ACETALS AND RELATED FUNCTIONS

An acetal function can adopt any of the nine gauche conformations described in Fig. 1. Conformers A, B, and D are the mirror images of conforraers A.  

Stereoelectronic effects have long been recognized to influence the configuration and the conformation of acetals, particularly in carbohydrates where these effects were first discovered and discussed in terras of the anonieric and the exo-anomeric effects (1-3). The term anomeric effect introduced by Lemieux in 195B (4) refers to the tendency of an alkoxy group at C-1 of a pyranose ring to assume the axial rather than the equatorial orientation despite unfavorable steric interactions, whereas the term exo-anomeric effect also introduced by the same author (5) concerns the preferred orientation of the 0-R bond of the alkoxy group at the anomeric center. 

In practice, it makes little difference whether one believes that the anomeric effect is destabilizing or stabilizing since one can arrive at the same conclusion concerning the relative stability of conformers or isomers. For instance, for a molecule which can take the two conformations and 

if the anomeric effect is stabilizing, one concludes that Z is more stable than 2 because 2 has a stabilizing anomeric effect. If one accepts that the anoraeric effect is destabilizing, one concludes that 2 is more stable than 2 because 2 bas a destabilizing anoraeric effect. 

It is possible and probably very likely that both types of electronic effects are occurring in the acetal function. In other words, 2 could be more stable than ] because 2 is stabilized relative to 2 by a partial electron transfer and 2 is destabilized relative to 2 by electronic repulsions. There is presently no experimental technique to differentiate between the two effects. At the present time, many chemists, including myself, prefer to consider the anoraeric effect as a stabilizing rather than a destabilizing effect. The main reason is that the concept of stabilization of a system through electronic delocalization is a well established principle in organic chemistry. The resonance theory is indeed based on this principle. I believe that this concept rather than the dipole - dipole or electron pair - electron pair repulsions allows the organic chemist to rationalize his results better. 

---

Acetals and Related Functions 5 B, and respectively. The remaining conformers , , and F possess a plane of symmetry. Consequently, the acetal function can in principle exist in the six different conformations A-F. Experimental evidence will be described which shows that the relative stabilities of these various conformers depend on stereoelectronic effects and the standard steric interactions. 

---