Anomers common sugars

Even at the inception of ALPH, however, there was one system already known where the pattern of reactivity apparently contradicted it. It was well known in carbohydrate chemistry that protected a-glycopyranosyl halides were less labile than their p-anomers, at least for the common sugars in the D series (Haynes and Newth, 1955). The pyranose ring in these compounds is strongly biased towards the 4CX conformation, so that the a-halides are axial and the P-halides are equatorial. The a-halides have an antiperiplanar... 

Although the crystalline forms of a- and /3-D-glucose are quite stable, in solution each form slowly changes into an equilibrium mixture of both. The process can be observed as a decrease in the optical rotation of the a anomer (+112°) or an increase for the /3 anomer (+18.7°) to the equilibrium value of 52.5°. The phenomenon is known as mutarotation and commonly is observed for reducing sugars. Both acids and bases catalyze mutarotation the mechanism, Equation 20-1, is virtually the same as described for acid- and base-catalyzed hemiacetal and hemiketal equilibria of aldehydes and ketones (see Section 15-4E) ... 

Glycosyl bromides are most commonly prepared by treatment of a per-0-acylated sugar derivative with a solution of HBr in acetic acid (Scheme 4.2b). The more stable a-anomer is usually obtained in high yield. In this reaction, the anomeric acetyl moiety is converted into a good leaving... 

When the substituent in 54 or 55 is axial, and also in the case of the a-anomer of a sugar, it is clear that the destabilizing C-O dipole effects, present when oxygen of OMe is equatorial, are minimized when this oxygen becomes axial. The absence of any significant dipolar repulsions in the latter case has been proposed as the cause of the anomeric effect (see Perrin et al.1). Though its cause is still uncertain the anomeric effect is well documented and it commonly has a magnitude of ca. 8-10 kJ mol, and is solvent dependent. 

Natural sialic acids (Schauer 1982 1991) are derivatives of 5-amino-3,5-dideoxy- D-glycero-D-galacto-nonu osonic acid 12.1. This awkward name has been replaced by neuraminic acid . The most common derivative is N-acetyl-neuraminic acid 12.2 whose configuration is easy to memorize because, in the Fischer representation, 12.3, it is presented as an aldolic condensation product of N-acetylmannosamine (2-acetamido-2-deoxy-D-mannose) and pyruvic acid. When the expression sialic acid is used without any other precision, it is in reference to derivative 12.2. It exists in the free state or glycosidated in the d-conformation, which allows an equatorial disposition of the three-carbon side chain. Structure 12.2 represents the stable /3-anomer of the free sugar with an axial anomeric hydroxyl group and all-equatorial non-anomeric substituents. An X-ray spectrum of this crystallized /3- anomer confirms this conformation and reveals, moreover, that the side chain has the zig-zag conformation with two... 

We will illustrate the phenomenon for the crystallization of lactose. Lactose is a reducing sugar, and in solution the a and p anomers are in equilibrium with each other the ratio p over a is about 1.6. The crystallization of a-lactose monohydrate, the most common crystalline form, has been studied in detail. Figure 15.10 depicts a crystal as formed at... 

In 1950, Kuhn recorded the spectra of many carbohydrates, ten of which were crystalline sugars in Nujol mulls. Except for a-o-glucose, the type of anomer used was not mentioned. Nor did Urbanski et al. (1959) specify anomeric form when they recorded the spectra of six sugars. Tipson and Isbell (1962) have since identified these compounds by comparing their spectra with those of authentic anomeric forms of those sugars. Such studies for the identification of compounds are among the most common applications of infrared spectroscopy. 

---