width1 = 157 | height1 = 170
| image2 = Alpha-D-glucose-from-xtal-1979-3D-balls.png | width2 = 1014 | height2 = 1100
| image3 = Beta-D-Glucopyranose.svg | width3 = 157 | height3 = 170
| image4 = Beta-D-glucose-from-xtal-3D-balls.png | width4 = 1034 | height4 = 1100
| image5 = Alpha-D-Glucofuranose.svg | width5 = 173 | height5 = 187
| image6 = Alpha-D-Glucofuranose Molekülbaukasten 9134 (crop).jpg | width6 = 3073 | height6 = 3074
| image7 = Beta-D-Glucofuranose.svg | width7 = 173 | height7 = 187
| image8 = Beta-D-Glucofuranose Molekülbaukasten 9136 (crop).jpg | width8 = 3089 | height8 = 2873
| header = Cyclic forms of glucose
| footer = From left to right: [[Haworth projection]]s and [[ball-and-stick model|ball-and-stick]] structures of the α- and β- [[anomer]]s of {{small|D}}-glucopyranose (top row) and {{small|D}}-glucofuranose (bottom row)
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In solutions, the open-chain form of glucose (either "{{small|D}}-" or "{{small|L}}-") exists in equilibrium with several [[Carbohydrate#Ring-straight chain isomerism|cyclic isomers]], each containing a ring of carbons closed by one oxygen atom. In aqueous solution, however, more than 99% of glucose molecules exist as [[pyranose]] forms. The open-chain form is limited to about 0.25%, and [[furanose]] forms exist in negligible amounts. The terms "glucose" and "{{small|D}}-glucose" are generally used for these cyclic forms as well. The ring arises from the open-chain form by an intramolecular [[nucleophilic addition]] reaction between the aldehyde group (at C-1) and either the C-4 or C-5 hydroxyl group, forming a [[hemiacetal]] linkage, {{chem2|\sC(OH)H\sO\s}}.

The reaction between C-1 and C-5 yields a six-membered [[heterocycle|heterocyclic]] system called a pyranose, which is a monosaccharide sugar (hence "-ose") containing a derivatised [[pyran]] skeleton. The (much rarer) reaction between C-1 and C-4 yields a five-membered furanose ring, named after the cyclic ether [[furan]]. In either case, each carbon in the ring has one hydrogen and one hydroxyl attached, except for the last carbon (C-4 or C-5) where the hydroxyl is replaced by the remainder of the open molecule (which is {{chem2|\s(C(CH2OH)HOH)\sH}} or {{chem2|\s(CHOH)\sH}} respectively).

The ring-closing reaction can give two products, denoted "α-" and "β-". When a glucopyranose molecule is drawn in the [[Haworth projection]], the designation "α-" means that the hydroxyl group attached to C-1 and the {{chem2|\sCH2OH}} group at C-5 lies on opposite sides of the ring's plane (a[[cis–trans isomerism|'' trans'']] arrangement), while "β-" means that they are on the same side of the plane (a[[cis–trans isomerism|'' cis'']] arrangement). Therefore, the open-chain isomer {{small|D}}-glucose gives rise to four distinct cyclic isomers: α-{{small|D}}-glucopyranose, β-{{small|D}}-glucopyranose, α-{{small|D}}-glucofuranose, and β-{{small|D}}-glucofuranose. These five structures exist in equilibrium and interconvert, and the interconversion is much more rapid with acid [[catalysis]].

[[File:Alpha-D-glucose and beta-D-glucose acid-catalyzed mechanism.svg|centre|500px|class=skin-invert-image|Widely proposed arrow-pushing mechanism for acid-catalyzed dynamic equilibrium between the α- and β- [[anomer]]s of D-glucopyranose]]
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| image1 = ALPHA-D-Glucopyranose V.1.png | width1 = 2240 | height1 = 1662
| image2 = BETA-D-Glucopyranose V.1.png | width2 = 2608| height2 = 1420
| footer = [[Chair conformation]]s of α- (left) and β- (right) {{small|D}}-glucopyranose
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The other open-chain isomer {{small|L}}-glucose similarly gives rise to four distinct cyclic forms of {{small|L}}-glucose, each the mirror image of the corresponding {{small|D}}-glucose.

The glucopyranose ring (α or β) can assume several non-planar shapes, analogous to the "chair" and "boat" conformations of [[cyclohexane]]. Similarly, the glucofuranose ring may assume several shapes, analogous to the "envelope" conformations of [[cyclopentane]].

In the solid state, only the glucopyranose forms are observed.

Some derivatives of glucofuranose, such as [[1,2-O-isopropylidene-D-glucofuranose|1,2-''O''-isopropylidene-{{sc|D}}-glucofuranose]] are stable and can be obtained pure as crystalline solids.<ref name=taka1979>{{cite journal | last1 = Takagi | first1 = S. | last2 = Jeffrey | first2 = G. A. | year = 1979 | title = 1,2-O-isopropylidene-D-glucofuranose | journal = Acta Crystallographica Section B | volume = B35 | issue = 6| pages = 1522–1525 | doi = 10.1107/S0567740879006968 | bibcode = 1979AcCrB..35.1522T }}</ref><ref name=biel1999>{{cite journal | last1 = Bielecki | first1 = Mia | last2 = Eggert | first2 = Hanne | last3 = Christian Norrild | first3 = Jens | year = 1999 | title = A fluorescent glucose sensor binding covalently to all five hydroxy groups of α-D-glucofuranose. A reinvestigation | journal = Journal of the Chemical Society, Perkin Transactions | volume = 2 | issue = 3 | pages = 449–456 | doi = 10.1039/A808896I }}</ref> For example, reaction of α-D-glucose with [[p-tolylboronic acid|''para''-tolylboronic acid]] {{chem2|H3C\s(C6H4)\sB(OH)2}} reforms the normal pyranose ring to yield the 4-fold ester α-D-glucofuranose-1,2:3,5-bis(''p''-tolylboronate).<ref name=chan2006>{{cite journal | last1 = Chandran | first1 = Sreekanth K. | last2 = Nangia | first2 = Ashwini | year = 2006 | title = Modulated crystal structure (Z{{prime}} = 2) of α-d-glucofuranose-1,2:3,5-bis(p-tolyl)boronate | journal = CrystEngComm | volume = 8 | issue = 8| pages = 581–585 | doi = 10.1039/B608029D }}</ref>