Anomeric reference atom

The anomeric reference atom and the anomeric conftgurational symbol... 

Note. In cyclic forms of sugars, the configuration at the anomeric chiral centre is defined in relation to the anomeric reference atom (see 2-Carb-6.2). 

The new centre of chirality generated by hemiacetal ring closure is called the anomeric centre. The two stereoisomers are referred to as anomers, designated a or (i according to the configurational relationship between the anomeric centre and a specified anomeric reference atom. 

Carb-6.2. The anomeric reference atom and the anomeric configurational symbol (a or p)... 

The anomeric symbol a or p, followed by a hyphen, is placed immediately before the configurational symbol D or L of the trivial name or of the configurational prefix denoting the group of chiral carbon atoms that includes the anomeric reference atom. 

Note. For simple aldoses up to aldohexoses, and ketoses up to hept-2-uloses, the anomeric reference atom and the configurational atom are the same. 

Acetamido-3,5-dicleoxy-D-g/ycero-a-D gafacfo-non-2-ulopyranosonic acid (W-acetyl-a-neuraminic acid, a-Neu5Ac), drawn in three ways (note that C-7 is the anomeric reference atom)... 

Fig. 1. Chemical structures for simple sialic acids in different views, (a) S-amino-S.S-dideoxy-D-gfycero-D-ga/acro-non-2-ulosonic acid (Neu, open chain, Fischer projection formula) (b) 5-acetaraido-3,5-dideoxy-D-g/> cero-a-D-gfl/acto-non-2-ulopyranosonic acid (a-Neu5Ac, Fischer projection formula, note that C7 is the anomeric reference atom) (c) a-Neu5Ac (Haworth projection formula) (d) a-Neu5Ac ( 5 chair conformation) (e) 3-deoxy-D-g/ycero-3-D-gfl/acto-non-2-ulopyranosonic acid (P-Kdn, 5 chair conformation). Note that the D-notation is part of the trivial name.

The configurational symbols a, p refer each to the so-called anomeric reference atom that in the above examples is identical with the configurational atom. Accordingly, the cis-relationship between the HO group attached to the anomeric C atom and the oxygen attached to the reference atom translates into a and trans into jS. 

The situation in which the anomeric OH is retained but H is lost from the anomeric carbon atom is indicated by use of the ending -yl without locants in conjunction with the prefix 1-hydroxy- (not by the ending -1-C-yl ). N.B. In this case, the anomeric prefix a or P refers to the free valency, not the OH group. 

Chemical shift (p.p.m.) of model compounds in H20 relative to internal 1,4-dioxane (67.86 p.p.m.). Chemical shifts for these compounds are given at pH 5.5-7.5. Estimated precision for the chemical shifts is 0.05 p.p.m.h See Refs. 82 and 83. See Ref. 20.d These assignments may have to be interchanged. See Ref. 21 numbers in the brackets below the given chemical shift values refer to those published in Ref. 86. f See Ref. 19. The chemical shift for the -anomeric carbon atom was found to be 100.6 p.p.m. and was determined from an anomeric mixture of this compound. The existence of the a-Man — Ser unit was confirmed by the l]CH value (169 Hz) obtained for this compound. See Ref. 84. 

The chemical structures of five dextrans were partially determined by methylation, and found to be branched molecules having the following types of substitution (a) 6-0 and 3,6-di-O, (b) 6-0, 3-0, and 3,6-di-O, (c) 6-0,3,6-di-O, and 2,3-di-O, (d) 6-0, 4-0, and 3,4-di-O, and (e) 6-0 and 2,3-di-O. At 27° and pH 7 (external, Me4Si standard), the 13C shifts ofO-substituted, non-anomeric carbon atoms were C-2 (76.5), C-3 (81.6), and C-4 (79.5). The C-l resonances were also recorded, and may be used for reference purposes. Some variation of chemical shifts, relative to each other, was observed with changing temperature. (The work serves to emphasize the importance of accurately measuring the temperature of the solution when determining chemical shifts.102)... 

In the furanose forms, the four carbon atoms involved in the ring are numbered 1 to 4, starting with the anomeric carbon atom.3,22 The symbol D or L now distinguishes between the two furanoses derived from one apiose, the respective symbol referring to the reference (highest-numbered), asymmetric carbon atom (C-3) of the cyclic forms, as depicted in formulas 2 and 3. After cyclization, the ring-... 

Five-carbon sugars, such as D-ribose (Topic Gl) and D-deoxyribose (Topic FI), and six-carbon ketose sugars (ketohexoses), such as D-fructose, form rings called furanoses (Fig. 6a) by comparison with the compound furan (Fig. 6b). Again furanoses can exists in both a and (5 forms (Fig. 6a) except here the nomenclature refers to the hydroxyl group attached to C-2 which is the anomeric carbon atom. 

Two configurations known as anomers may result from the formation of the ring. These are distinguished by the anomeric prefixes a- and p-, which relate the configuration of the anomeric carbon atom to the configuration at a reference chiral carbon atom (normally the highest-numbered chiral carbon atom). For example, consider the glucopyranoses ... 

The same nomenclature applies to the furanose ring form of fructose, except that a and p refer to the hydroxyl groups attached to C-2, the anomeric carbon atom (see Figure 11.5). Fructose forms both pyranose and furanose rings. The pyranose form predominates in fructose free in solution, and the furanose form predominates in many fructose derivatives (Figure 11.6). Pentoses such as d-ribose and 2-deoxy-d-ribose form furanose rings, as we have seen in the structure of these units in RNA and DNA. 

Anomeric. Referring to a cyclic stereoisomer of the carbohydrate series with isomerism involving only the arrangement of atoms or groups at the aldehyde or ketone position. 

The monosaccharide D-glucose, whose chemistry is representative of all aldoses containing four or more carbon atoms, exists predominantly in the two pyranosc forms 4 and 5. These are six-membered hemiacetals formed by the reversible cyclization of the acyclic polyhydroxy aldehyde 3 (Eq. 23.1). In the cyclic forms 4 and 5, the ring carbon that is derived from the carbonyl group is referred to as the anomeric carbon atom. The specific rotation, [a] (Sec. 7.5), of a-D-(+)-glucose (4) is +112 whereas that of the -anomer 5 is +19°. When crystals of either pure 4 or pure 5 are dissolved in water, the [a]p changes to an equilibrium value of +52.7°. This process is termed mutarotation. At equilibrium in water, the a- and p-forms are present in the ratio of 36 64 only about 0.03% of D-glucose is in the acyclic form 3. 

Pd(0)-catalyzed allylation of purine derivatives with cyclopentenol esters and other alicydic allylic esters is a key step in the preparation of natural and unnatural carbanucleosides with antiviral activity. Carbanucleosides cannot be hydrolyzed as conventional nucleosides are since the anomeric carbon atom is absent therefore, their activity and stability render carbanucleosides valuable compounds. Although much work has been performed with aUylic acetates, a few examples of the application of allylic carbonates in carbanucleoside synthesis have been reported and their number will increase in the future. Two earlier examples by Trost and co-workers are in Scheme 30, including the numbering system of the purine skeleton. Other examples refer to more couplings of purines with flve-membered or with six-membered... 

---