Anomeric effect in carbohydrates

Anhydrous hydrogen fluoride, application of for the structural analysis of polysaccharides, 47, 167-202 Anomeric and exo-anomeric effects in carbohydrate chemistry, 47, 45-123 Anomeric-oxygen activation for glycoside synthesis, trichloroacetimidate method, 50,21-123... 

Stoddart, J F, Stereochemistry of Carbohydrates, Wiley-Interscience, New York, 1971, pp. 1-249. Tvaroska, I, Bleha, T, Anomeric and Exo-Anomeric Effects in Carbohydrate Chemistry, Adv. Carbohydr. Chem. Biochem. A1, 45-123, 1989. 

Figure 4.17 The anomeric effect in carbohydrates leads to energetic preference of the electronegative substituent (X = OH, F) in the axial orientation by ca. 6 kcal mol [36], because of stabilizing overlap. With X in the sterically more favorable equatorial position this overlap is not possible.

Cossd-Barbi A, Watson DG, Dubois JE (1989) Anomeric effect in carbohydrates non-equivalence of endocyclic oxygen lone pairs. Tetrahedron Lett 30 163-166... 

An extensive review has appeared on the anomeric and exo-anomeric effects in carbohydrate chemistry. A further review of the anomeric effect asserts that n- a contributions to the former effect are small relative to those of n- n destabilising components. Further consideration has been given to the effect following analysis of 529 crystal structures of carbohydrates, in particular the C-O bond lengths and C-O-C and O-C-O bond angles as they depend on dihedral angles in the sequence C-O-C-O-C. The work is an extension of an earlier study (Vol. 18, p.2, ref. 4). ... 

The incorporation of heteroatoms can result in stereoelectronic effects that have a pronounced effect on conformation and, ultimately, on reactivity. It is known from numerous examples in carbohydrate chemistry that pyranose sugars substituted with an electron-withdrawing group such as halogen or alkoxy at C-1 are often more stable when the substituent has an axial, rather than an equatorial, orientation. This tendency is not limited to carbohydrates but carries over to simpler ring systems such as 2-substituted tetrahydropyrans. The phenomenon is known as the anomeric ect, because it involves a substituent at the anomeric position in carbohydrate pyranose rings. Scheme 3.1 lists... 

Carba-sugars (pseudo-sugars) and their derivatives, chemistry of, 48, 21 - 90 Carbohydrate chemistry anomeric and exo-anomeric effects in, 47, 45-123... 

On the basis of the above results and discussion, the glycosides can now be considered. Efforts have been made previously to evaluate the magnitude of the anomeric effect by undertaking equilibration studies between equatorial and axial isomers at the anomeric center in carbohydrates (48), in monosubstituted 2-alkoxytetrahydropyrans (49, 50) and in more rigid systems (51). The anomeric effect has been evaluated to be of the order of 1.2 to 1.8 kcal/mol from these studies. In these evaluations, the conformation of the OR group in the axial and in the equatorial isomer was not considered the influence of the exo-anomeric effect was therefore neglected (3). Nevertheless, these studies demonstrated the importance of the anomeric effect. 

In closing this short and incomplete, theoretical excursion into the field of reaction stereoselectivity in carbohydrates, we have also to mention that some other aspects in this article have been treated only partially for instance, the anomeric effect in rings other than six-membered, or in carbohydrate radicals. It is hoped that the present article has shown that many questions on the nature and various manifestations of the anomeric and related effects have already been answered. The problems still remaining to be solved will attract growing attention. " We believe that this survey. 

In addition to steric effects and electrostatic effects, the conformational analysis of carbohydrates requires two stereoelectronic effects to be taken into consideration the gauche effect and the anomeric effect, in its various manifestations. Both of these effects can be considered as aspects of no-bond resonance. 

As indicated at the beginning of this section, heteroatom hyperconjugation was first recognized in carbohydrates. The anomeric effect has been particularly well studied in cyclic systems, such as found in carbohydrates. We return to the anomeric effect in cyclic systems in Topic 2.3. 

Woodcock HL, Moran D, Pastor RW, McKerell AD Jr, Brooks BR (2007) Ab initio modeling of glycosyl torsions and anomeric effects in a model carbohydrate 2-ethoxy tetrahydropyran. Biophys J 93 1-10... 

Tvaroska, L, Carver, J. P. (1996). Ab Initio Molecular Orbital Calculation of Carbohydrate Model Compounds. 5. Anomeric, xo-Anomeric, and Reverse Anomeric Effects in C-, N-, and S-Glycosyl Compounds. Journal of Physical Chemistry, 100, 11305-11313. 

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