An anomer is an epimer at an (hemi)-acetal carbon like $\ce{R2C(OR')2}$.
Why is the 1'-position of a nucleoside referred to as the anomeric center despite the lack of acetal functional groups?
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1$\begingroup$ They're just like sugars, just with nitrogen instead of O. One could call them hemiaminals maybe. $\endgroup$– MithoronCommented Oct 24, 2018 at 16:13
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$\begingroup$ Well sure, but I don't think that justifies calling the isomers 'anomers'. $\endgroup$– JacobCommented Oct 24, 2018 at 16:29
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2 Answers
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The C1 position of carbohydrates and carbohydrate derivatives is generally referred to as the anomeric center and compounds differing only in their configuration at this position are called anomers. In nucleosides, the 1'-position may not be part of a hemiacetal or acetal functional group, but it is at the same oxidation state. The IUPAC Gold Book defines anomers as:
Diastereoisomers of glycosides, hemiacetals or related cyclic forms of sugars, or related molecules differing in configuration only at C-1 of an aldose, C-2 of a 2-ketose, etc.
The fact that the definition includes "glycosides" and "related cyclic forms of sugars, or related molecules" points towards why it is acceptable to describe the 1'-position as the anomeric center. Furthermore, this position in nucleosides is at the same oxidation state as a hemiacetal or acetal. Therefore, there are structural and chemical reactivity similarities between these functional groups and the 1'-position of nucleosides.
The correct name for this R1C(OR2)(NR3R4)R5 functional group is a hemiaminal ether. Hemiaminals have a free hydroxyl group: R1C(OH)(NR2R3)R4.
Also of note is that the anomeric effect refers to the preference for polar groups to be axial on pyranose derivatives, where the polar group may be not only oxygen substituents (eg. hemiacetals, acetals), but also other derivatives such as halides (eg. glycosyl halides). In all these cases, C1 would be referred to as the anomeric center.
answered Oct 25, 2018 at 11:42
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To add to the correct anomer definition cited by Mr. Organic Chemistry Explained, also note, that from the carbohydrate chemistry nomenclature standpoint, these are N-glycosides, more correctly N-glycosyl compounds (or glycosylamines), and the α/β (not a/b, like in your image) anomeric configuration notation is used in the names, see in Nomenclature of Carbohydrates (Recommendations 1996):
2-Carb-33. Glycosides and glycosyl compounds
(…)
2-Carb-33.6. N-Glycosyl compounds (glycosylamines)
N-Glycosyl derivatives are conveniently named as glycosylamines. In the case of complex heterocyclic amines, such as nucleosides, the same approach is used.
Examples:
(…)
1-β-ᴅ-ribofuranosyluracil
(trivial name uridine)
