D-glucopyranose
= D-glucopyranose
{c}
<D-glucopyranose> is the six-membered-ring <hemiacetal> form of D-<glucose>. The ring contains five <carbon> atoms and one <oxygen> atom; the sixth <carbon>, C6, lies outside the ring. Its name specifies both the glucose configuration and the ring form, but without an α or β prefix it does not select a single <anomer>.<Nomenclature of Carbohydrates (Recommendations 1996) by Alan D. McNaught>[Sections2-Carb-5 and2-Carb-6: cyclic representations, α-D-glucopyranose and anomeric descriptors]{ref}
Ring closure connects the C5 <hydroxy group> to the C1 <aldehyde> carbon of open-chain D-<glucose>. C1 becomes the <anomeric carbon>. No <chemical atoms> are lost in forming this cyclic <hemiacetal>, so both <anomers> retain the <molecular formula> $\mathrm{C_6H_{12}O_6}$.
In the conventional <Haworth projection> of D-<glucose>, the anomeric <hydroxy group> and the C5 hydroxymethyl substituent are on opposite faces in α-D-glucopyranose and on the same face in β-D-glucopyranose. This relative relationship is more useful than saying that α always means “down”: how a structure is drawn and which sugar configuration is involved must also be specified.<Nomenclature of Carbohydrates (Recommendations 1996) by Alan D. McNaught>[Section2-Carb-5: Haworth representation and trans relationship in α-D-glucopyranose; section2-Carb-6.2: α/β descriptors]{ref}
An original information-counting comparison uses three labels. $\mathrm{C_6H_{12}O_6}$ gives composition; D-glucopyranose additionally specifies a particular sugar configuration and six-membered ring; α-D-glucopyranose further fixes the anomeric configuration. The α and β forms therefore provide an immediate pair of structures with the same elemental count. Neither the <molecular formula> nor the number of ring atoms alone determines the <stereochemistry>.
Replacing the anomeric <hydroxy group> by $\mathrm{OCH_3}$ gives the corresponding methyl <glycoside>; choosing the α configuration yields <methyl α-D-glucopyranoside>. Free <D-glucopyranose> and that <acetal> derivative differ in chemical identity even though their names share the same sugar-derived root.<Nomenclature of Carbohydrates (Recommendations 1996) by Alan D. McNaught>[Section2-Carb-33.1, named methyl glycoside example and mixed-acetal definition]{ref}