External anomeric torsional effect

Discussion in the literature regarding the exo-anomeric effect has to do with the anomeric effect when it occurs with a pyranose ring, where one oxygen is in the ring and the other one is external (exo) to the ring. It was thought earlier that this particular arrangement led to unusual characteristics for the anomeric effect that seemed somewhat different from the ordinary anomeric effect found with simple molecules such as dimethoxymethane. Earlier, we showed that the so-called exo-anomeric effect is really just an ordinary anomeric effect, but it may occur in more complicated mole- 

In Structure 6 are shown the pertinent conformers of dimethoxymethane. A capital letter is used to indicate the bond about which the conformation is of particular interest (the 3,4 bond). 

Where does this reduction of the barrier for the methyl rotation in dimethoxymethane come from Structure 7 shows the aG conformation in which this barrier reduction is important. 

TABLE 8.3. Methyl Rotational Barriers (C4-C5) in Dimethoxymethane (kcal/mol) 

Structure 7. External anomeric torsional effect aG dimethoxymethane. 

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Figure 8.3. Ramachandran plot of the CBA potential surface by MM40 (without the external anomeric torsional effect). (From Lii, Chen, Johnson, French, and Alllnger. Copyright 2005 by Elsevier. Reprinted by permission of the authors.)...

In order to reproduce this external anomeric torsional effect in MM4, a torsion-torsion interaction (between the 2,3,4,5 and 3,4,5,6 bonds in Structure 8) was used. It was worked out so that the results on dimethoxymethane and on methoxyeth-oxymethane as obtained by MMP2 calculations were reproduced with MM4. (It is... 

Without the external anomeric or Bohlmann torsional effects. With the external anomeric and Bohlmann torsional effects. 

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