Anomeric effect molecular orbital model

Two-electron/two-orbital h5q>erconjugative interactions of the t3q>e responsible for the anomeric effect depend on the relative orientation of bonds and lone pairs in a molecule and are also inversely proportional to the energy difference between the interacting orbitals. Spectroscopic manifestations of stereoelectronic interactions are particularly useful experimental signatures of these effects, which can be utilized for testing molecular models. Empirical observations together with theoretical interpretations in cyclohexane and six-membered heterocycles confirm the relevance of ffc uax Hax/ nx Hax (X = O or N), [Pg.210]

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. 

Calculated rotamer states for the acetoxymethyl group at C-5 of gluco-and galacto-pyranosyl rings have been compared with those obtained from A"-ray crystallography and found to be in good agreement. Molecular orbital calculations on dimethoxymethane, a model for the anomeric centre of methyl pyranosides, have produced results consistent with the anomeric and the exn-anomeric effects. The results were compared with evidence obtained by neutron diffraction. Dynamic H n.m.r. methods applied to the... 

Molecular orbital calculations have been carried out on 2-fluoro- and 2-chloro-tetrahydropyrans as models for examining the anomeric effect in gtycosyl halides. The calculated population of the axial conformer decreased as the solvent polarity increased, which indicated a consequent decrease in the anomeric effect.The synthesis of a series of 2-(perfluoroall yl) ethyl glycosides is mentioned in Chapter 3, and some 3 -fluorinated ribonucleosides are covered in Chapter 20. The syntheses of 2-fluoro-L-daunosaniine and 2-fluoro-D-ristosamine are discussed in Chapter 9, while 2 -fluorocarminontydn is described in Chapter 19, and some fluorinated 2-deoxy KDO analogues are covered in Chapter 16. 

Ab Initio molecular orbital calculations on methanedithiol and thiolmethanol have been used as models for the anomeric effect in S-C-S and 0-C-S carbohydrate systems. The lower anomeric effect in sulphur systems was attributed to lower dipolar contributions to the total energy compared to the oxygen systems. 

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