Glycopyranosyl oxacarbenium ions, importance

For exact mH values and other geometric parameters for these species, see Table II. Following from the terminology used with other glycopyranosyl oxacarbenium ions, this conformation is called BO. The 5Si inverted conformer is called Bl, and in this case, since three conformers were found and not the usual two, the 3H4 conformer is termed B2. For both Bl and B2, 0-4 is quasi-equatorial and the classic Hassel-Ottar analysis of mH fails. This is why it was important to check all three mH minima. In this case Bl has mH = —159.3° as the minimum and B2 has mH = —35.1° as its minimum. 

The 5,S conformation of glycopyranosyl oxacarbenium ions with 0-2 equatorial and the remaining substituents quasi-axial is important, especially for gluco, galacto, and related configurations. 

The syn conformation about C-2-0-2 is an important feature of glycopyranosyl oxacarbenium ions. 

CBS calculations of important quantities, notably BO versus Bl, and B1 versus B2, and so on, for a selection of glycopyranosyl oxacarbenium ions. Such studies should give a more accurate number for the following effects (i) O-2-equatorial effect, (ii) 0-2 syn versus anti. Experimental studies of such glycopyranosyl oxacarbenium ions should also be encouraged. 

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