Glycopyranosyl oxacarbenium

COMPUTATIONAL STUDIES OF THE ROLE OF GLYCOPYRANOSYL OXACARBENIUM IONS IN GLYCOBIOLOGY AND GLYCOCHEMISTRY... 

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. 

Following from the discussions in Section II.3, one of the many factors that affect the reactivity and stability of the glycopyranosyl oxacarbenium ion is the orientation and the conformation of the side chains attached to C-2, C-3, C-4, and C-5. In this study the implications for reactivity of the different stereochemistry and hence orientation at C-4 of galacto- versus g/nco-configured ions (2 versus 3) are one of the main considerations. This section considers some of these implications. 

Selected points from the vast glycosylation literature have been presented which are intended to complement the results shown for galactopyranosyl oxacarbenium ions related to 2 and 17. A number of little recognized parameters previously described for other glycopyranosyl oxacarbenium ions have been identified for 2 and 17 that help to suggest their generality. A summary is made below ... 

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. 

Facial selectivity of nucleophilic attack on glycopyranosyl oxacarbenium ions is determined by ring conformation and H-bonding. The latter depends especially on the lifetime of the oxacarbenium ions. 

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. 

Since encounter complexes of alcohol donors and glycopyranosyl oxacarbenium ions were not found, nor were CIPs of partially ionized glycopyranosyl donors, it is hypothesized that including solvent molecules in such studies is necessary. Such studies may require methodology development, but are included here since these are extrapolations of existing studies. 

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