Effect of oxocarbonium ion structure

Effect of structure on pH-independent hydrolyses of acetals relative rates at 39°C, at constant leaving group acidity 

Comparison made at 15°C with R = Et [76], data for 2-( p-nitrophenoxy) tetrahydropyran [77] extrapolated to this temperature, and then extended to a leaving group pK, of 16 using a ]8 g value of — 1.18 for aryloxytetrahydropyrans obtained at 39 C [54]. 

Data on nitrogen leaving groups are less extensive and are summarised in Table 4. Note that in the case of the a-gluco and a-xylo compounds the sugar rings adopt normally disfavoured conformations. 

From the similar reactivities of tetrahydropyranyl and tetrahydrofuranyl derivatives (Table 3) it is clear that the difference between a 5-membered and a 6-membered ring per se cannot account for the increased reactivity of furanosyl derivatives, as exhibited for example in the departure of nicotinamide from NAD (Table 4), and the 200-400-fold faster departure of HjPO from a-D-ribofuranosyl phosphate than from a-D-glucopyranosyl phosphate [183]. It appears rather that relief of non-bonded interactions on forming an oxocarbonium ion is the main cause of the difference in the reactivity of the various glycosyl derivatives here considered. Conversion of an equatorial hydroxyl of a pyranoside to an axial hydroxyl group - 

---