Conformations, anomeric effect chair, observation

In contrast, the mannosyl radical 8 does not undergo such a conformational change, and the observed a-attack results from the shielding effect of the axial C-2 substituent in the chair conformation and the stereoelectronic effects mentioned earlier. In radicals 7b and 8 the C-O bonds adjacent to the radical center are coplanar with the singly occupied orbital. This reminds us of the anomeric effect in which an interaction between the nonbonding electron pair of the ring oxygen and the LUMO of the C-O bond stabilizes the conformation. 

N.m.r. studies on pyranoid sugar derivatives containing an epoxide ring indicate that the compounds adopt a half-chair conformation similar to that observed for unsaturated, pyranoid sugars. From an analysis of the spin-coupling data for some methyl 2,3- and 3,4-anhydroaldopyranosides, it was found that the particular half-chair form adopted is determined by the anomeric effect and the tendency of the 5-(hydroxymethyl) or 5-methyl group to assume the equatorial... 

There is a well-known conformational effect observed in pyran derivatives called the anomeric effect. Whereas methoxycyclohexane (A) exists primarily in the chair conformation with the OMe unit equatorial, 2-methoxypyran (B) exists primarily in the chair conformation where the OMe is axial. Although the real explanation for this is nontrivial, suggest a simple reason that is consistent with why B might exist with an axial OMe unit. 

Paulsen has discussed the influence of jy/z-l,3-diaxial interactions and the anomeric effect on the conformations of various pyranoid derivatives. High proportions of the C4 forms, indicating that there are strong syn-d ax a interactions in the Ci forms, were observed for 2,4-diazido-2,4,6-trideoxy-a-D-idopyranoside and the corresponding 2,4-diamino dihydrochloride derivative. Strong 5y -diaxial interactions in the usual chair forms of l,6-anhydro-2,4-... 

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