Toward the origin of the conformational preference of 2-methoxyoxane, a model useful to study the anomeric effect

Abstract

The potential energy curves resulting from the C 2 -O exo bond rotation of 2-OMeOxane were calculated. One minimum was found for the axial conformer at MP2/6-31G(d,p); B3LYP/6-31G(d,p); and HF/6-31G(d,p) levels, and two minima in the equatorial conformer. The difference in the entropy of mixing between both conformers has a positive value, close to zero, which means that the entropy is higher for the equatorial conformer. The Scheme of rotation of the methoxy group at position 2 of oxane with six conformers, usually employed to describe this conformational process, is inadequate.

A True Reverse Anomeric Effect Does Exist After All: A Hydrogen Bonding Stereocontrolling Effect in 2-Iminoaldoses

“…As discussed above, the origin of mutarotation in imines derived from 2-amino-2-deoxyaldoses can be traced on the one hand to anomerization equilibria and on the other to the existence of imine-enamine structures via tautomerization. Among the stereoconformational aspects controlling the molecular arrangements observed in carbohydrates, the anomeric, exoanomeric, and gauche effects are the most prominent . Not by chance, the interpretation of the anomeric effects and other associated stereoelectronic effects in six-membered saturated heterocycles has been extensively documented. ,− …”

“…Among the stereoconformational aspects controlling the molecular arrangements observed in carbohy- drates, the anomeric, exoanomeric, 40 and gauche effects 41 are the most prominent. 42 Not by chance, the interpretation of the anomeric effects and other associated stereoelectronic effects in six-membered saturated heterocycles has been extensively documented. 40,43−46 Even if the first observations of "anomalous effects" date back to early studies on mutarotation, 46 the preferential stabilization of pyranose rings when they contain an axial electronegative substituent at C1 is contrary to expectations based on considerations of steric or solvation factors.…”

Conformational Analysis of Nine-Membered Cyclic Acetals. Stereoelectronic Effect in 2,4- and 3,5-Benzodioxonine Derivatives

“…In this approach, the electronic wave function of the molecule is treated as a linear combination of localized orbitals, either Lewis occupied, or non-Lewis unoccupied. Examination of delocalizations between these orbitals and evaluation of the energies associated with their interactions is a common technique. − ,20b,− It can be accomplished either by inspecting the electronic energy partitioning obtained from second-order perturbation theory 13,34 or by performing selective deletions of off-diagonal elements of the Fock matrix. , …”

“…Experimental methods are by far more common in studying modifications to the geometrical and thermodynamical parameters affected by the stereoelectronic effect, [3][4][5][6][7][8] while the theoretical approach is the method of choice to investigate the fundamental mechanisms underlying this phenomenon. [9][10][11][12][13][14] Conformational preferences of the acetal moiety C-O-C- O-C are in principle governed by a stereoelectronic effect defined as a delocalization of an oxygen lone pair into a C-O antibonding orbital (n O f σ C-O *). The highest stability of such a fragment is a consequence of the most effective overlap between the above-mentioned orbitals, which can be achieved for either a gauche+/gauche+ (g+g+) or gauche+/gauche-(g+g-) atom arrangement.…”

“…For the last five decades the stereoelectronic effect, being one of the key structural factors recognized in a vast number of natural and synthetic compounds, has attracted considerable attention among both experimental and theoretical chemists. , Numerous attempts have been undertaken to investigate consequences and to understand the origin and nature of this effect. Experimental methods are by far more common in studying modifications to the geometrical and thermodynamical parameters affected by the stereoelectronic effect, − while the theoretical approach is the method of choice to investigate the fundamental mechanisms underlying this phenomenon. − …”