Anomeric stabilization relative energies

MM2(91), MMX, and MM2 molecular mechanics calculations have been performed to determine the relative steric energy of (25)-2-hydroxymethyl-9-methylperhydro[2,l-h][l,3]oxazine. MM2(91) calculations predict that the major isomer should be a cis-fused conformation, overestimating the influence of anomeric stabilization, but the other calculations give more realistic predictions that the major isomer should be (2S)(5R)(9aS)-trans fused (95JOC2989). 

Take a look at the following reaction and note the formation of the two isomeric products. Predict, on account of relative energy calculation, the ratio of the two isomers. Please consider the anomeric stabilization arising from electron pair orbital on sulfur the same as that arising from oxygen. 

In cases where substitution of the spiroacetal means that there is little difference between the relative energies of the conformations, a mixture is usually obtained. Given that most spiroacetal-containing natural products are doubly anomerically stabilized, this method is ideal for, and has found extensive use in, their synthesis. 

The paucity of information on the mechanism of reactions, and on the structure of the transition state, and the role of the anomeric effect in its stabilization, constitutes the main reason why qualitative interpretation of reactivity as shown in the aforementioned examples is still very rare. An alternative, more-popular estimation of the relative reaction-rates of con-formers is based on the lone-pair orbital interactions, and their symmetry and energy in the ground state, and could be loosely associated with the perturbation theory of chemical reactivity. ... 

Fig. 11. —Hypothetical Role of Pseudo-Acyclic Intermediates in Mutarotation Reactions. [The curves qualitatively represent the changes in free energy of a sugar for which the /3-pyranose is more stable than the a-pyranose (curve 1). The pyranose-furanose interconversion (curve 2) is faster than either the a-/3-pyranose anomerization (curve 1) or the a-/8-furanose anomerization (curve 3). The relative stabilities of the isomers are represented in the decreasing order /3-pyranose > a-pyranose > /3-furanose > a-furanose.]...

Syn-anomeric effect In order to achieve a better understanding of the conformational dependence of stereoelectronic effects in anomeric systems, Dunitz and coworkers asked whether a syn-oriented lone pair could provide a similar, if perhaps weaker, effect on structure and reactivity relative to an anti-oriented lone pair and play a role when the antiperiplanar orientation is inaccessible. Indeed, it was found that the syn anomeric effect is weaker bnt is still a strongly stabilizing effect in a-fluoroamines. Furthermore, it has similar structural features an elongated C-F bond and a shortened C-N bond. Taking into account the presence of two eclipsed CH/NH contacts and approximating their energies as -1 kcal/mol (similar to the cost... 

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